ATMEGA64A Datasheet by Microchip Technology

AtmeE
ATmega64A
ATmega64A
DATASHEET COMPLETE
Introduction
The Atmel® ATmega64A is a low-power CMOS 8-bit microcontroller based
on the AVR® enhanced RISC architecture. By executing powerful instructions
in a single clock cycle, the ATmega64A achieves throughputs close to
1MIPS per MHz. This empowers system designer to optimize the device for
power consumption versus processing speed.
Features
•High-performance, Low-power Atmel AVR 8-bit Microcontroller
•Advanced RISC Architecture
–130 Powerful Instructions - Most Single-clock Cycle Execution
–32 × 8 General Purpose Working Registers + Peripheral Control
Registers
–Fully Static Operation
–Up to 16MIPS Throughput at 16MHz
–On-chip 2-cycle Multiplier
•High Endurance Non-volatile Memory segments
–64Kbytes of In-System Self-programmable Flash program
memory
–2Kbytes EEPROM
–4Kbytes Internal SRAM
–Write/Erase cycles: 10,000 Flash/100,000 EEPROM
–Data retention: 20 years at 85°C/100 years at 25°C(1)
–Optional Boot Code Section with Independent Lock Bits
• In-System Programming by On-chip Boot Program
• True Read-While-Write Operation
–Up to 64 Kbytes Optional External Memory Space
–Programming Lock for Software Security
–SPI Interface for In-System Programming
•JTAG (IEEE std. 1149.1 Compliant) Interface
–Boundary-scan Capabilities According to the JTAG Standard
–Extensive On-chip Debug Support
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–Programming of Flash, EEPROM, Fuses and Lock Bits through the JTAG Interface
•Atmel QTouch® library support
–Capacitive touch buttons, sliders and wheels
–Atmel QTouch and QMatrix acquisition
–Up to 64 sense channels
•Peripheral Features
–Two 8-bit Timer/Counters with Separate Prescalers and Compare Modes
–Two Expanded 16-bit Timer/Counters with Separate Prescaler, Compare Mode and Capture
Mode
–Real Time Counter with Separate Oscillator
–Two 8-bit PWM Channels
–6 PWM Channels with Programmable Resolution from 1 to 16 Bits
–Output Compare Modulator
–8-channel, 10-bit ADC
• 8 Single-ended Channels
• 7 Differential Channels
• 2 Differential Channels with Programmable Gain at 1x, 10x, or 200x
–Byte-oriented Two-wire Serial Interface
–Dual Programmable Serial USARTs
–Master/Slave SPI Serial Interface
–Programmable Watchdog Timer with On-chip Oscillator
–On-chip Analog Comparator
•Special Microcontroller Features
–Power-on Reset and Programmable Brown-out Detection
–Internal Calibrated RC Oscillator
–External and Internal Interrupt Sources
–Six Sleep Modes: Idle, ADC Noise Reduction, Power-save, Power-down, Standby, and
Extended Standby
–Software Selectable Clock Frequency
–ATmega103 Compatibility Mode Selected by a Fuse
–Global Pull-up Disable
•I/O and Packages
–53 Programmable I/O Lines
–64-lead TQFP and 64-pad QFN/MLF
•Operating Voltages
–2.7 - 5.5V
•Speed Grades
–0 - 16MHz
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Table of Contents
Introduction......................................................................................................................1
Features.......................................................................................................................... 1
1. Description.................................................................................................................9
2. Configuration Summary...........................................................................................10
3. Ordering Information................................................................................................ 11
4. Block Diagram......................................................................................................... 12
5. ATmega103 and ATmega64A Compatibility............................................................ 13
5.1. ATmega103 Compatibility Mode.................................................................................................13
6. Pin Configurations................................................................................................... 14
6.1. Pin Descriptions..........................................................................................................................14
7. Resources................................................................................................................18
8. Data Retention.........................................................................................................19
9. About Code Examples.............................................................................................20
10. Capacitive Touch Sensing....................................................................................... 21
11. AVR CPU Core........................................................................................................ 22
11.1. Overview.....................................................................................................................................22
11.2. ALU – Arithmetic Logic Unit........................................................................................................23
11.3. Status Register...........................................................................................................................23
11.4. General Purpose Register File................................................................................................... 25
11.5. Stack Pointer.............................................................................................................................. 26
11.6. Instruction Execution Timing...................................................................................................... 26
11.7. Reset and Interrupt Handling..................................................................................................... 27
12. AVR Memories.........................................................................................................30
12.1. Overview.....................................................................................................................................30
12.2. In-System Reprogrammable Flash Program Memory................................................................30
12.3. SRAM Data Memory...................................................................................................................31
12.4. EEPROM Data Memory............................................................................................................. 33
12.5. I/O Memory.................................................................................................................................34
12.6. External Memory Interface......................................................................................................... 34
12.7. Register Description................................................................................................................... 41
13. System Clock and Clock Options............................................................................ 52
13.1. Clock Systems and their Distribution..........................................................................................52
13.2. Clock Sources............................................................................................................................ 53
Atmel
13.3. Default Clock Source..................................................................................................................54
13.4. Crystal Oscillator........................................................................................................................ 54
13.5. Low-frequency Crystal Oscillator................................................................................................55
13.6. External RC Oscillator................................................................................................................ 56
13.7. Calibrated Internal RC Oscillator................................................................................................56
13.8. External Clock............................................................................................................................ 57
13.9. Timer/Counter Oscillator.............................................................................................................58
13.10. Register Description...................................................................................................................58
14. Power Management and Sleep Modes................................................................... 61
14.1. Sleep Modes...............................................................................................................................61
14.2. Idle Mode....................................................................................................................................62
14.3. ADC Noise Reduction Mode.......................................................................................................62
14.4. Power-down Mode......................................................................................................................62
14.5. Power-save Mode.......................................................................................................................62
14.6. Standby Mode............................................................................................................................ 63
14.7. Extended Standby Mode............................................................................................................ 63
14.8. Minimizing Power Consumption................................................................................................. 63
14.9. Register Description................................................................................................................... 65
15. System Control and Reset.......................................................................................67
15.1. Resetting the AVR...................................................................................................................... 67
15.2. Reset Sources............................................................................................................................67
15.3. Internal Voltage Reference.........................................................................................................71
15.4. Watchdog Timer......................................................................................................................... 71
15.5. Timed Sequences for Changing the Configuration of the Watchdog Timer............................... 72
15.6. Register Description................................................................................................................... 73
16. Interrupts................................................................................................................. 77
16.1. Interrupt Vectors in ATmega64A.................................................................................................77
16.2. Register Description................................................................................................................... 82
17. External Interrupts................................................................................................... 85
17.1. Register Description................................................................................................................... 85
18. I/O Ports.................................................................................................................. 92
18.1. Overview.....................................................................................................................................92
18.2. Ports as General Digital I/O........................................................................................................93
18.3. Alternate Port Functions.............................................................................................................96
18.4. Register Description..................................................................................................................111
19. Timer/Counter3, Timer/Counter2, and Timer/Counter1 Prescalers....................... 134
19.1. Overview...................................................................................................................................134
19.2. Internal Clock Source............................................................................................................... 134
19.3. Prescaler Reset........................................................................................................................134
19.4. External Clock Source..............................................................................................................134
19.5. Register Description................................................................................................................. 135
20. 16-bit Timer/Counter (Timer/Counter1 and Timer/Counter3).................................137
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20.1. Features................................................................................................................................... 137
20.2. Overview...................................................................................................................................137
20.3. Accessing 16-bit Registers.......................................................................................................140
20.4. Timer/Counter Clock Sources.................................................................................................. 142
20.5. Counter Unit............................................................................................................................. 142
20.6. Input Capture Unit.................................................................................................................... 143
20.7. Output Compare Units..............................................................................................................146
20.8. Compare Match Output Unit.....................................................................................................147
20.9. Modes of Operation..................................................................................................................148
20.10. Timer/Counter Timing Diagrams.............................................................................................. 156
20.11. Register Description................................................................................................................. 157
21. 8-bit Timer/Counter0 with PWM and Asynchronous Operation............................. 192
21.1. Features................................................................................................................................... 192
21.2. Overview...................................................................................................................................192
21.3. Timer/Counter Clock Sources.................................................................................................. 193
21.4. Counter Unit............................................................................................................................. 193
21.5. Output Compare Unit................................................................................................................194
21.6. Compare Match Output Unit.....................................................................................................196
21.7. Modes of Operation..................................................................................................................197
21.8. Timer/Counter Timing Diagrams...............................................................................................201
21.9. Asynchronous Operation of the Timer/Counter........................................................................ 203
21.10. Timer/Counter Prescaler.......................................................................................................... 204
21.11. Register Description................................................................................................................. 205
22. 8-bit Timer/Counter2 with PWM.............................................................................215
22.1. Features................................................................................................................................... 215
22.2. Overview...................................................................................................................................215
22.3. Timer/Counter Clock Sources.................................................................................................. 216
22.4. Counter Unit............................................................................................................................. 216
22.5. Output Compare Unit................................................................................................................217
22.6. Compare Match Output Unit.....................................................................................................219
22.7. Modes of Operation..................................................................................................................220
22.8. Timer/Counter Timing Diagrams...............................................................................................224
22.9. Register Description................................................................................................................. 225
23. Output Compare Modulator (OCM1C2).................................................................233
23.1. Overview...................................................................................................................................233
23.2. Description................................................................................................................................233
24. SPI – Serial Peripheral Interface........................................................................... 235
24.1. Features................................................................................................................................... 235
24.2. Overview...................................................................................................................................235
24.3. SS Pin Functionality................................................................................................................. 238
24.4. Data Modes.............................................................................................................................. 239
24.5. Register Description................................................................................................................. 240
25. USART...................................................................................................................245
25.1. Features................................................................................................................................... 245
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25.2. Overview...................................................................................................................................245
25.3. Clock Generation......................................................................................................................247
25.4. Frame Formats.........................................................................................................................250
25.5. USART Initialization..................................................................................................................251
25.6. Data Transmission – The USART Transmitter......................................................................... 252
25.7. Data Reception – The USART Receiver.................................................................................. 255
25.8. Asynchronous Data Reception.................................................................................................258
25.9. Multi-Processor Communication Mode.....................................................................................261
25.10. Examples of Baud Rate Setting............................................................................................... 262
25.11. Register Description................................................................................................................. 265
26. TWI - Two-wire Serial Interface............................................................................. 274
26.1. Features................................................................................................................................... 274
26.2. Overview...................................................................................................................................274
26.3. Two-Wire Serial Interface Bus Definition..................................................................................276
26.4. Data Transfer and Frame Format.............................................................................................277
26.5. Multi-master Bus Systems, Arbitration and Synchronization....................................................280
26.6. Using the TWI...........................................................................................................................281
26.7. Multi-master Systems and Arbitration.......................................................................................298
26.8. Register Description................................................................................................................. 299
27. Analog Comparator............................................................................................... 306
27.1. Overview...................................................................................................................................306
27.2. Analog Comparator Multiplexed Input...................................................................................... 306
27.3. Register Description................................................................................................................. 307
28. ADC - Analog to Digital Converter......................................................................... 311
28.1. Features....................................................................................................................................311
28.2. Overview...................................................................................................................................311
28.3. Starting a Conversion...............................................................................................................313
28.4. Prescaling and Conversion Timing...........................................................................................314
28.5. Changing Channel or Reference Selection.............................................................................. 317
28.6. ADC Noise Canceler................................................................................................................ 318
28.7. ADC Conversion Result............................................................................................................322
28.8. Register Description................................................................................................................. 324
29. JTAG Interface and On-chip Debug System..........................................................335
29.1. Features................................................................................................................................... 335
29.2. Overview...................................................................................................................................335
29.3. TAP – Test Access Port............................................................................................................336
29.4. TAP Controller.......................................................................................................................... 337
29.5. Using the Boundary-scan Chain...............................................................................................338
29.6. Using the On-chip Debug System............................................................................................ 338
29.7. On-chip Debug Specific JTAG Instructions.............................................................................. 339
29.8. Using the JTAG Programming Capabilities.............................................................................. 340
29.9. Bibliography..............................................................................................................................340
29.10. IEEE 1149.1 (JTAG) Boundary-scan........................................................................................340
29.11. Data Registers..........................................................................................................................341
29.12. Boundry-scan Specific JTAG Instructions................................................................................ 343
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29.13. Boundary-scan Chain...............................................................................................................344
29.14. ATmega64A Boundary-scan Order.......................................................................................... 354
29.15. Boundary-scan Description Language Files............................................................................ 363
29.16. Register Description.................................................................................................................363
30. BTLDR - Boot Loader Support – Read-While-Write Self-Programming................ 366
30.1. Features................................................................................................................................... 366
30.2. Overview...................................................................................................................................366
30.3. Application and Boot Loader Flash Sections............................................................................366
30.4. Read-While-Write and No Read-While-Write Flash Sections...................................................367
30.5. Boot Loader Lock Bits.............................................................................................................. 369
30.6. Entering the Boot Loader Program...........................................................................................370
30.7. Addressing the Flash During Self-Programming...................................................................... 371
30.8. Self-Programming the Flash.....................................................................................................372
30.9. Register Description................................................................................................................. 380
31. Memory Programming...........................................................................................383
31.1. Program and Data Memory Lock Bits.......................................................................................383
31.2. Fuse Bits...................................................................................................................................384
31.3. Signature Bytes........................................................................................................................ 386
31.4. Calibration Byte........................................................................................................................ 386
31.5. Page Size................................................................................................................................. 387
31.6. Parallel Programming...............................................................................................................387
31.7. Parallel Programming Parameters, Pin Mapping, and Commands..........................................394
31.8. Serial Downloading...................................................................................................................396
31.9. Serial Programming Pin Mapping.............................................................................................396
31.10. Programming Via the JTAG Interface.......................................................................................400
32. Electrical Characteristics – TA = -40°C to 85°C.....................................................415
32.1. DC Characteristics....................................................................................................................415
32.2. Speed Grades.......................................................................................................................... 417
32.3. Clock Characteristics................................................................................................................417
32.4. System and Reset Characteristics........................................................................................... 418
32.5. Two-wire Serial Interface Characteristics................................................................................. 419
32.6. Parallel Programming Characteristics...................................................................................... 421
32.7. SPI Timing Characteristics....................................................................................................... 422
32.8. ADC Characteristics................................................................................................................. 424
32.9. External Data Memory Timing.................................................................................................. 427
33. Electrical Characteristics – TA = -40°C to 105°C...................................................433
33.1. DC Characteristics....................................................................................................................433
34. Typical Characteristics – TA = -40°C to 85°C........................................................436
34.1. Active Supply Current...............................................................................................................436
34.2. Idle Supply Current...................................................................................................................440
34.3. Power-Down Supply Current....................................................................................................443
34.4. Power-Save Supply Current.....................................................................................................444
34.5. Standby Supply Current........................................................................................................... 445
34.6. Pin Pull-up................................................................................................................................ 446
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34.7. Pin Driver Strength................................................................................................................... 449
34.8. Pin Thresholds and Hysteresis.................................................................................................451
34.9. BOD Thresholds and Analog Comparator Offset..................................................................... 454
34.10. Internal Oscillator Speed..........................................................................................................455
34.11. Current Consumption of Peripheral Units.................................................................................462
34.12. Current Consumption in Reset and Reset Pulse width............................................................ 464
35. Typical Characteristics – TA = -40°C to 105°C......................................................466
35.1. Active Supply Current...............................................................................................................466
35.2. Idle Supply Current...................................................................................................................469
35.3. Power-down Supply Current.....................................................................................................472
35.4. Pin Pull-up................................................................................................................................ 473
35.5. Pin Driver Strength................................................................................................................... 475
35.6. Pin Thresholds and Hysteresis.................................................................................................477
35.7. BOD Thresholds and Analog Comparator Offset..................................................................... 480
35.8. Internal Oscillator Speed.......................................................................................................... 482
35.9. Current Consumption of Peripheral Units.................................................................................487
35.10. Current Consumption in Reset and Reset Pulsewidth............................................................. 490
36. Register Summary.................................................................................................492
37. Instruction Set Summary....................................................................................... 495
38. Packaging Information...........................................................................................500
38.1. 64A........................................................................................................................................... 500
38.2. 64M1.........................................................................................................................................501
39. Errata.....................................................................................................................502
39.1. ATmega64A Rev. D..................................................................................................................502
40. Datasheet Revision History................................................................................... 504
40.1. 8160E - 07/2015.......................................................................................................................504
40.2. 8160D - 02/2013.......................................................................................................................504
40.3. 8160C - 07/2009.......................................................................................................................504
40.4. 8160B - 03/2009.......................................................................................................................504
40.5. 8160A - 08/2008.......................................................................................................................504
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1. Description
The AVR core combines a rich instruction set with 32 general purpose working registers. All the 32
registers are directly connected to the Arithmetic Logic Unit (ALU), allowing two independent registers to
be accessed in one single instruction executed in one clock cycle. The resulting architecture is more code
efficient while achieving throughputs up to ten times faster than conventional CISC microcontrollers.
The ATmega64A provides the following features: 64 Kbytes In-System Programmable Flash with Read-
While- Write capabilities, 2 Kbytes EEPROM, 4 Kbytes SRAM, 53 general purpose I/O lines, 32 general
purpose working registers, Real Time Counter (RTC), four flexible Timer/Counters with compare modes
and PWM, two USARTs, a byte oriented Two-wire Serial Interface, an 8-channel, 10-bit ADC with optional
differential input stage with programmable gain, programmable Watchdog Timer with internal Oscillator,
an SPI serial port, IEEE std. 1149.1 compliant JTAG test interface, also used for accessing the On-chip
Debug system and programming, and six software selectable power saving modes. The Idle mode stops
the CPU while allowing the SRAM, Timer/Counters, SPI port, and interrupt system to continue
functioning. The Power-down mode saves the register contents but freezes the Oscillator, disabling all
other chip functions until the next interrupt or Hardware Reset. In Power-save mode, the asynchronous
timer continues to run, allowing the user to maintain a timer base while the rest of the device is sleeping.
The ADC Noise Reduction mode stops the CPU and all I/O modules except asynchronous timer and
ADC, to minimize switching noise during ADC conversions. In Standby mode, the crystal/resonator
Oscillator is running while the rest of the device is sleeping. This allows very fast start-up combined with
low power consumption. In Extended Standby mode, both the main Oscillator and the asynchronous timer
continue to run.
The device is manufactured using Atmel’s high-density non-volatile memory technology. The On-chip ISP
Flash allows the program memory to be reprogrammed In-System through an SPI serial interface, by a
conventional nonvolatile memory programmer, or by an On-chip Boot program running on the AVR core.
The Boot Program can use any interface to download the Application Program in the Application Flash
memory. Software in the Boot Flash section will continue to run while the Application Flash section is
updated, providing true Read-While-Write operation. By combining an 8-bit RISC CPU with In-System
Self-Programmable Flash on a monolithic chip, the Atmel ATmega64A is a powerful microcontroller that
provides a highly-flexible and cost-effective solution to many embedded control applications.
The ATmega64A AVR is supported with a full suite of program and system development tools including: C
compilers, macro assemblers, program debugger/simulators, in-circuit emulators, and evaluation kits.
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2. Configuration Summary
Features ATmega64A
Pin count 64
Flash (KB) 64
SRAM (KB) 4
EEPROM (KB) 2
General Purpose I/O pins 53
SPI 1
TWI (I2C) 1
USART 2
ADC 10-bit, up to 76.9ksps (15ksps at max resolution)
ADC channels 6 (8 in TQFP and QFN/MLF packages)
AC propagation delay Typ 400ns
8-bit Timer/Counters 2
16-bit Timer/Counters 1
PWM channels 8
RC Oscillator +/-3%
VREF Bandgap
Operating voltage 2.7 - 5.5V
Max operating frequency 16MHz
Temperature range -55°C to +125°C
JTAG Yes
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3. Ordering Information
Speed (MHz) Power Supply Ordering Code(2) Package(1) Operational Range
16 2.7 - 5.5V
ATmega64A-AU
ATmega64A-AUR(3)
ATmega64A-MU
ATmega64A-MUR(3)
64A
64A
64M1
64M1
Industrial (-40oC to 85oC)
ATmega64A-AN
ATmega64A-ANR(3)
ATmega64A-MN
ATmega64A-MNR(3)
64A
64A
64M1
64M1
Extended (-40oC to 105oC)(4)
Note: 
1. This device can also be supplied in wafer form. Please contact your local Atmel sales office for
detailed ordering information and minimum quantities.
2. Pb-free packaging, complies to the European Directive for Restriction of Hazardous Substances
(RoHS directive). Also Halide free and fully Green.
3. Tape and Reel
4. See characterization specifications at 105°C
Package Type
64A 64-lead, Thin (1.0mm) Plastic Gull Wing Quad Flat Package (TQFP)
64M1 64-pad, 9 x 9 x 1.0mm body, lead pitch 0.50mm, Quad Flat No-Lead/Micro Lead Frame Package
(QFN/MLF)
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4. Block Diagram
Figure 4-1 Block Diagram
CPU
ADC ADC[7:0]
AREF
I/O
PORTS
D
A
T
A
B
U
S
SRAM
OCD FLASH
NVM
programming
JTAG
TC 0
(8-bit async)
SPI
AC
AIN0
AIN1
ACO
ADCMUX
EEPROM
EEPROMIF
TC 3
(16-bit)
OC3A/B
T3
ICP3
TWI
SDA
SCL
USART 1
RxD1
TxD1
XCK1
Internal
Reference
Watchdog
Timer
Power
management
and clock
control
VCC
GND
Power
Supervision
POR/BOD &
RESET
TOSC2
XTAL2
RESET
XTAL1
TOSC1
TCK
TMS
TDI
TDO
INT[7:0]
OC0
MISO
MOSI
SCK
SS
PA[7:0]
PB[7:0]
PC[7:0]
PD[7:0]
PE[7:0]
PF[7:0]
PG[4:0]
USART 0
RxD0
TxD0
XCK0
TC 1
(16-bit)
OC1A/B/C
T1
ICP1
TC 2
(8-bit)
T2
OC2
AD[7:0]
A[15:8]
RD/WR/ALE
ExtMem
ExtInt
SERPROG
PARPROG
PEN
PDI
PDO
SCK
Clock generation
1MHz int
osc
32.768kHz
XOSC
External
clock
8MHz
Crystal Osc
12MHz
External
RC Osc
8MHz
Calib RC
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5. ATmega103 and ATmega64A Compatibility
The ATmega64A is a highly complex microcontroller where the number of I/O locations supersedes the
64 I/O locations reserved in the AVR instruction set. To ensure backward compatibility with the
ATmega103, all I/O locations present in ATmega103 have the same location in ATmega64A. Most
additional I/O locations are added in an Extended I/O space starting from 0x60 to 0xFF, (that is, in the
ATmega103 internal RAM space). These locations can be reached by using LD/LDS/LDD and
ST/STS/STD instructions only, not by using IN and OUT instructions. The relocation of the internal RAM
space may still be a problem for ATmega103 users. Also, the increased number of interrupt vectors might
be a problem if the code uses absolute addresses. To solve these problems, an ATmega103 compatibility
mode can be selected by programming the fuse M103C. In this mode, none of the functions in the
Extended I/O space are in use, so the internal RAM is located as in ATmega103. Also, the Extended
Interrupt vectors are removed.
The Atmel AVR ATmega64A is 100% pin compatible with ATmega103, and can replace the ATmega103
on current Printed Circuit Boards. The application note “Replacing ATmega103 by ATmega64A” describes
what the user should be aware of replacing the ATmega103 by an ATmega64A.
5.1. ATmega103 Compatibility Mode
By programming the M103C fuse, the ATmega64A will be compatible with the ATmega103 regards to
RAM, I/O pins and interrupt vectors as described above. However, some new features in ATmega64A are
not available in this compatibility mode, these features are listed below:
•One USART instead of two, Asynchronous mode only. Only the eight least significant bits of the
Baud Rate Register is available.
•One 16 bits Timer/Counter with two compare registers instead of two 16-bit Timer/Counters with
three compare registers.
•Two-wire serial interface is not supported.
•Port C is output only.
•Port G serves alternate functions only (not a general I/O port).
•Port F serves as digital input only in addition to analog input to the ADC.
•Boot Loader capabilities is not supported.
•It is not possible to adjust the frequency of the internal calibrated RC Oscillator.
•The External Memory Interface can not release any Address pins for general I/O, neither configure
different wait-states to different External Memory Address sections.
•In addition, there are some other minor differences to make it more compatible to ATmega103:
•Only EXTRF and PORF exists in MCUCSR.
•Timed sequence not required for Watchdog Time-out change.
•External Interrupt pins 3 - 0 serve as level interrupt only.
•USART has no FIFO buffer, so data overrun comes earlier.
Unused I/O bits in ATmega103 should be written to 0 to ensure same operation in ATmega64A.
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Ell-III Atmel (g) PEN GND AREF GND VCC PAO (ADO) PAl (ADI) PAZ (ADZ) RESET VCC GN D :1 PAS (ADS) j PA4(AD4) 3 ms (ADS) j mums) j PA7(AD7) :1 P02 (ALE) j PC7 (A15) :1 PC6(A14) 3 PCS (A13) 3 PC4 (A12) :1 Pcs (A11) 3 PCZ (A10) :1 PC1(A9) :1 PCO(A8) 3 m1 (E) 3 mo (W)
6. Pin Configurations
Figure 6-1 Pinout ATmega64A
1
2
3
4
44
43
42
41
40
39
38
5
6
7
8
9
10
11
33
32
31
30
29
28
27
26
25
24
23
37
36
35
34
12
13
14
15
16
17
18
19
20
21
22
45
46
47
48
64
63
62
61
60
59
58
57
56
55
54
53
52
51
50
49
RESET
VCC
GND
GND
VCC
AREF
AVCC
GND
PA3 (AD3)
PA4 (AD4)
PA5 (AD5)
PA6 (AD6)
PA7 (AD7)
PG2 (ALE)
PC7 (A15)
PC6 (A14)
PC5 (A13)
PC4 (A12)
PC3 (A11)
PC2 (A10)
PC1 (A9)
PC0 (A8)
PG1 (RD)
PG0 (WR)
PA0 (AD0)
PA1 (AD1)
PA2 (AD2)
PF7 (ADC7/TDI)
PF6 (ADC6/TDO)
PF5 (ADC5/TMS)
PF4 (ADC4/TCK)
PF3 (ADC3)
PF2 (ADC2)
PF1 (ADC1)
PF0 (ADC0)
XTAL2
XTAL1
(TOSC1) PG4
(TOSC2) PG3
(OC2/OC1C) PB7
(SCL/INT0) PD0
(SDA/INT1) PD1
(RXD1/INT2) PD2
(TXD1/INT3) PD3
(ICP1) PD4
(XCK1) PD5
(T1) PD6
(T2) PD7
PEN
(RXD0/PDI) PE0
(TXD0/PDO) PE1
(XCK0/AIN0) PE2
(OC3A/AIN1) PE3
(OC3B/INT4) PE4
(OC3C/INT5) PE5
(T3/INT6) PE6
(ICP3/INT7) PE7
(SS) PB0
(SCK) PB1
(MOSI) PB2
(MISO) PB3
(OC0) PB4
(OC1A) PB5
(OC1B) PB6
Power
Ground
Programming/debug
Digital
Analog
Crystal/Osc
External Memory
Note:  The Pinout figure applies to both TQFP and MLF packages. The bottom pad under the QFN/MLF
package should be soldered to ground.
6.1. Pin Descriptions
6.1.1. VCC
Digital supply voltage.
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6.1.2. GND
Ground.
6.1.3. Port A (PA7:PA0)
Port A is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port A
output buffers have symmetrical drive characteristics with both high sink and source capability. As inputs,
Port A pins that are externally pulled low will source current if the pull-up resistors are activated. The Port
A pins are tristated when a reset condition becomes active, even if the clock is not running.
Port A also serves the functions of various special features of the ATmega64A as listed in Alternate
Functions of Port A.
Related Links
Alternate Functions of Port A on page 98
6.1.4. Port B (PB7:PB0)
Port B is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port B
output buffers have symmetrical drive characteristics with both high sink and source capability. As inputs,
Port B pins that are externally pulled low will source current if the pull-up resistors are activated. The Port
B pins are tristated when a reset condition becomes active, even if the clock is not running.
Port B also serves the functions of various special features of the ATmega64A as listed in Alternate
Functions of Port B.
Related Links
Alternate Functions of Port B on page 100
6.1.5. Port C (PC7:PC0)
Port C is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port C
output buffers have symmetrical drive characteristics with both high sink and source capability. As inputs,
Port C pins that are externally pulled low will source current if the pull-up resistors are activated. The Port
C pins are tristated when a reset condition becomes active, even if the clock is not running.
Port C also serves the functions of special features of the ATmega64A as listed in Alternate Functions of
Port C. In ATmega103 compatibility mode, Port C is output only, and the port C pins are not tri-stated
when a reset condition becomes active.
Note:  The Atmel AVR ATmega64A is by default shipped in ATmega103 compatibility mode. Thus, if the
parts are not programmed before they are put on the PCB, PORTC will be output during first power up,
and until the ATmega103 compatibility mode is disabled.
Related Links
Alternate Functions of Port C on page 102
6.1.6. Port D (PD7:PD0)
Port D is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port D
output buffers have symmetrical drive characteristics with both high sink and source capability. As inputs,
Port D pins that are externally pulled low will source current if the pull-up resistors are activated. The Port
D pins are tristated when a reset condition becomes active, even if the clock is not running.
Port D also serves the functions of various special features of the ATmega64A as listed in Alternate
Functions of Port D.
Related Links
Alternate Functions of Port D on page 103
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6.1.7. Port E (PE7:PE0)
Port E is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port E
output buffers have symmetrical drive characteristics with both high sink and source capability. As inputs,
Port E pins that are externally pulled low will source current if the pull-up resistors are activated. The Port
E pins are tristated when a reset condition becomes active, even if the clock is not running.
Port E also serves the functions of various special features of the ATmega64A as listed in Alternate
Functions of Port E.
Related Links
Alternate Functions of Port E on page 105
6.1.8. Port F (PF7:PF0)
Port F serves as the analog inputs to the A/D Converter.
Port F also serves as an 8-bit bi-directional I/O port, if the A/D Converter is not used. Port pins can
provide internal pull-up resistors (selected for each bit). The Port F output buffers have symmetrical drive
characteristics with both high sink and source capability. As inputs, Port F pins that are externally pulled
low will source current if the pull-up resistors are activated. The Port F pins are tri-stated when a reset
condition becomes active, even if the clock is not running. If the JTAG interface is enabled, the pull-up
resistors on pins PF7(TDI), PF5(TMS), and PF4(TCK) will be activated even if a Reset occurs.
The TDO pin is tri-stated unless TAP states that shift out data are entered.
Port F also serves the functions of the JTAG interface.
In ATmega103 compatibility mode, Port F is an input Port only.
Related Links
Alternate Functions of Port F on page 108
6.1.9. Port G (PG4:PG0)
Port G is a 5-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port G
output buffers have symmetrical drive characteristics with both high sink and source capability. As inputs,
Port G pins that are externally pulled low will source current if the pull-up resistors are activated. The Port
G pins are tristated when a reset condition becomes active, even if the clock is not running.
Port G also serves the functions of various special features.
The port G pins are tri-stated when a reset condition becomes active, even if the clock is not running.
In Atmel AVR ATmega103 compatibility mode, these pins only serves as strobes signals to the external
memory as well as input to the 32kHz Oscillator, and the pins are initialized to PG0 = 1, PG1 = 1, and
PG2 = 0 asynchronously when a reset condition becomes active, even if the clock is not running. PG3
and PG4 are oscillator pins.
Related Links
Alternate Functions of Port G on page 110
6.1.10. RESET
Reset input. A low level on this pin for longer than the minimum pulse length will generate a reset, even if
the clock is not running. The minimum pulse length is given in System and Reset Characteristics. Shorter
pulses are not guaranteed to generate a reset.
Related Links
System and Reset Characteristics on page 418
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6.1.11. XTAL1
Input to the inverting Oscillator amplifier and input to the internal clock operating circuit.
6.1.12. XTAL2
Output from the inverting Oscillator amplifier.
6.1.13. AVCC
AVCC is the supply voltage pin for Port F and the A/D Converter. It should be externally connected to VCC,
even if the ADC is not used. If the ADC is used, it should be connected to VCC through a low-pass filter.
6.1.14. AREF
AREF is the analog reference pin for the A/D Converter.
6.1.15. PEN
PEN is a programming enable pin for the SPI Serial Programming mode, and is internally pulled high. By
holding this pin low during a Power-on Reset, the device will enter the SPI Serial Programming mode.
PEN has no function during normal operation.
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7. Resources
A comprehensive set of development tools, application notes and datasheets are available for download
on http://www.atmel.com/avr.
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8. Data Retention
Reliability Qualification results show that the projected data retention failure rate is much less than 1 PPM
over 20 years at 85°C or 100 years at 25°C.
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9. About Code Examples
This datasheet contains simple code examples that briefly show how to use various parts of the device.
These code examples assume that the part specific header file is included before compilation. Be aware
that not all C compiler vendors include bit definitions in the header files and interrupt handling in C is
compiler dependent. Please confirm with the C compiler documentation for more details.
For I/O registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI” instructions
must be replaced with instructions that allow access to extended I/O. Typically “LDS” and “STS”
combined with “SBRS”, “SBRC”, “SBR”, and “CBR”.
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10. Capacitive Touch Sensing
The Atmel QTouch Library provides a simple to use solution to realize touch sensitive interfaces on most
Atmel AVR microcontrollers. The QTouch Library includes support for the QTouch and QMatrix®
acquisition methods.
Touch sensing can be added to any application by linking the appropriate Atmel QTouch Library for the
AVR Microcontroller. This is done by using a simple set of APIs to define the touch channels and sensors,
and then calling the touch sensing API’s to retrieve the channel information and determine the touch
sensor states.
The QTouch Library is FREE and downloadable from the Atmel website at the following location:
www.atmel.com/qtouchlibrary. For implementation details and other information, refer to the Atmel
QTouch Library User Guide - also available for download from the Atmel website.
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11. AVR CPU Core
11.1. Overview
This section discusses the Atmel AVR core architecture in general. The main function of the CPU core is
to ensure correct program execution. The CPU must therefore be able to access memories, perform
calculations, control peripherals, and handle interrupts.
Figure 11-1 Block Diagram of the AVR MCU Architecture
Flash
Program
Memory
Instruction
Register
Instruction
Decoder
Program
Counter
Control Line s
32 x 8
General
Purpose
Registrers
ALU
Status
and Control
I/O Line s
EEPROM
Data Bus 8-bit
Data
SRAM
Direct Addre s s ing
Indire ct Addre s s ing
Interrupt
Unit
SPI
Unit
Watchdog
Timer
Analog
Comparator
i/O Module 2
i/O Module1
i/O Module n
In order to maximize performance and parallelism, the AVR uses a Harvard architecture – with separate
memories and buses for program and data. Instructions in the Program memory are executed with a
single level pipelining. While one instruction is being executed, the next instruction is pre-fetched from the
Program memory. This concept enables instructions to be executed in every clock cycle. The Program
memory is In-System Reprogrammable Flash memory.
The fast-access Register File contains 32 x 8-bit general purpose working registers with a single clock
cycle access time. This allows single-cycle Arithmetic Logic Unit (ALU) operation. In a typical ALU
operation, two operands are output from the Register File, the operation is executed, and the result is
stored back in the Register File – in one clock cycle.
Six of the 32 registers can be used as three 16-bit indirect address register pointers for Data Space
addressing – enabling efficient address calculations. One of the these address pointers can also be used
as an address pointer for look up tables in Flash Program memory. These added function registers are
the 16-bit X-, Y-, and Z-register, described later in this section.
The ALU supports arithmetic and logic operations between registers or between a constant and a
register. Single register operations can also be executed in the ALU. After an arithmetic operation, the
Status Register is updated to reflect information about the result of the operation.
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The Program flow is provided by conditional and unconditional jump and call instructions, able to directly
address the whole address space. Most AVR instructions have a single 16-bit word format. Every
Program memory address contains a 16- or 32-bit instruction.
Program Flash memory space is divided in two sections, the Boot program section and the Application
program section. Both sections have dedicated Lock Bits for write and read/write protection. The SPM
instruction that writes into the Application Flash memory section must reside in the Boot program section.
During interrupts and subroutine calls, the return address Program Counter (PC) is stored on the Stack.
The Stack is effectively allocated in the general data SRAM, and consequently the Stack size is only
limited by the total SRAM size and the usage of the SRAM. All user programs must initialize the SP in the
reset routine (before subroutines or interrupts are executed). The Stack Pointer SP is read/write
accessible in the I/O space. The data SRAM can easily be accessed through the five different addressing
modes supported in the AVR architecture.
The memory spaces in the AVR architecture are all linear and regular memory maps.
A flexible interrupt module has its control registers in the I/O space with an additional global interrupt
enable bit in the Status Register. All interrupts have a separate Interrupt Vector in the Interrupt Vector
table. The interrupts have priority in accordance with their Interrupt Vector position. The lower the
Interrupt Vector address, the higher the priority.
The I/O memory space contains 64 addresses for CPU peripheral functions as Control Registers, SPI,
and other I/O functions. The I/O Memory can be accessed directly, or as the Data Space locations
following those of the Register File, 0x20 - 0x5F. In addition, the ATmega64A has Extended I/O space
from $60 in SRAM where only the ST/STS/STD and LD/LDS/LDD instructions can be used.
11.2. ALU – Arithmetic Logic Unit
The high-performance Atmel AVR ALU operates in direct connection with all the 32 general purpose
working registers. Within a single clock cycle, arithmetic operations between general purpose registers or
between a register and an immediate are executed. The ALU operations are divided into three main
categories – arithmetic, logical, and bit-functions. Some implementations of the architecture also provide
a powerful multiplier supporting both signed/unsigned multiplication and fractional format. See the
“Instruction Set” section for a detailed description.
11.3. Status Register
The Status Register contains information about the result of the most recently executed arithmetic
instruction. This information can be used for altering program flow in order to perform conditional
operations. Note that the Status Register is updated after all ALU operations, as specified in the
Instruction Set Reference. This will in many cases remove the need for using the dedicated compare
instructions, resulting in faster and more compact code.
The Status Register is not automatically stored when entering an interrupt routine and restored when
returning from an interrupt. This must be handled by software.
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11.3.1. SREG – The AVR Status Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  SREG
Offset:  0x3F
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x5F
Bit 7 6 5 4 3 2 1 0
I T H S V N Z C
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bit 7 – I: Global Interrupt Enable
The Global Interrupt Enable bit must be set for the interrupts to be enabled. The individual interrupt
enable control is then performed in separate control registers. If the Global Interrupt Enable Register is
cleared, none of the interrupts are enabled independent of the individual interrupt enable settings. The I-
bit is cleared by hardware after an interrupt has occurred, and is set by the RETI instruction to enable
subsequent interrupts. The I-bit can also be set and cleared by the application with the SEI and CLI
instructions, as described in the Instruction Set Reference.
Bit 6 – T: Bit Copy Storage
The Bit Copy instructions BLD (Bit LoaD) and BST (Bit STore) use the T-bit as source or destination for
the operated bit. A bit from a register in the Register File can be copied into T by the BST instruction, and
a bit in T can be copied into a bit in a register in the Register File by the BLD instruction.
Bit 5 – H: Half Carry Flag
The Half Carry Flag H indicates a Half Carry in some arithmetic operations. Half Carry is useful in BCD
arithmetic. See the “Instruction Set Description” for detailed information.
Bit 4 – S: Sign Bit, S = N ⊕ V
The S-bit is always an exclusive or between the Negative Flag N and the Two’s Complement Overflow
Flag V. See the “Instruction Set Description” for detailed information.
Bit 3 – V: Two’s Complement Overflow Flag
The Two’s Complement Overflow Flag V supports two’s complement arithmetics. See the “Instruction Set
Description” for detailed information.
Bit 2 – N: Negative Flag
The Negative Flag N indicates a negative result in an arithmetic or logic operation. See the “Instruction
Set Description” for detailed information.
Bit 1 – Z: Zero Flag
The Zero Flag Z indicates a zero result in an arithmetic or logic operation. See the “Instruction Set
Description” for detailed information.
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Bit 0 – C: Carry Flag
The Carry Flag C indicates a Carry in an arithmetic or logic operation. See the “Instruction Set
Description” for detailed information.
11.4. General Purpose Register File
The Register File is optimized for the Atmel AVR Enhanced RISC instruction set. In order to achieve the
required performance and flexibility, the following input/output schemes are supported by the Register
File:
•One 8-bit output operand and one 8-bit result input.
•Two 8-bit output operands and one 8-bit result input.
•Two 8-bit output operands and one 16-bit result input.
•One 16-bit output operand and one 16-bit result input.
The following figure shows the structure of the 32 general purpose working registers in the CPU.
Figure 11-2 AVR CPU General Purpose Working Registers
7
0
Addr.
R0
0x00
R1
0x01
R2
0x02
…
R13
0x0D
Genera l
R14
0x0E
Purpose
R15
0x0F
Working
R16
0x10
Registers
R17
0x11
…
R26
0x1A
X-re giste r Low Byte
R27
0x1B
X-re giste r High Byte
R28
0x1C
Y-re giste r Low Byte
R29
0x1D
Y-re giste r High Byte
R30
0x1E
Z-regis te r Low Byte
R31
0x1F
Z-regis te r High Byte
Most of the instructions operating on the Register File have direct access to all registers, and most of
them are single cycle instructions.
As shown in the figure above, each register is also assigned a Data memory address, mapping them
directly into the first 32 locations of the user Data Space. Although not being physically implemented as
SRAM locations, this memory organization provides great flexibility in access of the registers, as the X-,
Y-, and Z-pointer Registers can be set to index any register in the file.
11.4.1. The X-register, Y-register and Z-register
The registers R26:R31 have some added functions to their general purpose usage. These registers are
16-bit address pointers for indirect addressing of the Data Space. The three indirect address registers X,
Y and Z are defined as described in the following figure.
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A5 X‘H x1. in: «mm R30 «mm En I5 l4 IS 12 n Atmel
Figure 11-3 The X-, Y- and Z-Registers
15
XL
0
X-re gis te r
7
0
7
0
R27 (0x1B)
R26 (0x1A)
15
YL
0
Y-re gis te r
7
0
7
0
R29 (0x1D)
R28 (0x1C)
15
ZL
0
Z-re gis te r
7
0
7
0
R31 (0x1F)
R30 (0x1E)
In the different addressing modes these address registers have functions as fixed displacement,
automatic increment, and automatic decrement (see the Instruction Set Reference for details).
11.5. Stack Pointer
The Stack is mainly used for storing temporary data, for storing local variables and for storing return
addresses after interrupts and subroutine calls. The Stack Pointer Register always points to the top of the
Stack. Note that the Stack is implemented as growing from higher memory locations to lower memory
locations. This implies that a Stack PUSH command decreases the Stack Pointer. If software reads the
Program Counter from the Stack after a call or an interrupt, unused bits (bit 15) should be masked out.
The Stack Pointer points to the data SRAM Stack area where the subroutine and interrupt Stacks are
located. This Stack space in the data SRAM must be defined by the program before any subroutine calls
are executed or interrupts are enabled. The Stack Pointer must be set to point above 0x60. The Stack
Pointer is decremented by one when data is pushed onto the Stack with the PUSH instruction, and it is
decremented by two when the return address is pushed onto the Stack with subroutine call or interrupt.
The Stack Pointer is incremented by one when data is popped from the Stack with the POP instruction,
and it is incremented by two when data is popped from the Stack with return from subroutine RET or
return from interrupt RETI.
The AVR Stack Pointer is implemented as two 8-bit registers in the I/O space. The number of bits actually
used is implementation dependent. Note that the data space in some implementations of the AVR
architecture is so small that only SPL is needed. In this case, the SPH Register will not be present.
Bit
15
14
13
12
11
10
9
8
0x3E
S P15
S P14
S P13
S P12
S P11
S P10
S P9
S P8
S PH
0x3D
S P7
S P6
S P5
S P4
S P3
S P2
S P1
S P0
S PL
7
6
5
4
3
2
1
0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
11.6. Instruction Execution Timing
This section describes the general access timing concepts for instruction execution. The Atmel AVR CPU
is driven by the CPU clock clkCPU, directly generated from the selected clock source for the chip. No
internal clock division is used.
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The following figure shows the parallel instruction fetches and instruction executions enabled by the
Harvard architecture and the fast-access Register File concept. This is the basic pipelining concept to
obtain up to 1 MIPS per MHz with the corresponding unique results for functions per cost, functions per
clocks, and functions per power-unit.
Figure 11-4 The Parallel Instruction Fetches and Instruction Executions
clk
1s t Ins truction Fe tch
1s t Instruction Execute
2nd Ins truction Fe tch
2nd Ins truction Execute
3rd Ins truction Fetch
3rd Ins truction Execute
4th Ins truction Fetch
T1 T2 T3 T4
CPU
The next figure shows the internal timing concept for the Register File. In a single clock cycle an ALU
operation using two register operands is executed, and the result is stored back to the destination
register.
Figure 11-5 Single Cycle ALU Operation
Total Exe cution Time
Re gis te r Operands Fetch
ALU Opera tion Execute
Re sult Write Back
T1 T2 T3 T4
clkCP U
11.7. Reset and Interrupt Handling
The Atmel AVR provides several different interrupt sources. These interrupts and the separate Reset
Vector each have a separate Program Vector in the Program memory space. All interrupts are assigned
individual enable bits which must be written logic one together with the Global Interrupt Enable bit in the
Status Register in order to enable the interrupt. Depending on the Program Counter value, interrupts may
be automatically disabled when Boot Lock Bits BLB02 or BLB12 are programmed. This feature improves
software security. See the section Memory Programming for details.
The lowest addresses in the Program memory space are by default defined as the Reset and Interrupt
Vectors. The complete list of Vectors is shown in Interrupts . The list also determines the priority levels of
the different interrupts. The lower the address the higher is the priority level. RESET has the highest
priority, and next is INT0 – the External Interrupt Request 0. The Interrupt Vectors can be moved to the
start of the boot Flash section by setting the Interrupt Vector Select (IVSEL) bit in the MCU Control
Register (MCUCR). Refer to Interrupts for more information. The Reset Vector can also be moved to the
start of the boot Flash section by programming the BOOTRST Fuse, see Boot Loader Support – Read-
While-Write Self-Programming.
When an interrupt occurs, the Global Interrupt Enable I-bit is cleared and all interrupts are disabled. The
user software can write logic one to the I-bit to enable nested interrupts. All enabled interrupts can then
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interrupt the current interrupt routine. The I-bit is automatically set when a Return from Interrupt
instruction – RETI – is executed.
There are basically two types of interrupts. The first type is triggered by an event that sets the Interrupt
Flag. For these interrupts, the Program Counter is vectored to the actual Interrupt Vector in order to
execute the interrupt handling routine, and hardware clears the corresponding Interrupt Flag. Interrupt
Flags can also be cleared by writing a logic one to the flag bit position(s) to be cleared. If an interrupt
condition occurs while the corresponding interrupt enable bit is cleared, the Interrupt Flag will be set and
remembered until the interrupt is enabled, or the flag is cleared by software. Similarly, if one or more
interrupt conditions occur while the global interrupt enable bit is cleared, the corresponding Interrupt
Flag(s) will be set and remembered until the global interrupt enable bit is set, and will then be executed by
order of priority.
The second type of interrupts will trigger as long as the interrupt condition is present. These interrupts do
not necessarily have Interrupt Flags. If the interrupt condition disappears before the interrupt is enabled,
the interrupt will not be triggered.
When the AVR exits from an interrupt, it will always return to the main program and execute one more
instruction before any pending interrupt is served.
Note that the Status Register is not automatically stored when entering an interrupt routine, nor restored
when returning from an interrupt routine. This must be handled by software.
When using the CLI instruction to disable interrupts, the interrupts will be immediately disabled. No
interrupt will be executed after the CLI instruction, even if it occurs simultaneously with the CLI instruction.
The following example shows how this can be used to avoid interrupts during the timed EEPROM write
sequence.
Assembly Code Example
in r16, SREG ; store SREG value
cli ; disable interrupts during timed sequence
sbi EECR, EEMWE ; start EEPROM write
sbi EECR, EEWE
out SREG, r16 ; restore SREG value (I-bit)
C Code Example
char cSREG;
cSREG = SREG; /* store SREG value */
/* disable interrupts during timed sequence */
_CLI();
EECR |= (1<<EEMWE); /* start EEPROM write */
EECR |= (1<<EEWE);
SREG = cSREG; /* restore SREG value (I-bit) */
When using the SEI instruction to enable interrupts, the instruction following SEI will be executed before
any pending interrupts, as shown in the following example.
Assembly Code Example
sei ; set global interrupt enable
sleep ; enter sleep, waiting for interrupt
; note: will enter sleep before any pending
; interrupt(s)
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C Code Example
_enable_interrupt(); /* set global interrupt enable */
_SLEEP(); /* enter sleep, waiting for interrupt */
/* note: will enter sleep before any pending interrupt(s) */
Related Links
Memory Programming on page 383
Interrupts on page 77
BTLDR - Boot Loader Support – Read-While-Write Self-Programming on page 366
11.7.1. Interrupt Response Time
The interrupt execution response for all the enabled Atmel AVR interrupts is four clock cycles minimum.
After four clock cycles, the Program Vector address for the actual interrupt handling routine is executed.
During this 4-clock cycle period, the Program Counter is pushed onto the Stack. The Vector is normally a
jump to the interrupt routine, and this jump takes three clock cycles. If an interrupt occurs during
execution of a multi-cycle instruction, this instruction is completed before the interrupt is served. If an
interrupt occurs when the MCU is in sleep mode, the interrupt execution response time is increased by
four clock cycles. This increase comes in addition to the start-up time from the selected sleep mode.
A return from an interrupt handling routine takes four clock cycles. During these four clock cycles, the
Program Counter (2 bytes) is popped back from the Stack, the Stack Pointer is incremented by 2, and the
I-bit in SREG is set.
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12. AVR Memories
12.1. Overview
This section describes the different memories in the Atmel AVR ATmega64A. The AVR architecture has
two main memory spaces, the Data memory and the Program Memory space. In addition, the
ATmega64A features an EEPROM Memory for data storage. All three memory spaces are linear and
regular.
12.2. In-System Reprogrammable Flash Program Memory
The ATmega64A contains 64K bytes On-chip In-System Reprogrammable Flash memory for program
storage. Since all AVR instructions are 16- or 32-bits wide, the Flash is organized as 32K x 16 bits. For
software security, the Flash Program memory space is divided into two sections, Boot Program section
and Application Program section.
The Flash memory has an endurance of at least 10,000 write/erase cycles. The ATmega64A Program
Counter (PC) is 15 bits wide, thus addressing the 32K Program memory locations. The operation of Boot
Program section and associated Boot Lock Bits for software protection are described in detail in Boot
Loader Support – Read-While-Write Self-Programming. Memory Programming contains a detailed
description on Flash Programming in SPI, JTAG, or Parallel Programming mode.
Constant tables can be allocated within the entire Program memory address space (see the LPM – Load
Program memory instruction description).
Timing diagrams for instruction fetch and execution are presented in Instruction Execution Timing.
Figure 12-1 Program Memory Map
$0000
$7FFF
Applica tion Fla s h Section
Boot Flas h S e ction
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12.3. SRAM Data Memory
The Atmel AVR ATmega64A supports two different configurations for the SRAM data memory as listed in
the table below
Table 12-1 Memory Configurations
Configuration Internal SRAM Data Memory External SRAM Data Memory
Normal mode 4096 up to 64K
ATmega103 Compatibility mode 4000 up to 64K
Figure 12-2 Data Memory Map on page 32 shows how the ATmega64A SRAM Memory is organized.
The ATmega64A is a complex microcontroller with more peripheral units than can be supported within the
64 location reserved in the Opcode for the IN and OUT instructions. For the Extended I/O space from
0x60 - 0xFF in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used. The Extended
I/O space does not exist when the ATmega64A is in the ATmega103 compatibility mode.
In normal mode, the first 4352 Data Memory locations address both the Register file, the I/O Memory,
Extended I/O Memory, and the internal data SRAM. The first 32 locations address the Register file, the
next 64 location the standard I/O memory, then 160 locations of Extended I/O memory, and the next 4096
locations address the internal data SRAM.
In ATmega103 compatibility mode, the first 4096 Data Memory locations address both the Register file,
the I/O Memory and the internal data SRAM. The first 32 locations address the Register file, the next 64
location the standard I/O memory, and the next 4000 locations address the internal data SRAM.
An optional external data SRAM can be used with the ATmega64A. This SRAM will occupy an area in the
remaining address locations in the 64K address space. This area starts at the address following the
internal SRAM. The Register file, I/O, Extended I/O and Internal SRAM occupies the lowest 4352bytes in
normal mode, and the lowest 4096 bytes in the ATmega103 compatibility mode (Extended I/O not
present), so when using 64 Kbytes (65536 bytes) of External Memory, 61184 bytes of External Memory
are available in normal mode, and 61440 bytes in ATmega103 compatibility mode. Refer to External
Memory Interface on page 34 for details on how to take advantage of the external memory map.
When the addresses accessing the SRAM memory space exceeds the internal data memory locations,
the external data SRAM is accessed using the same instructions as for the internal data memory access.
When the internal data memories are accessed, the read and write strobe pins (PG0 and PG1) are
inactive during the whole access cycle. External SRAM operation is enabled by setting the SRE bit in the
MCUCR Register.
Accessing external SRAM takes one additional clock cycle per byte compared to access of the internal
SRAM. This means that the commands LD, ST, LDS, STS, LDD, STD, PUSH, and POP take one
additional clock cycle. If the Stack is placed in external SRAM, interrupts, subroutine calls and returns
take three clock cycles extra because the two-byte program counter is pushed and popped, and external
memory access does not take advantage of the internal pipe-line memory access. When external SRAM
interface is used with wait-state, onebyte external access takes two, three, or four additional clock cycles
for one, two, and three wait-states respectively. Interrupts, subroutine calls and returns will need five,
seven, or nine clock cycles more than specified in the instruction set manual for one, two, and three wait-
states.
The five different addressing modes for the data memory cover: Direct, Indirect with Displacement,
Indirect, Indirect with Pre-decrement, and Indirect with Post-increment. In the Register file, registers R26
to R31 feature the indirect addressing pointer registers.
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32 Registers 32 Regiswm
The direct addressing reaches the entire data space.
The Indirect with Displacement mode reaches 63 address locations from the base address given by the
Y- or Zregister.
When using register indirect addressing modes with automatic pre-decrement and post-increment, the
address registers X, Y, and Z are decremented or incremented.
The 32 general purpose working registers, 64 I/O registers, and the 4096 bytes of internal data SRAM in
the Atmel AVR ATmega64A are all accessible through all these addressing modes. The Register file is
described in General Purpose Register File.
Figure 12-2 Data Memory Map
Me mo ry Co nfiguration B
32 Registers
64 I/O Regis ters
Interna l S RAM
(4000 x 8)
$0000 - $001F
$0020 - $005F
$1000
$0FFF
$FFFF
$0060
Data Memory
Exte rnal S RAM
(0 - 64K x 8)
Me mo ry Co nfiguration A
32 Registers
64 I/O Regis ters
Interna l S RAM
(4096 x 8)
$0000 - $001F
$0020 - $005F
$1100
$10FF
$FFFF
$0060 - $00FF
Data Memory
Exte rnal S RAM
(0 - 64K x 8)
160 Ext I/O Re g.
$0100
Related Links
General Purpose Register File on page 25
12.3.1. Data Memory Access Times
This section describes the general access timing concepts for internal memory access. The internal data
SRAM access is performed in two clkCPU cycles as described in the figure below.
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Figure 12-3 On-chip Data SRAM Access Cycles
clk
WR
RD
Data
Data
Addre s s Addre ss Va lid
T1 T2 T3
Compute Addre s s
Re a d Write
CPU
Memory Vccess Instruction Next Ins truction
12.4. EEPROM Data Memory
The Atmel AVR ATmega64A contains 2Kbytes of data EEPROM memory. It is organized as a separate
data space, in which single bytes can be read and written. The EEPROM has an endurance of at least
100,000 write/erase cycles. The access between the EEPROM and the CPU is described below,
specifying the EEPROM Address Registers, the EEPROM Data Register, and the EEPROM Control
Register.
Memory Programming contains a detailed description on EEPROM Programming in SPI, JTAG, or
Parallel Programming mode.
Related Links
Memory Programming on page 383
12.4.1. EEPROM Read/Write Access
The EEPROM Access Registers are accessible in the I/O space.
The write access time for the EEPROM is given in Table 12-2 EEPROM Programming Time on page
46. A self-timing function, however, lets the user software detect when the next byte can be written. If
the user code contains instructions that write the EEPROM, some precautions must be taken. In heavily
filtered power supplies, VCC is likely to rise or fall slowly on Power-up/down. This causes the device for
some period of time to run at a voltage lower than specified as minimum for the clock frequency used.
See Preventing EEPROM Corruption on page 34 for details on how to avoid problems in these
situations.
In order to prevent unintentional EEPROM writes, a specific write procedure must be followed. Refer to
the description of the EEPROM Control Register for details on this.
When the EEPROM is read, the CPU is halted for four clock cycles before the next instruction is
executed. When the EEPROM is written, the CPU is halted for two clock cycles before the next instruction
is executed.
12.4.2. EEPROM Write during Power-down Sleep Mode
When entering Power-down sleep mode while an EEPROM write operation is active, the EEPROM write
operation will continue, and will complete before the Write Access time has passed. However, when the
write operation is completed, the Oscillator continues running, and as a consequence, the device does
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not enter Power-down entirely. It is therefore recommended to verify that the EEPROM write operation is
completed before entering Power-down.
12.4.3. Preventing EEPROM Corruption
During periods of low VCC, the EEPROM data can be corrupted because the supply voltage is too low for
the CPU and the EEPROM to operate properly. These issues are the same as for board level systems
using EEPROM, and the same design solutions should be applied.
An EEPROM data corruption can be caused by two situations when the voltage is too low. First, a regular
write sequence to the EEPROM requires a minimum voltage to operate correctly. Second, the CPU itself
can execute instructions incorrectly, if the supply voltage is too low.
EEPROM data corruption can easily be avoided by following this design recommendation:
Keep the AVR RESET active (low) during periods of insufficient power supply voltage. This can be done
by enabling the internal Brown-out Detector (BOD). If the detection level of the internal BOD does not
match the needed detection level, an external low VCC Reset Protection circuit can be used. If a reset
occurs while a write operation is in progress, the write operation will be completed provided that the
power supply voltage is sufficient.
12.5. I/O Memory
The I/O space definition of the ATmega64A is shown in Register Summary.
All ATmega64A I/Os and peripherals are placed in the I/O space. The I/O locations are accessed by the
IN and OUT instructions, transferring data between the 32 general purpose working registers and the I/O
space. I/O Registers within the address range 0x00 - 0x1F are directly bit-accessible using the SBI and
CBI instructions. In these registers, the value of single bits can be checked by using the SBIS and SBIC
instructions. Refer to the instruction set section for more details. When using the I/O specific commands
IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When addressing I/O Registers as data space
using LD and ST instructions, 0x20 must be added to these addresses. The ATmega64A is a complex
microcontroller with more peripheral units than can be supported within the 64 location reserved in
Opcode for the IN and OUT instructions. For the Extended I/O space from 0x60 - 0xFF in SRAM, only the
ST/STS/STD and LD/LDS/LDD instructions can be used. The Extended I/O space is replaced with SRAM
locations when the ATmega64A is in the ATmega103 compatibility mode.
For compatibility with future devices, reserved bits should be written to zero if accessed. Reserved I/O
memory addresses should never be written.
Some of the Status Flags are cleared by writing a logical one to them. Note that the CBI and SBI
instructions will operate on all bits in the I/O Register, writing a one back into any flag read as set, thus
clearing the flag. The CBI and SBI instructions work with registers 0x00 to 0x1F only.
The I/O and Peripherals Control Registers are explained in later sections.
Related Links
Register Summary on page 492
12.6. External Memory Interface
12.6.1. Features
•Four different wait-state settings (including no wait-state).
•Independent wait-state setting for different external Memory sectors (configurable sector size).
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Extemal Memo (0—60K x 8) Atmel
•The number of bits dedicated to address high byte is selectable.
•Bus-keepers on data lines to minimize current consumption (optional).
12.6.2. Overview
With all the features the External Memory Interface provides, it is well suited to operate as an interface to
memory devices such as External SRAM and Flash, and peripherals such as LCD-display, A/D, and D/A.
When the eXternal MEMory (XMEM) is enabled, address space outside the internal SRAM becomes
available using the dedicated External Memory pins (refer to figure in Pin Configurations, table Port A
Pins Alternate Functions in section Alternate Functions of Port A, table Port C Pins Alternate Functions in
section Alternate Functions of Port C and table Port G Pins Alternate Functions in section Alternate
Functions of Port G). The memory configuration is shown in the figure below.
Figure 12-4 External Memory with Sector Select
Me mory Co nfiguration A
0x0000
0x10FF
Exte rnal Memory
(0-60K x 8)
0xFFFF
Interna l memory
SRL[2..0]
SRW11
SRW10
SRW01
SRW00
Lower se ctor
Upper s e ctor
0x1100
Me mory Co nfiguration B
0x0000
Exte rnal Memory
(0-60K x 8)
0xFFFF
Interna l memory
SRW10
0x0FFF
0x1000
Note: 
Atmel AVR ATmega64A in non ATmega103 compatibility mode: Memory Configuration A is available
(Memory Configuration B N/A)
ATmega64A in ATmega103 compatibility mode: Memory Configuration B is available (Memory
Configuration A N/A)
Related Links
Pin Configurations on page 14
Alternate Functions of Port A on page 98
Alternate Functions of Port G on page 110
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12.6.3. ATmega103 Compatibility
Both External Memory Control Registers (XMCRA and XMCRB) are placed in Extended I/O space. In
ATmega103 compatibility mode, these registers are not available, and the features selected by these
registers are not available. The device is still ATmega103 compatible, as these features did not exist in
ATmega103. The limitations in ATmega103 compatibility mode are:
•Only two wait-states settings are available (SRW1n = 0b00 and SRW1n = 0b01).
•The number of bits that are assigned to address high byte are fixed.
•The External Memory section can not be divided into sectors with different wait-state settings.
•Bus-keeper is not available.
•RD, WR and ALE pins are output only (Port G in ATmega64A).
12.6.4. Using the External Memory Interface
The interface consists of:
•AD7:0: Multiplexed low-order address bus and data bus.
•A15:8: High-order address bus (configurable number of bits).
•ALE: Address latch enable.
•RD: Read strobe.
•WR: Write strobe.
The control bits for the External Memory Interface are located in three registers, the MCU Control
Register – MCUCR, the External Memory Control Register A – XMCRA, and the External Memory Control
Register B – XMCRB.
When the XMEM interface is enabled, the XMEM interface will override the setting in the data direction
registers that corresponds to the ports dedicated to the XMEM interface. For details about the port
override, see the alternate functions in section I/O Ports. The XMEM interface will auto-detect whether an
access is internal or external. If the access is external, the XMEM interface will output address, data, and
the control signals on the ports according to Figure 12-6 External Data Memory Cycles without Wait-state
(SRWn1=0 and SRWn0=0) on page 38 (this figure shows the wave forms without wait-states). When
ALE goes from high-to-low, there is a valid address on AD7:0. ALE is low during a data transfer. When
the XMEM interface is enabled, also an internal access will cause activity on address, data and ALE
ports, but the RD and WR strobes will not toggle during internal access. When the External Memory
Interface is disabled, the normal pin and data direction settings are used. Note that when the XMEM
interface is disabled, the address space above the internal SRAM boundary is not mapped into the
internal SRAM. Figure 12-5 External SRAM Connected to the Atmel AVR on page 37 illustrates how to
connect an external SRAM to the AVR using an octal latch (typically “74 × 573” or equivalent) which is
transparent when G is high.
Related Links
I/O Ports on page 92
12.6.5. Address Latch Requirements
Due to the high-speed operation of the XRAM interface, the address latch must be selected with care for
system frequencies above 8MHz @ 4V and 4MHz @ 2.7V. When operating at conditions above these
frequencies, the typical old style 74HC series latch becomes inadequate. The External Memory Interface
is designed in compliance to the 74AHC series latch. However, most latches can be used as long they
comply with the main timing parameters. The main parameters for the address latch are:
•D to Q propagation delay (tPD).
•Data setup time before G low (tSU).
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•Data (address) hold time after G low (TH).
The External Memory Interface is designed to guaranty minimum address hold time after G is asserted
low of th = 5ns. Refer to tLAXX_LD/tLLAXX_ST in all the tables in section External Data Memory Timing. The
D-to-Q propagation delay (tPD) must be taken into consideration when calculating the access time
requirement of the external component. The data setup time before G low (tSU) must not exceed address
valid to ALE low (tAVLLC) minus PCB wiring delay (dependent on the capacitive load).
Figure 12-5 External SRAM Connected to the Atmel AVR
D[7:0]
A[7:0]
A[15:8]
RD
WR
SRAM
D Q
G
AD7:0
ALE
A15:8
RD
WR
AVR
12.6.6. Pull-up and Bus-keeper
The pull-ups on the AD7:0 ports may be activated if the corresponding Port register is written to one. To
reduce power consumption in sleep mode, it is recommended to disable the pull-ups by writing the Port
register to zero before entering sleep.
The XMEM interface also provides a bus-keeper on the AD7:0 lines. The bus-keeper can be disabled and
enabled in software as described in XMCRB on page 51. When enabled, the bus-keeper will ensure a
defined logic level (zero or one) on the AD7:0 bus when these lines would otherwise be tri-stated by the
XMEM interface.
12.6.7. Timing
External Memory devices have different timing requirements. To meet these requirements, the Atmel AVR
ATmega64A XMEM interface provides four different wait-states as shown in Table 12-4 Wait States(1) on
page 50. It is important to consider the timing specification of the External Memory device before
selecting the wait-state. The most important parameters are the access time for the external memory
compared to the set-up requirement of the ATmega64A. The access time for the External Memory is
defined to be the time from receiving the chip select/address until the data of this address actually is
driven on the bus. The access time cannot exceed the time from the ALE pulse must be asserted low until
data is stable during a read sequence (See tLLRL+ tRLRH - tDVRH in the tables in section External Data
Memory Timing). The different wait-states are set up in software. As an additional feature, it is possible to
divide the external memory space in two sectors with individual wait-state settings. This makes it possible
to connect two different memory devices with different timing requirements to the same XMEM interface.
For XMEM interface timing details, please refer to the tables and figures in section External Data Memory
Timing.
Note that the XMEM interface is asynchronous and that the waveforms in the following figures are related
to the internal system clock. The skew between the internal and external clock (XTAL1) is not guaranteed
(varies between devices temperature, and supply voltage). Consequently, the XMEM interface is not
suited for synchronous operation.
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gm EXA )Cxi 3 )C] Prdmam XAddrcss X : X l X X: Atmel
Figure 12-6 External Data Memory Cycles without Wait-state (SRWn1=0 and SRWn0=0)
ALE
T1 T2 T3
Write
Re a d
WR
T4
A15:8 Addres s
Prev. a ddr.
DA7:0 Addre s s DataPrev. data XX
RD
DA7:0 (XMBK = 0) Data
Prev. data Addres s
DataPrev. data Addres s
DA7:0 (XMBK = 1)
Sys te m Clock (CLKCPU)
XXXXX XXXXXXXX
Note:  1. SRWn1 = SRW11 (upper sector) or SRW01 (lower sector), SRWn0 = SRW10 (upper sector) or
SRW00 (lower sector). The ALE pulse in period T4 is only present if the next instruction accesses the
RAM (internal or external).
Figure 12-7 External Data Memory Cycles with SRWn1 = 0 and SRWn0 = 1(1)
ALE
T1 T2 T3
Write
Re a d
WR
T5
A15:8 Addre ss
Prev. a ddr.
DA7:0 Addre ss Da taPrev. da ta XX
RD
DA7:0 (XMBK = 0) Data
Prev. da ta Addre s s
DataPrev. data Addre s s
DA7:0 (XMBK = 1)
Sys te m Clock (CLKCPU)
T4
Note:  1. SRWn1 = SRW11 (upper sector) or SRW01 (lower sector), SRWn0 = SRW10 (upper sector) or
SRW00 (lower sector).
The ALE pulse in period T5 is only present if the next instruction accesses the RAM (internal or external).
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Figure 12-8 External Data Memory Cycles with SRWn1 = 1 and SRWn0 = 0(1)
ALE
T1 T2 T3
Write
Read
WR
T6
A15:8 Addre s s
Prev. addr.
DA7:0 Addre s s DataPrev. data XX
RD
DA7:0 (XMBK = 0) Data
Prev. data Addre ss
DataPrev. data Addre ss
DA7:0 (XMBK = 1)
System Clock (CLKCP U)
T4 T5
Note:  1. SRWn1 = SRW11 (upper sector) or SRW01 (lower sector), SRWn0 = SRW10 (upper sector) or
SRW00 (lower sector).
The ALE pulse in period T6 is only present if the next instruction accesses the RAM (internal or external).
Figure 12-9 External Data Memory Cycles with SRWn1 = 1 and SRWn0 = 1(1)
ALE
T1 T2 T3
Write
Read
WR
T7
A15:8 Address
Prev. a ddr.
DA7:0 Address DataPrev. da ta XX
RD
DA7:0 (XMBK = 0) Data
Prev. da ta Address
DataPrev. da ta Addre ss
DA7:0 (XMBK = 1)
Syste m Clock (CLK
CPU)
T4 T5 T6
Note:  1. SRWn1 = SRW11 (upper sector) or SRW01 (lower sector), SRWn0 = SRW10 (upper sector) or
SRW00 (lower sector). The ALE pulse in period T7 is only present if the next instruction accesses the
RAM (internal or external).
12.6.8. Using all Locations of External Memory Smaller than 64 Kbytes
Since the external memory is mapped after the internal memory as shown in Figure 12-4 External
Memory with Sector Select on page 35, the external memory is not addressed when addressing the first
4,352 bytes of data space. It may appear that the first 4,352 bytes of the external memory are
inaccessible (external memory addresses 0x0000 to 0x10FF). However, when connecting an external
memory smaller than 64K bytes, for example 32K bytes, these locations are easily accessed simply by
addressing from address 0x8000 to 0x90FF. Since the External Memory Address bit A15 is not connected
to the external memory, addresses 0x8000 to 0x90FF will appear as addresses 0x0000 to 0x10FF for the
external memory. Addressing above address 0x90FF is not recommended, since this will address an
external memory location that is already accessed by another (lower) address. To the Application
software, the external 32K bytes memory will appear as one linear 32K bytes address space from 0x1100
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AAAAA
to 0x90FF. This is illustrated in the figure below. Memory configuration B refers to the ATmega103
compatibility mode, configuration A to the non-compatible mode.
When the device is set in Atmel AVR ATmega103 compatibility mode, the internal address space is 4,096
bytes. This implies that the first 4,096 bytes of the external memory can be accessed at addresses
0x8000 to 0x8FFF. To the Application software, the external 32 Kbytes memory will appear as one linear
32 Kbytes address space from 0x1000 to 0x8FFF.
Figure 12-10 Address Map with 32Kbytes External Memory
0x0000
0x10FF
0xFFFF
0x1100
0x7FFF
0x8000
0x90FF
0x9100
0x0000
0x10FF
0x1100
0x7FFF
Memory Configuration A Memory Configuration B
Internal Memory
(Unused)
AVR Memory Map Externa l 32K SRAM
External
Memory
0x0000
0x0FFF
0xFFFF
0x1000
0x7FFF
0x8000
0x8FFF
0x9000
0x0000
0x0FFF
0x1000
0x7FFF
Internal Memory
(Unused)
AVR Memory Map Externa l 32K SRAM
External
Memory
12.6.9. Using all 64 Kbytes Locations of External Memory
Since the External Memory is mapped after the Internal Memory as shown in Figure 12-4 External
Memory with Sector Select on page 35, only 60Kbytes of External Memory is available by default
(address space 0x0000 to 0x10FF is reserved for internal memory). However, it is possible to take
advantage of the entire External Memory by masking the higher address bits to zero. This can be done by
using the XMMn bits and control by software the most significant bits of the address. By setting Port C to
output 0x00, and releasing the most significant bits for normal Port Pin operation, the Memory Interface
will address 0x0000 - 0x1FFF. See the following code examples.
Assembly Code Example(1)
; OFFSET is defined to 0x2000 to ensure
; external memory access
; Configure Port C (address high byte) to
; output 0x00 when the pins are released
; for normal Port Pin operation
ldi r16, 0xFF
out DDRC, r16
ldi r16, 0x00
out PORTC, r16
; release PC7:5
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ldi r16, (1<<XMM1)|(1<<XMM0)
sts XMCRB, r16
; write 0xAA to address 0x0001 of external
; memory
ldi r16, 0xaa
sts 0x0001+OFFSET, r16
; re-enable PC7:5 for external memory
ldi r16, (0<<XMM1)|(0<<XMM0)
sts XMCRB, r16
; store 0x55 to address (OFFSET + 1) of
; external memory
ldi r16, 0x55
sts 0x0001+OFFSET, r16
C Code Example(1)
#define OFFSET 0x2000
void XRAM_example(void)
{
unsigned char *p = (unsigned char *) (OFFSET + 1);
DDRC = 0xFF;
PORTC = 0x00;
XMCRB = (1<<XMM1) | (1<<XMM0);
*p = 0xaa;
XMCRB = 0x00;
*p = 0x55;
}
Note:  1. See About Code Examples.
Care must be exercised using this option as most of the memory is masked away.
Related Links
About Code Examples on page 20
12.7. Register Description
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12.7.1. EEARL – The EEPROM Address Register Low
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  EEARL
Offset:  0x1E
Reset:  0xXX
Property:
 
When addressing I/O Registers as data space the offset address is 0x3E
Bit 7 6 5 4 3 2 1 0
EEAR7 EEAR6 EEAR5 EEAR4 EEAR3 EEAR2 EEAR1 EEAR0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset x x x x x x x x
Bits 7:0 – EEARn: EEPROM Address [n = 7:0]
The EEPROM Address Registers – EEARH and EEARL – specify the EEPROM address in the 2Kbytes
EEPROM space. The EEPROM data bytes are addressed linearly between 0 and 2048. The initial value
of EEAR is undefined. A proper value must be written before the EEPROM may be accessed.
.
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12.7.2. EEARH – The EEPROM Address Register High
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  EEARH
Offset:  0x1F
Reset:  0xXX
Property:
 
When addressing I/O Registers as data space the offset address is 0x3F
Bit 7 6 5 4 3 2 1 0
EEAR10 EEAR9 EEAR8
Access R/W R/W R/W
Reset x x x
Bit 2 – EEAR10: EEPROM Address
Bit 1 – EEAR9: EEPROM Address
Bit 0 – EEAR8: EEPROM Address
Refer to EEARL on page 42.
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12.7.3. EEDR – The EEPROM Data Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  EEDR
Offset:  0x1D
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x3D
Bit 7 6 5 4 3 2 1 0
EEDR7 EEDR6 EEDR5 EEDR4 EEDR3 EEDR2 EEDR1 EEDR0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – EEDRn: EEPROM Data [n = 7:0]
For the EEPROM write operation, the EEDR Register contains the data to be written to the EEPROM in
the address given by the EEAR Register. For the EEPROM read operation, the EEDR contains the data
read out from the EEPROM at the address given by EEAR.
•EEDR[7] is MSB
•EEDR[0] is LSB
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12.7.4. EECR – The EEPROM Control Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  EECR
Offset:  0x1C
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x3C
Bit 7 6 5 4 3 2 1 0
EERIE EEMWE EEWE EERE
Access R/W R/W R/W R/W
Reset 0 0 x 0
Bit 3 – EERIE: EEPROM Ready Interrupt Enable
Writing EERIE to one enables the EEPROM Ready Interrupt if the I-bit in SREG is set. Writing EERIE to
zero disables the interrupt. The EEPROM Ready interrupt generates a constant interrupt when EEWE is
cleared.
Bit 2 – EEMWE: EEPROM Master Write Enable
The EEMWE bit determines whether setting EEWE to one causes the EEPROM to be written. When
EEMWE is set, setting EEWE within four clock cycles will write data to the EEPROM at the selected
address. If EEMWE is zero, setting EEWE will have no effect. When EEMWE has been written to one by
software, hardware clears the bit to zero after four clock cycles. See the description of the EEWE bit for
an EEPROM write procedure.
Bit 1 – EEWE: EEPROM Write Enable
The EEPROM Write Enable Signal EEWE is the write strobe to the EEPROM. When address and data
are correctly set up, the EEWE bit must be written to one to write the value into the EEPROM. The
EEMWE bit must be written to one before a logical one is written to EEWE, otherwise no EEPROM write
takes place. The following procedure should be followed when writing the EEPROM (the order of steps 3
and 4 is not essential):
1. Wait until EEWE becomes zero.
2. Wait until SPMEN in SPMCSR becomes zero.
3. Write new EEPROM address to EEAR (optional).
4. Write new EEPROM data to EEDR (optional).
5. Write a logical one to the EEMWE bit while writing a zero to EEWE in EECR.
6. Within four clock cycles after setting EEMWE, write a logical one to EEWE.
The EEPROM can not be programmed during a CPU write to the Flash memory. The software must
check that the Flash programming is completed before initiating a new EEPROM write. Step 2 is only
relevant if the software contains a boot loader allowing the CPU to program the Flash. If the Flash is
never being updated by the CPU, step 2 can be omitted. See Boot Loader Support – Read-While-Write
Self-Programming for details about boot programming.
Caution: An interrupt between step 5 and step 6 will make the write cycle fail, since the EEPROM Master
Write Enable will time-out. If an interrupt routine accessing the EEPROM is interrupting another EEPROM
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access, the EEAR or EEDR Register will be modified, causing the interrupted EEPROM access to fail. It
is recommended to have the Global Interrupt Flag cleared during all the steps to avoid these problems.
When the write access time has elapsed, the EEWE bit is cleared by hardware. The user software can
poll this bit and wait for a zero before writing the next byte. When EEWE has been set, the CPU is halted
for two cycles before the next instruction is executed.
Bit 0 – EERE: EEPROM Read Enable
The EEPROM Read Enable Signal EERE is the read strobe to the EEPROM. When the correct address
is set up in the EEAR Register, the EERE bit must be written to a logic one to trigger the EEPROM read.
The EEPROM read access takes one instruction, and the requested data is available immediately. When
the EEPROM is read, the CPU is halted for four cycles before the next instruction is executed.
The user should poll the EEWE bit before starting the read operation. If a write operation is in progress, it
is neither possible to read the EEPROM, nor to change the EEAR Register.
The calibrated Oscillator is used to time the EEPROM accesses. The following table lists the typical
programming time for EEPROM access from the CPU.
Table 12-2 EEPROM Programming Time
Symbol Number of Calibrated RC Oscillator Cycles(1) Typ Programming Time
EEPROM Write (from CPU) 8448 8.5ms
Note:  1. Uses 1MHz clock, independent of CKSEL Fuse settings.
The following code examples show one assembly and one C function for writing to the EEPROM. The
examples assume that interrupts are controlled (for example by disabling interrupts globally) so that no
interrupts will occur during execution of these functions. The examples also assume that no Flash boot
loader is present in the software. If such code is present, the EEPROM write function must also wait for
any ongoing SPM command to finish.
Assembly Code Example
EEPROM_write:
; Wait for completion of previous write
sbic EECR,EEWE
rjmp EEPROM_write
; Set up address (r18:r17) in address register
out EEARH, r18
out EEARL, r17
; Write data (r16) to data register
out EEDR,r16
; Write logical one to EEMWE
sbi EECR,EEMWE
; Start eeprom write by setting EEWE
sbi EECR,EEWE
ret
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C Code Example
void EEPROM_write(unsigned int uiAddress, unsigned char ucData)
{
/* Wait for completion of previous write */
while(EECR & (1<<EEWE))
;
/* Set up address and data registers */
EEAR = uiAddress;
EEDR = ucData;
/* Write logical one to EEMWE */
EECR |= (1<<EEMWE);
/* Start eeprom write by setting EEWE */
EECR |= (1<<EEWE);
}
The next code examples show assembly and C functions for reading the EEPROM. The examples
assume that interrupts are controlled so that no interrupts will occur during execution of these functions.
Assembly Code Example
EEPROM_read:
; Wait for completion of previous write
sbic EECR,EEWE
rjmp EEPROM_read
; Set up address (r18:r17) in address register
out EEARH, r18
out EEARL, r17
; Start eeprom read by writing EERE
sbi EECR,EERE
; Read data from data register
in r16,EEDR
ret
C Code Example
unsigned char EEPROM_read(unsigned int uiAddress)
{
/* Wait for completion of previous write */
while(EECR & (1<<EEWE))
;
/* Set up address register */
EEAR = uiAddress;
/* Start eeprom read by writing EERE */
EECR |= (1<<EERE);
/* Return data from data register */
return EEDR;
}
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12.7.5. MCUCR – MCU Control Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  MCUCR
Offset:  0x35
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x55
Bit 7 6 5 4 3 2 1 0
SRE SRW10
Access R/W R/W
Reset 0 0
Bit 7 – SRE: External SRAM/XMEM Enable
Writing SRE to one enables the External Memory Interface. The pin functions AD7:0, A15:8, ALE, WR,
and RD are activated as the alternate pin functions. The SRE bit overrides any pin direction settings in
the respective data direction registers. Writing SRE to zero, disables the External Memory Interface and
the normal pin and data direction settings are used.
Bit 6 – SRW10: Wait-state Select Bit
For a detailed description in non-ATmega103 compatibility mode, see common description for the SRWn
bits below (XMCRA description). In ATmega103 compatibility mode, writing SRW10 to one enables the
wait-state and one extra cycle is added during read/write strobe as shown in Figure 12-7 External Data
Memory Cycles with SRWn1 = 0 and SRWn0 = 1(1) on page 38.
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12.7.6. XMCRA – External Memory Control Register A
Name:  XMCRA
Offset:  0x6D
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
SRL2 SRL1 SRL0 SRW01 SRW00 SRW11
Access R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0
Bits 6:4 – SRLn: Wait-state Sector Limit [n = 2:0]
It is possible to configure different wait-states for different External Memory addresses. The external
memory address space can be divided in two sectors that have separate wait-state bits. The SRL2,
SRL1, and SRL0 bits select the split of the sectors, refer to the next table and Table 12-4 Wait States(1)
on page 50. By default, the SRL2, SRL1, and SRL0 bits are set to zero and the entire external memory
address space is treated as one sector. When the entire SRAM address space is configured as one
sector, the wait-states are configured by the SRW11 and SRW10 bits.
Table 12-3 Sector limits with different settings of SRL2:0
SRL2 SRL1 SRL0 Sector Limits
0 0 0 Lower sector = N/A
Upper sector = 0x1100 - 0xFFFF
0 0 1 Lower sector = 0x1100 - 0x1FFF
Upper sector = 0x2000 - 0xFFFF
0 1 0 Lower sector = 0x1100 - 0x3FFF
Upper sector = 0x4000 - 0xFFFF
0 1 1 Lower sector = 0x1100 - 0x5FFF
Upper sector = 0x6000 - 0xFFFF
1 0 0 Lower sector = 0x1100 - 0x7FFF
Upper sector = 0x8000 - 0xFFFF
1 0 1 Lower sector = 0x1100 - 0x9FFF
Upper sector = 0xA000 - 0xFFFF
1 1 0 Lower sector = 0x1100 - 0xBFFF
Upper sector = 0xC000 - 0xFFFF
1 1 1 Lower sector = 0x1100 - 0xDFFF
Upper sector = 0xE000 - 0xFFFF
Bits 3:2 – SRW0n: Wait-state Select Bits for Lower Sector [n = 1:0]
The SRW01 and SRW00 bits control the number of wait-states for the lower sector of the external
memory address space, see table below.
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Table 12-4 Wait States(1)
SRWn1 SRWn0 Wait States
0 0 No wait-states
0 1 Wait one cycle during read/write strobe
1 0 Wait two cycles during read/write strobe
1 1 Wait two cycles during read/write and wait one cycle before driving
out new address
Note:  1. n = 0 or 1 (lower/upper sector). For further details of the timing and wait-states of the External
Memory Interface, see Figures 13-6 through Figures 13-9 for how the setting of the SRW bits affects the
timing.
Bit 1 – SRW11: Wait-state Select Bits for Upper Sector
The SRW11 and SRW10 (bit 6 in MCUCR) bits control the number of wait-states for the upper sector of
the external memory address space, see Table 12-4 Wait States(1) on page 50.
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12.7.7. XMCRB – External Memory Control Register B
Name:  XMCRB
Offset:  0x6C
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
XMBK XMM2 XMM1 XMM0
Access R/W R/W R/W R/W
Reset 0 0 0 0
Bit 7 – XMBK: External Memory Bus-keeper Enable
Writing XMBK to one enables the bus keeper on the AD7:0 lines. When the bus keeper is enabled, it will
ensure a defined logic level (zero or one) on AD7:0 when they would otherwise be tri-stated. Writing
XMBK to zero disables the bus keeper. XMBK is not qualified with SRE, so even if the XMEM interface is
disabled, the bus keepers are still activated as long as XMBK is one.
Bits 2:0 – XMMn: External Memory High Mask [n = 2:0]
When the External Memory is enabled, all Port C pins are default used for the high address byte. If the
full 60Kbytes address space is not required to access the External Memory, some, or all, Port C pins can
be released for normal Port Pin function as described in the table below. As described in Using all 64
Kbytes Locations of External Memory on page 40, it is possible to use the XMMn bits to access all
64Kbytes locations of the External Memory.
Table 12-5 Port C Pins Released as Normal Port Pins when the External Memory is Enabled
XMM2 XMM1 XMM0 # Bits for External Memory Address Released Port
Pins
0 0 0 8 (Full 60 Kbytes space) None
0 0 1 7 PC7
0 1 0 6 PC7 - PC6
0 1 1 5 PC7 - PC5
1 0 0 4 PC7 - PC4
1 0 1 3 PC7 - PC3
1 1 0 2 PC7 - PC2
1 1 1 No Address high bits Full Port C
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13. System Clock and Clock Options
13.1. Clock Systems and their Distribution
The figure below presents the principal clock systems in the AVR and their distribution. All of the clocks
need not be active at a given time. In order to reduce power consumption, the clocks to modules not
being used can be halted by using different sleep modes, as described in Power Management and Sleep
Modes on page 61. The clock systems are detailed in the following figure.
Figure 13-1 Clock Distribution
Gene ral I/O
Module s
Asynchronous
Timer/Counte r ADC CPU Core RAM
clkI/O
clkASY
AVR Clock
Control Unit
clkCP U
Flash a nd
EEP ROM
clkFLASH
clkADC
Source Clock
Wa tchdog Timer
Wa tchdog
Oscillator
Reset Logic
Clock
Multiplexer
Wa tchdog Clock
Calibrated RC
Oscillator
Timer/Counte r
Oscillator
Crys ta l
Oscillator
Low-Freque ncy
Crys ta l Os cilla tor
Externa l RC
Oscillator Exte rnal Clock
13.1.1. CPU Clock – clkCPU
The CPU clock is routed to parts of the system concerned with operation of the AVR core. Examples of
such modules are the General Purpose Register File, the Status Register and the Data memory holding
the Stack Pointer. Halting the CPU clock inhibits the core from performing general operations and
calculations.
13.1.2. I/O Clock – clkI/O
The I/O clock is used by the majority of the I/O modules, like Timer/Counters, SPI, and USART. The I/O
clock is also used by the External Interrupt module, but note that some external interrupts are detected by
asynchronous logic, allowing such interrupts to be detected even if the I/O clock is halted. Also note that
address recognition in the TWI module is carried out asynchronously when clkI/O is halted, enabling TWI
address reception in all sleep modes.
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13.1.3. Flash Clock – clkFLASH
The Flash clock controls operation of the Flash interface. The Flash clock is usually active simultaneously
with the CPU clock.
13.1.4. Asynchronous Timer Clock – clkASY
The Asynchronous Timer clock allows the Asynchronous Timer/Counter to be clocked directly from an
external 32kHz clock crystal. The dedicated clock domain allows using this Timer/Counter as a real-time
counter even when the device is in sleep mode.
13.1.5. ADC Clock – clkADC
The ADC is provided with a dedicated clock domain. This allows halting the CPU and I/O clocks in order
to reduce noise generated by digital circuitry. This gives more accurate ADC conversion results.
13.2. Clock Sources
The device has the following clock source options, selectable by Flash Fuse Bits as shown below. The
clock from the selected source is input to the AVR clock generator, and routed to the appropriate
modules.
Table 13-1 Device Clocking Options Select
Device Clocking Option CKSEL3:0(1)
External Crystal/Ceramic Resonator 1111 - 1010
External Low-frequency Crystal 1001
External RC Oscillator 1000 - 0101
Calibrated Internal RC Oscillator 0100 - 0001
External Clock 0000
Note:  1. For all fuses “1” means unprogrammed while “0” means programmed.
The various choices for each clocking option is given in the following sections. When the CPU wakes up
from Power-down or Power-save, the selected clock source is used to time the start-up, ensuring stable
Oscillator operation before instruction execution starts. When the CPU starts from reset, there is as an
additional delay allowing the power to reach a stable level before commencing normal operation. The
Watchdog Oscillator is used for timing this real-time part of the start-up time. The number of WDT
Oscillator cycles used for each time-out is shown in the table below. The frequency of the Watchdog
Oscillator is voltage dependent as shown in Typical Characteristics.
Table 13-2 Number of Watchdog Oscillator Cycles
Typical Time-out (VCC = 5.0V) Typical Time-out (VCC = 3.0V) Number of Cycles
4.1ms 4.3ms 4K (4,096)
65ms 69ms 64K (65,536)
Related Links
Typical Characteristics – TA = -40°C to 85°C on page 436
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13.3. Default Clock Source
The device is shipped with CKSEL = “0001” and SUT = “10”. The default clock source setting is therefore
the Internal RC Oscillator with longest startup time. This default setting ensures that all users can make
their desired clock source setting using an In-System or Parallel Programmer.
13.4. Crystal Oscillator
XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be configured for
use as an On-chip Oscillator, as shown in the figure below. Either a quartz crystal or a ceramic resonator
may be used. The CKOPT Fuse selects between two different Oscillator amplifier modes. When CKOPT
is programmed, the Oscillator output will oscillate a full rail-to-rail swing on the output. This mode is
suitable when operating in a very noisy environment or when the output from XTAL2 drives a second
clock buffer. This mode has a wide frequency range. When CKOPT is unprogrammed, the Oscillator has
a smaller output swing. This reduces power consumption considerably. This mode has a limited
frequency range and it cannot be used to drive other clock buffers.
For resonators, the maximum frequency is 8MHz with CKOPT unprogrammed and 16MHz with CKOPT
programmed. C1 and C2 should always be equal for both crystals and resonators. The optimal value of
the capacitors depends on the crystal or resonator in use, the amount of stray capacitance, and the
electromagnetic noise of the environment. Some initial guidelines for choosing capacitors for use with
crystals are given in the next table. For ceramic resonators, the capacitor values given by the
manufacturer should be used.
Figure 13-2 Crystal Oscillator Connections
XTAL2
XTAL1
GND
C2
C1
The Oscillator can operate in three different modes, each optimized for a specific frequency range. The
operating mode is selected by the fuses CKSEL3:1 as shown in the following table.
Table 13-3 Crystal Oscillator Operating Modes
CKOPT(1) CKSEL3:1 Frequency Range(MHz) Recommended Range for Capacitors C1 and C2
for Use with Crystals (pF)
1 101(2) 0.4 - 0.9 –
1 110 0.9 - 3.0 12 - 22
1 111 3.0 - 8.0 12 - 22
0 101, 110, 111 1.0 -16.0 12 - 22
Note: 
1. When CKOPT is programmed (0), the oscillator output will be a full rail-to-rail swing on the output.
2. This option should not be used with crystals, only with ceramic resonators.
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The CKSEL0 Fuse together with the SUT1:0 Fuses select the start-up times as shown in the next table.
Table 13-4 Start-up Times for the Crystal Oscillator Clock Selection
CKSEL0 SUT1:0 Start-up Time
from Power-down
and Power-save
Additional Delay
from Reset
(VCC = 5.0V)
Recommended Usage
0 00 258 CK(1) 4.1ms Ceramic resonator, fast rising power
0 01 258 CK(1) 65ms Ceramic resonator, slowly rising power
0 10 1K CK(2) – Ceramic resonator, BOD enabled
0 11 1K CK(2) 4.1ms Ceramic resonator, fast rising power
1 00 1K CK(2) 65ms Ceramic resonator, slowly rising power
1 01 16K CK – Crystal Oscillator, BOD enabled
1 10 16K CK 4.1ms Crystal Oscillator, fast rising power
1 11 16K CK 65ms Crystal Oscillator, slowly rising power
Note: 
1. These options should only be used when not operating close to the maximum frequency of the
device, and only if frequency stability at start-up is not important for the application. These options
are not suitable for crystals.
2. These options are intended for use with ceramic resonators and will ensure frequency stability at
start-up. They can also be used with crystals when not operating close to the maximum frequency
of the device, and if frequency stability at start-up is not important for the application.
13.5. Low-frequency Crystal Oscillator
To use a 32.768kHz watch crystal as the clock source for the device, the Low-frequency Crystal Oscillator
must be selected by setting the CKSEL Fuses to “1001”. The crystal should be connected as shown in
Figure 13-2 Crystal Oscillator Connections on page 54. By programming the CKOPT Fuse, the user can
enable internal capacitors on XTAL1 and XTAL2, thereby removing the need for external capacitors. The
internal capacitors have a nominal value of 36pF.
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in the table
below.
Table 13-5 Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
SUT1:0 Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(VCC = 5.0V)
Recommended Usage
00 1K CK(1) 4.1ms Fast rising power or BOD enabled
01 1K CK(1) 65ms Slowly rising power
10 32K CK 65ms Stable frequency at start-up
11 Reserved
Note:  1. These options should only be used if frequency stability at start-up is not important for the
application.
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13.6. External RC Oscillator
For timing insensitive applications, the external RC configuration shown in the figure below can be used.
The frequency is roughly estimated by the equation f = 1/(3RC). C should be at least 22pF. By
programming the CKOPT Fuse, the user can enable an internal 36pF capacitor between XTAL1 and
GND, thereby removing the need for an external capacitor.
Figure 13-3 External RC Configuration
XTAL2
XTAL1
GND
C
R
VCC
NC
The Oscillator can operate in four different modes, each optimized for a specific frequency range. The
operating mode is selected by the fuses CKSEL3:0 as shown in the following table.
Table 13-6 External RC Oscillator Operating Modes
CKSEL3:0 Frequency Range (MHz)
0101 0.1 - 0.9
0110 0.9 - 3.0
0111 3.0 - 8.0
1000 8.0 - 12.0
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in the table
below.
Table 13-7 Start-up Times for the External RC Oscillator Clock Selection
SUT1:0 Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(VCC = 5.0V)
Recommended Usage
00 18 CK – BOD enabled
01 18 CK 4.1ms Fast rising power
10 18 CK 65ms Slowly rising power
11 6 CK(1) 4.1ms Fast rising power or BOD enabled
Note:  1. This option should not be used when operating close to the maximum frequency of the device.
13.7. Calibrated Internal RC Oscillator
The calibrated internal RC Oscillator provides a fixed 1.0, 2.0, 4.0, or 8.0MHz clock. All frequencies are
nominal values at 5V and 25°C. This clock may be selected as the system clock by programming the
CKSEL Fuses as shown in the following table. If selected, it will operate with no external components.
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The CKOPT Fuse should always be unprogrammed when using this clock option. During reset, hardware
loads the 1MHz calibration byte into the OSCCAL Register and thereby automatically calibrates the RC
Oscillator. At 5V, 25°C and 1.0MHz Oscillator frequency selected, this calibration gives a frequency within
± 3% of the nominal frequency. Using calibration methods as described in application notes available at
www.atmel.com/avr it is possible to achieve ± 1% accuracy at any given VCC and Temperature. When this
Oscillator is used as the chip clock, the Watchdog Oscillator will still be used for the Watchdog Timer and
for the Reset Time-out. For more information on the pre-programmed calibration value, see the section
Calibration Byte.
Table 13-8 Internal Calibrated RC Oscillator Operating Modes
CKSEL3:0 Nominal Frequency (MHz)
0001(1) 1.0
0010 2.0
0011 4.0
0100 8.0
Note:  1. The device is shipped with this option selected.
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in the
following table. XTAL1 and XTAL2 should be left unconnected (NC).
Table 13-9 Start-up Times for the Internal Calibrated RC Oscillator Clock Selection
SUT1:0 Start-up Time from Power-down
and Power-save
Additional Delay from Reset
(VCC = 5.0V)
Recommended Usage
00 6 CK – BOD enabled
01 6 CK 4.1ms Fast rising power
10(1) 6 CK 65ms Slowly rising power
11 Reserved
Note:  1. The device is shipped with this option selected.
Related Links
Calibration Byte on page 386
13.8. External Clock
To drive the device from an external clock source, XTAL1 should be driven as shown in the figure below.
To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”. By
programming the CKOPT Fuse, the user can enable an internal 36pF capacitor between XTAL1 and
GND.
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Nc xTALz 7 XTALI Atmel
Figure 13-4 External Clock Drive Configuration
EXTERNAL
CLOCK
SIGNAL
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in the
following table.
Table 13-10 Start-up Times for the External Clock Selection
SUT1:0 Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(VCC = 5.0V)
Recommended Usage
00 6 CK – BOD enabled
01 6 CK 4.1ms Fast rising power
10 6 CK 65ms Slowly rising power
11 Reserved
When applying an external clock, it is required to avoid sudden changes in the applied clock frequency to
ensure stable operation of the MCU. A variation in frequency of more than 2% from one clock cycle to the
next can lead to unpredictable behavior. It is required to ensure that the MCU is kept in Reset during such
changes in the clock frequency.
13.9. Timer/Counter Oscillator
For AVR microcontrollers with Timer/Counter Oscillator pins (TOSC1 and TOSC2), the crystal is
connected directly between the pins. No external capacitors are needed. The Oscillator is optimized for
use with a 32.768kHz watch crystal. Applying an external clock source to TOSC1 is not recommended.
Note:  1. The Timer/Counter Oscillator uses the same type of crystal oscillator as Low-Frequency
Oscillator and the internal capacitors have the same nominal value of 36pF.
13.10. Register Description
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13.10.1. XDIV – XTAL Divide Control Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
The XTAL Divide Control Register is used to divide the Source clock frequency by a number in the range
2 - 129. This feature can be used to decrease power consumption when the requirement for processing
power is low.
Name:  XDIV
Offset:  0x3C
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x5C
Bit 7 6 5 4 3 2 1 0
XDIVEN XDIV6 XDIV5 XDIV4 XDIV3 XDIV2 XDIV1 XDIV0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bit 7 – XDIVEN: XTAL Divide Enable
When the XDIVEN bit is written one, the clock frequency of the CPU and all peripherals (clkI/O, clkADC,
clkCPU, clkFLASH) is divided by the factor defined by the setting of XDIV6 - XDIV0. This bit can be written
run-time to vary the clock frequency as suitable to the application.
Bits 6:0 – XDIVn: XTAL Divide Select Bits [n = 6:0]
These bits define the division factor that applies when the XDIVEN bit is set (one). If the value of these
bits is denoted d, the following formula defines the resulting CPU and peripherals clock frequency fCLK:
CLK =Source clock
129 – d
The value of these bits can only be changed when XDIVEN is zero. When XDIVEN is written to one, the
value written simultaneously into XDIV6:XDIV0 is taken as the division factor. When XDIVEN is written to
zero, the value written simultaneously into XDIV6:XDIV0 is rejected. As the divider divides the master
clock input to the MCU, the speed of all peripherals is reduced when a division factor is used.
When the system clock is divided, Timer/Counter0 can be used with Asynchronous clock only. The
frequency of the asynchronous clock must be lower than 1/4th of the frequency of the scaled down
Source clock. Otherwise, interrupts may be lost, and accessing the Timer/Counter0 registers may fail.
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13.10.2. OSCCAL – The Oscillator Calibration Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  OSCCAL
Offset:  0x31
Reset:  0x00
Property:
 
Bit 7 6 5 4 3 2 1 0
CAL7 CAL6 CAL5 CAL4 CAL3 CAL2 CAL1 CAL0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset x x x x x x x x
Bits 7:0 – CALn: Oscillator Calibration Value [n = 7:0]
Writing the calibration byte to this address will trim the Internal Oscillator to remove process variations
from the Oscillator frequency. During Reset, the 1MHz calibration value which is located in the signature
row High byte (address 0x00) is automatically loaded into the OSCCAL Register. If the internal RC is
used at other frequencies, the calibration values must be loaded manually. This can be done by first
reading the signature row by a programmer, and then store the calibration values in the Flash or
EEPROM. Then the value can be read by software and loaded into the OSCCAL Register. When
OSCCAL is zero, the lowest available frequency is chosen. Writing non-zero values to this register will
increase the frequency of the Internal Oscillator. Writing 0xFF to the register gives the highest available
frequency. The calibrated Oscillator is used to time EEPROM and Flash access. If EEPROM or Flash is
written, do not calibrate to more than 10% above the nominal frequency. Otherwise, the EEPROM or
Flash write may fail. Note that the Oscillator is intended for calibration to 1.0, 2.0, 4.0, or 8.0MHz. Tuning
to other values is not guaranteed, as indicated in the following table.
Table 13-11 Internal RC Oscillator Frequency Range
OSCCAL Value Min Frequency in Percentage of
Nominal Frequency (%)
Max Frequency in Percentage of
Nominal Frequency (%)
0x00 50 100
0x7F 75 150
0xFF 100 200
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14. Power Management and Sleep Modes
14.1. Sleep Modes
Sleep modes enable the application to shut down unused modules in the MCU, thereby saving power.
The AVR provides various sleep modes allowing the user to tailor the power consumption to the
application’s requirements.
Figure Clock Distribution in section Clock Systems and their Distribution presents the different clock
systems in the ATmega64A, and their distribution. The figure is helpful in selecting an appropriate sleep
mode. The table below shows the different clock options and their wake-up sources.
Table 14-1 Active Clock Domains and Wake-up Sources in the Different Sleep Modes
Active Clock Domains Oscillators Wake-up Sources
Sleep
Mode
clkCPU clkFLASH clkIO clkADC clkASY Main
Clock
Source
Enabled
Timer
Osc.
Enabled
INT1/
INT0
TWIAddress
Match
Timer0 SPM/
EEPROM
Ready
ADC Other
I/O
Idle X X X X X(2) X X X X X X
ADC
Noise
Reduction
X X X X(2) X(3) X X X X
Power-
down
X(3) X
Power-
save
X(2) X(2) X(3) X X(2)
Standby(1
)X X(3) X
Extended
Standby(1
)
X(2) X X(2) X(3) X X(2)
Note: 
1. External Crystal or resonator selected as clock source.
2. If AS0 bit in ASSR is set.
3. Only INT3:0 or level interrupt INT7:4.
To enter any of the six sleep modes, the SE bit in MCUCR must be written to logic one and a SLEEP
instruction must be executed. The SM2, SM1, and SM0 bits in the MCUCR Register select which sleep
mode (Idle, ADC Noise Reduction, Power-down, Power-save, Standby, or Extended Standby) will be
activated by the SLEEP instruction. See Table 14-2 Sleep Mode Select on page 66 for a summary.
If an enabled interrupt occurs while the MCU is in a sleep mode, the MCU wakes up. The MCU is then
halted for four cycles in addition to the start-up time, it executes the interrupt routine, and resumes
execution from the instruction following SLEEP. The contents of the Register File and SRAM are
unaltered when the device wakes up from sleep. If a reset occurs during sleep mode, the MCU wakes up
and executes from the Reset Vector.
Related Links
Clock Systems and their Distribution on page 52
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14.2. Idle Mode
When the SM2:0 bits are written to 000, the SLEEP instruction makes the MCU enter Idle mode, stopping
the CPU but allowing SPI, USART, Analog Comparator, ADC, Two-wire Serial Interface, Timer/Counters,
Watchdog, and the interrupt system to continue operating. This sleep mode basically halts clkCPU and
clkFLASH, while allowing the other clocks to run.
Idle mode enables the MCU to wake up from external triggered interrupts as well as internal ones like the
Timer Overflow and USART Transmit Complete interrupts. If wake-up from the Analog Comparator
interrupt is not required, the Analog Comparator can be powered down by setting the ACD bit in the
Analog Comparator Control and Status Register – ACSR. This will reduce power consumption in Idle
mode. If the ADC is enabled, a conversion starts automatically when this mode is entered.
14.3. ADC Noise Reduction Mode
When the SM2:0 bits are written to 001, the SLEEP instruction makes the MCU enter ADC Noise
Reduction mode, stopping the CPU but allowing the ADC, the external interrupts, the Two-wire Serial
Interface address watch, Timer/Counter0 and the Watchdog to continue operating (if enabled). This sleep
mode basically halts clkI/O, clkCPU, and clkFLASH, while allowing the other clocks to run.
This improves the noise environment for the ADC, enabling higher resolution measurements. If the ADC
is enabled, a conversion starts automatically when this mode is entered. Apart form the ADC Conversion
Complete interrupt, only an External Reset, a Watchdog Reset, a Brown-out Reset, a Two-wire Serial
Interface address match interrupt, a Timer/Counter0 interrupt, an SPM/EEPROM ready interrupt, an
External Level Interrupt on INT7:4, or an External Interrupt on INT3:0 can wake up the MCU from ADC
Noise Reduction mode.
14.4. Power-down Mode
When the SM2:0 bits are written to 010, the SLEEP instruction makes the MCU enter Power-down mode.
In this mode, the External Oscillator is stopped, while the External Interrupts, the Two-wire Serial
Interface address watch, and the Watchdog continue operating (if enabled). Only an External Reset, a
Watchdog Reset, a Brownout Reset, a Two-wire Serial Interface address match interrupt, an External
Level Interrupt on INT7:4, or an External Interrupt on INT3:0 can wake up the MCU. This sleep mode
basically halts all generated clocks, allowing operation of asynchronous modules only.
Note that if a level triggered interrupt is used for wake-up from Power-down mode, the changed level
must be held for some time to wake up the MCU. Refer to External Interrupts for details.
When waking up from Power-down mode, there is a delay from the wake-up condition occurs until the
wake-up becomes effective. This allows the clock to restart and become stable after having been
stopped. The wake-up period is defined by the same CKSEL Fuses that define the Reset Time-out
period, as described in Clock Sources.
Related Links
External Interrupts on page 85
Clock Sources on page 53
14.5. Power-save Mode
When the SM2:0 bits are written to 011, the SLEEP instruction makes the MCU enter Power-save mode.
This mode is identical to Power-down, with one exception:
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•If Timer/Counter0 is clocked asynchronously, i.e. the AS0 bit in ASSR is set, Timer/Counter0 will
run during sleep. The device can wake up from either Timer Overflow or Output Compare event
from Timer/Counter0 if the corresponding Timer/Counter0 interrupt enable bits are set in TIMSK,
and the global interrupt enable bit in SREG is set.
If the asynchronous timer is NOT clocked asynchronously, Power-down mode is recommended instead of
Power-save mode because the contents of the registers in the asynchronous timer should be considered
undefined after wake-up in Power-save mode if AS0 is 0.
This sleep mode basically halts all clocks except clkASY, allowing operation only of asynchronous
modules, including Timer/Counter0 if clocked asynchronously.
14.6. Standby Mode
When the SM2:0 bits are 110 and an external crystal/resonator clock option is selected, the SLEEP
instruction makes the MCU enter Standby mode. This mode is identical to Power-down with the exception
that the Oscillator is kept running. From Standby mode, the device wakes up in 6 clock cycles.
14.7. Extended Standby Mode
When the SM2:0 bits are 111 and an external crystal/resonator clock option is selected, the SLEEP
instruction makes the MCU enter Extended Standby mode. This mode is identical to Power-save mode
with the exception that the Oscillator is kept running. From Extended Standby mode, the device wakes up
in six clock cycles.
14.8. Minimizing Power Consumption
There are several issues to consider when trying to minimize the power consumption in an AVR
controlled system. In general, sleep modes should be used as much as possible, and the sleep mode
should be selected so that as few as possible of the device’s functions are operating. All functions not
needed should be disabled. In particular, the following modules may need special consideration when
trying to achieve the lowest possible power consumption.
Related Links
System Clock and Clock Options on page 52
14.8.1. Analog-to-Digital Converter (ADC)
If enabled, the ADC will be enabled in all sleep modes. To save power, the ADC should be disabled
before entering any sleep mode. When the ADC is turned off and on again, the next conversion will be an
extended conversion. Refer to Analog-to-Digital Converter for details on ADC operation.
Related Links
ADC - Analog to Digital Converter on page 311
14.8.2. Analog Comparator
When entering Idle mode, the Analog Comparator should be disabled if not used. When entering ADC
Noise Reduction mode, the Analog Comparator should be disabled. In the other sleep modes, the Analog
Comparator is automatically disabled. However, if the Analog Comparator is set up to use the Internal
Voltage Reference as input, the Analog Comparator should be disabled in all sleep modes. Otherwise,
the Internal Voltage Reference will be enabled, independent of sleep mode. Refer to Analog Comparator
for details on how to configure the Analog Comparator.
Related Links
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Analog Comparator on page 306
14.8.3. Brown-out Detector
If the Brown-out Detector is not needed in the application, this module should be turned off. If the Brown-
out Detector is enabled by the BODEN Fuse, it will be enabled in all sleep modes, and hence, always
consume power. In the deeper sleep modes, this will contribute significantly to the total current
consumption. Refer to Brown-out Detection for details on how to configure the Brown-out Detector.
Related Links
Brown-out Detection on page 70
14.8.4. Internal Voltage Reference
The Internal Voltage Reference will be enabled when needed by the Brown-out Detector, the Analog
Comparator or the ADC. If these modules are disabled as described in the sections above, the internal
voltage reference will be disabled and it will not be consuming power. When turned on again, the user
must allow the reference to start up before the output is used. If the reference is kept on in sleep mode,
the output can be used immediately. Refer to Internal Voltage Reference for details on the start-up time.
Related Links
Internal Voltage Reference on page 71
14.8.5. Watchdog Timer
If the Watchdog Timer is not needed in the application, this module should be turned off. If the Watchdog
Timer is enabled, it will be enabled in all sleep modes, and hence, always consume power. In the deeper
sleep modes, this will contribute significantly to the total current consumption. Refer to Watchdog Timer
for details on how to configure the Watchdog Timer.
Related Links
Watchdog Timer on page 71
14.8.6. Port Pins
When entering a sleep mode, all port pins should be configured to use minimum power. The most
important thing is then to ensure that no pins drive resistive loads. In sleep modes where the both the I/O
clock (clkI/O) and the ADC clock (clkADC) are stopped, the input buffers of the device will be disabled. This
ensures that no power is consumed by the input logic when not needed. In some cases, the input logic is
needed for detecting wake-up conditions, and it will then be enabled. Refer to the section Digital Input
Enable and Sleep Modes for details on which pins are enabled. If the input buffer is enabled and the input
signal is left floating or have an analog signal level close to VCC/2, the input buffer will use excessive
power.
Related Links
Digital Input Enable and Sleep Modes on page 96
14.8.7. JTAG Interface and On-chip Debug System
If the On-chip debug system is enabled by the OCDEN Fuse and the chip enter Power down or Power
save sleep mode, the main clock source remains enabled. In these sleep modes, this will contribute
significantly to the total current consumption. There are three alternative ways to avoid this:
•Disable OCDEN Fuse.
•Disable JTAGEN Fuse.
•Write one to the JTD bit in MCUCSR.
The TDO pin is left floating when the JTAG interface is enabled while the JTAG TAP controller is not
shifting data. If the hardware connected to the TDO pin does not pull up the logic level, power
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consumption will increase. Note that the TDI pin for the next device in the scan chain contains a pull-up
that avoids this problem. Writing the JTD bit in the MCUCSR register to one or leaving the JTAG fuse
unprogrammed disables the JTAG interface.
14.9. Register Description
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14.9.1. MCUCR – MCU Control Register
The MCU Control Register contains control bits for power management.
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  MCUCR
Offset:  0x35
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x55
Bit 7 6 5 4 3 2 1 0
SE SM1 SM0 SM2
Access R/W R/W R/W R/W
Reset 0 0 0 0
Bit 5 – SE: Sleep Enable
The SE bit must be written to logic one to make the MCU enter the sleep mode when the SLEEP
instruction is executed. To avoid the MCU entering the sleep mode unless it is the programmer’s purpose,
it is recommended to set the Sleep Enable (SE) bit to one just before the execution of the SLEEP
instruction.
Bits 4:3 – SMn: Sleep Mode n Select Bits [n=1:0]
These bits select between the five available sleep modes as shown in the table.
Table 14-2 Sleep Mode Select
SM2 SM1 SM0 Sleep Mode
0 0 0 Idle
0 0 1 ADC Noise Reduction
0 1 0 Power-down
0 1 1 Power-save
1 0 0 Reserved
1 0 1 Reserved
1 1 0 Standby(1)
1 1 0 Extended Standby (1)
Note:  1. Standby mode is only available with external crystals or resonators.
Bit 2 – SM2: Sleep Mode Select Bit 2
Refer to SMn: Sleep Mode n Select Bits above.
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15. System Control and Reset
15.1. Resetting the AVR
During Reset, all I/O Registers are set to their initial values, and the program starts execution from the
Reset Vector. If the program never enables an interrupt source, the Interrupt Vectors are not used, and
regular program code can be placed at these locations. This is also the case if the Reset Vector is in the
Application section while the Interrupt Vectors are in the boot section or vice versa. The circuit diagram in
the following section shows the Reset Logic. The Table in System and Reset Characteristics defines the
electrical parameters of the reset circuitry.
The I/O ports of the AVR are immediately reset to their initial state when a reset source goes active. This
does not require any clock source to be running.
After all reset sources have gone inactive, a delay counter is invoked, stretching the internal reset. This
allows the power to reach a stable level before normal operation starts. The time-out period of the delay
counter is defined by the user through the CKSEL Fuses. The different selections for the delay period are
presented in Clock Sources.
Related Links
System and Reset Characteristics on page 418
Clock Sources on page 53
15.2. Reset Sources
The ATmega64A has five sources of reset:
•Power-on Reset. The MCU is reset when the supply voltage is below the Power-on Reset threshold
(VPOT).
•External Reset. The MCU is reset when a low level is present on the RESET pin for longer than the
minimum pulse length.
•Watchdog Reset. The MCU is reset when the Watchdog Timer period expires and the Watchdog is
enabled.
•Brown-out Reset. The MCU is reset when the supply voltage VCC is below the Brown-out Reset
threshold (VBOT) and the Brown-out Detector is enabled.
•JTAG AVR Reset. The MCU is reset as long as there is a logic one in the Reset Register, one of the
scan chains of the JTAG system. Refer to the section IEEE 1149.1 (JTAG) Boundary-scan for
details.
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DATA BUS Bmwn »0u| BODEN Reset Cmm EODLEVEL INTERNAL RESET CKSEL[3:01 Atmel
Figure 15-1 Reset Logic
MCU Control a nd S tatus
Re giste r (MCUCS R)
Brown-Out
Re se t Circuit
BODEN
BODLEVEL
Dela y Counters
CKSEL[3:0]
CK
TIMEOUT
WDRF
BORF
EXTRF
PORF
DATA BUS
Clock
Ge ne ra tor
SP IKE
FILTER
Pull-up Res istor
JTRF
JTAG Re s et
Re giste r
Watchdog
Os cillator
SUT[1:0]
COUNTER RES ET
Watchdog
Timer
RES ET
Pull-up Res istor
PEN
Re se t Circuit
L
D Q
Q
Power-On Re s e t
Circuit
Related Links
IEEE 1149.1 (JTAG) Boundary-scan on page 340
15.2.1. Power-on Reset
A Power-on Reset (POR) pulse is generated by an On-chip detection circuit. The detection level is
defined in the table in System and Reset Characteristics. The POR is activated whenever VCC is below
the detection level. The POR circuit can be used to trigger the Start-up Reset, as well as to detect a
failure in supply voltage.
A Power-on Reset (POR) circuit ensures that the device is reset from Power-on. Reaching the Power-on
Reset threshold voltage invokes the delay counter, which determines how long the device is kept in
RESET after VCC rise. The RESET signal is activated again, without any delay, when VCC decreases
below the detection level.
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RESET RESET RESET TIME-OUT INTERNAL RESET
Figure 15-2 MCU Start-up, RESET Tied to VCC
V
RES ET
TIME-OUT
INTERNAL
RES ET
tTOUT
VPOT
VRST
CC
Figure 15-3 Figure: MCU Start-up, RESET Extended Externally
RES ET
TIME-OUT
INTERNAL
RES ET
tTOUT
VPOT
VRST
VCC
Related Links
System and Reset Characteristics on page 418
15.2.2. External Reset
An External Reset is generated by a low level on the RESET pin. Reset pulses longer than the minimum
pulse width (see table in System and Reset Characteristics) will generate a reset, even if the clock is not
running. Shorter pulses are not guaranteed to generate a reset. When the applied signal reaches the
Reset Threshold Voltage – VRST on its positive edge, the delay counter starts the MCU after the time-out
period tTOUT has expired.
Figure 15-4 External Reset During Operation
CC
Related Links
System and Reset Characteristics on page 418
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RESET WDT TIME-OUT RESET TIME-OUT INTERNAL RESET Atmel
15.2.3. Brown-out Detection
ATmega64A has an On-chip Brown-out Detection (BOD) circuit for monitoring the VCC level during
operation by comparing it to a fixed trigger level. The trigger level for the BOD can be selected by the fuse
BODLEVEL to be 2.7V (BODLEVEL unprogrammed), or 4.0V (BODLEVEL programmed). The trigger
level has a hysteresis to ensure spike free Brown-out Detection. The hysteresis on the detection level
should be interpreted as VBOT+ = VBOT + VHYST/2 and VBOT- = VBOT - VHYST/2.
The BOD circuit can be enabled/disabled by the fuse BODEN. When the BOD is enabled (BODEN
programmed), and VCC decreases to a value below the trigger level (VBOT- in the figure below), the
Brown-out Reset is immediately activated. When VCC increases above the trigger level (VBOT+ in the
figure below), the delay counter starts the MCU after the time-out period tTOUT has expired.
The BOD circuit will only detect a drop in VCC if the voltage stays below the trigger level for longer than
tBOD given in the table in System and Reset Characteristics.
Figure 15-5 Brown-out Reset During Operation
VCC
RES ET
TIME-OUT
INTERNAL
RES ET
VBOT-
VBOT+
tTOUT
Related Links
System and Reset Characteristics on page 418
15.2.4. Watchdog Reset
When the Watchdog times out, it will generate a short reset pulse of 1 CK cycle duration. On the falling
edge of this pulse, the delay timer starts counting the time-out period tTOUT. Refer to Watchdog Timer for
details on operation of the Watchdog Timer.
Figure 15-6 Watchdog Reset During Operation
CK
CC
Related Links
Watchdog Timer on page 71
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15.3. Internal Voltage Reference
ATmega64A features an internal bandgap reference. This reference is used for Brown-out Detection, and
it can be used as an input to the Analog Comparator or the ADC. The 2.56V reference to the ADC is
generated from the internal bandgap reference.
15.3.1. Voltage Reference Enable Signals and Start-up Time
The voltage reference has a start-up time that may influence the way it should be used. The start-up time
is given in the table in System and Reset Characteristics. To save power, the reference is not always
turned on. The reference is on during the following situations:
1. When the BOD is enabled (by programming the BODEN Fuse).
2. When the bandgap reference is connected to the Analog Comparator (by setting the ACBG bit in
ACSR).
3. When the ADC is enabled.
Thus, when the BOD is not enabled, after setting the ACBG bit or enabling the ADC, the user must
always allow the reference to start up before the output from the Analog Comparator or ADC is used. To
reduce power consumption in Power-down mode, the user can avoid the three conditions above to
ensure that the reference is turned off before entering Power-down mode.
Related Links
System and Reset Characteristics on page 418
15.4. Watchdog Timer
The Watchdog Timer is clocked from a separate On-chip Oscillator which runs at 1MHz. This is the typical
value at VCC = 5V. See characterization data for typical values at other VCC levels. By controlling the
Watchdog Timer prescaler, the Watchdog Reset interval can be adjusted as shown in Table 15-2 
Watchdog Timer Prescale Select on page 75. The WDR – Watchdog Reset – instruction resets the
Watchdog Timer. The Watchdog Timer is also reset when it is disabled and when a Chip Reset occurs.
Eight different clock cycle periods can be selected to determine the reset period. If the reset period
expires without another Watchdog Reset, the ATmega64A resets and executes from the Reset Vector.
For timing details on the Watchdog Reset, refer to Watchdog Reset on page 70.
To prevent unintentional disabling of the Watchdog or unintentional change of time-out period, 3 different
safety levels are selected by the Fuses M103C and WDTON as shown in the table. Safety level 0
corresponds to the setting in ATmega103. There is no restriction on enabling the WDT in any of the safety
levels. Refer Timed Sequences for Changing the Configuration of the Watchdog Timer on page 72
details.
Table 15-1 WDT Configuration as a Function of the Fuse Settings of M103C and WDTON.
M103C WDTON Safety
Level
WDT Initial
State
How to Disable
the WDT
How to
Change Time-
out
Unprogrammed Unprogrammed 1 Disabled Timed sequence Timed
sequence
Unprogrammed Programmed 2 Enabled Always enabled Timed
sequence
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Atmel WATCHDOG PRESCALER WATCHDOG RESET WDPO WDP1 WDPZ WDE 050/st Use/32K ()5ch osmium ass/255K OSC/SIQK oscn om osuznm MCU RESET
M103C WDTON Safety
Level
WDT Initial
State
How to Disable
the WDT
How to
Change Time-
out
Programmed Unprogrammed 0 Disabled Timed sequence No restriction
Programmed Programmed 2 Enabled Always enabled Timed
sequence
Figure 15-7 Watchdog Timer
WATCHDOG
OS CILLATOR
15.5. Timed Sequences for Changing the Configuration of the Watchdog Timer
The sequence for changing configuration differs slightly between the three safety levels. Separate
procedures are described for each level.
15.5.1. Safety Level 0
This mode is compatible with the Watchdog operation found in ATmega103. The Watchdog Timer is
initially disabled, but can be enabled by writing the WDE bit to 1 without any restriction. The time-out
period can be changed at any time without restriction. To disable an enabled Watchdog Timer, the
procedure described in the bit description for WDE in the WDTCR on page 75 must be followed.
15.5.2. Safety Level 1
In this mode, the Watchdog Timer is initially disabled, but can be enabled by writing the WDE bit to 1
without any restriction. A timed sequence is needed when changing the Watchdog Time-out period or
disabling an enabled Watchdog Timer. To disable an enabled Watchdog Timer, and/or changing the
Watchdog Time-out, the following procedure must be followed:
1. In the same operation, write a logic one to WDCE and WDE. A logic one must be written to WDE
regardless of the previous value of the WDE bit.
2. Within the next four clock cycles, in the same operation, write the WDE and WDP bits as desired,
but with the WDCE bit cleared.
15.5.3. Safety Level 2
In this mode, the Watchdog Timer is always enabled, and the WDE bit will always read as one. A timed
sequence is needed when changing the Watchdog Time-out period. To change the Watchdog Time-out,
the following procedure must be followed:
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1. In the same operation, write a logical one to WDCE and WDE. Even though the WDE always is set,
the WDE must be written to one to start the timed sequence.
2. Within the next four clock cycles, in the same operation, write the WDP bits as desired, but with the
WDCE bit cleared. The value written to the WDE bit is irrelevant.
15.6. Register Description
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15.6.1. MCUCSR – MCU Control and Status Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
The MCU Control and Status Register provides information on which reset source caused an MCU Reset.
Note: 
1. Only EXTRF and PORF are available in ATmega103 compatibility mode.
2. For Reset value, see bit description.
Name:  MCUCSR
Offset:  0x34
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x54
Bit 7 6 5 4 3 2 1 0
JTRF WDRF BORF EXTRF PORF
Access R/W R/W R/W R/W R/W
Reset - - - - -
Bit 4 – JTRF: JTAG Reset Flag
This bit is set if a reset is being caused by a logic one in the JTAG Reset Register selected by the JTAG
instruction AVR_RESET. This bit is reset by a Power-on Reset, or by writing a logic zero to the flag.
Bit 3 – WDRF: Watchdog Reset Flag
This bit is set if a Watchdog Reset occurs. The bit is reset by a Power-on Reset, or by writing a logic zero
to the flag.
Bit 2 – BORF: Brown-out Reset Flag
This bit is set if a Brown-out Reset occurs. The bit is reset by a Power-on Reset, or by writing a logic zero
to the flag.
Bit 1 – EXTRF: External Reset Flag
This bit is set if an External Reset occurs. The bit is reset by a Power-on Reset, or by writing a logic zero
to the flag.
Bit 0 – PORF: Power-on Reset Flag
This bit is set if a Power-on Reset occurs. The bit is reset only by writing a logic zero to the flag. To make
use of the Reset Flags to identify a reset condition, the user should read and then reset the MCUCSR as
early as possible in the program. If the register is cleared before another reset occurs, the source of the
reset can be found by examining the Reset Flags.
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15.6.2. WDTCR – Watchdog Timer Control Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  WDTCR
Offset:  0x21
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x41
Bit 7 6 5 4 3 2 1 0
WDCE WDE WDP2 WDP1 WDP0
Access R/W R/W R/W R/W R/W
Reset 0 0 0 0 0
Bit 4 – WDCE: Watchdog Change Enable
This bit must be set when the WDE bit is written to logic zero. Otherwise, the Watchdog will not be
disabled. Once written to one, hardware will clear this bit after four clock cycles. Refer to the description
of the WDE bit for a Watchdog disable procedure. In Safety Level 1 and 2, this bit must also be set when
changing the prescaler bits. Refer to Timed Sequences for Changing the Configuration of the Watchdog
Timer on page 72.
Bit 3 – WDE: Watchdog Enable
When the WDE is written to logic one, the Watchdog Timer is enabled, and if the WDE is written to logic
zero, the Watchdog Timer function is disabled. WDE can only be cleared if the WDCE bit has logic level
one. To disable an enabled Watchdog Timer, the following procedure must be followed:
1. In the same operation, write a logic one to WDCE and WDE. A logic one must be written to WDE
even though it is set to one before the disable operation starts.
2. Within the next four clock cycles, write a logic 0 to WDE. This disables the Watchdog.
In safety level 2, it is not possible to disable the Watchdog Timer, even with the algorithm described
above. Refer to Timed Sequences for Changing the Configuration of the Watchdog Timer on page 72.
Bits 2:0 – WDPn: Watchdog Timer Prescaler 2, 1, and 0 [n = 2:0]
The WDP2, WDP1, and WDP0 bits determine the Watchdog Timer prescaling when the Watchdog Timer
is enabled. The different prescaling values and their corresponding Timeout Periods are shown in the
table below.
Table 15-2 Watchdog Timer Prescale Select
WDP2 WDP1 WDP0 Number of WDT Oscillator
Cycles
Typical
Time-out at
VCC = 3.0V
Typical
Time-out at
VCC = 5.0V
0 0 0 16K (16,384) 17.1ms 16.3ms
0 0 1 32K (32,768) 34.3ms 32.5ms
0 1 0 64K (65,536) 68.5ms 65ms
0 1 1 128K (131,072) 0.14s 0.13s
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WDP2 WDP1 WDP0 Number of WDT Oscillator
Cycles
Typical
Time-out at
VCC = 3.0V
Typical
Time-out at
VCC = 5.0V
1 0 0 256K (262,144) 0.27s 0.26s
1 0 1 512K (524,288) 0.55s 0.52s
1 1 0 1,024K (1,048,576) 1.1s 1.0s
1 1 1 2,048K (2,097,152) 2.2s 2.1s
The following code example shows one assembly and one C function for turning off the WDT. The
example assumes that interrupts are controlled (for example by disabling interrupts globally) so that no
interrupts will occur during execution of these functions.
Assembly Code Example
WDT_off:
; Reset WDT
wdr
in r16, WDTCR
ldi r16, (1<<WDCE)|(1<<WDE)
; Write logical one to WDCE and WDE
ori r16, (1<<WDCE)|(1<<WDE)
out WDTCR, r16
; Turn off WDT
ldi r16, (0<<WDE)
out WDTCR, r16
ret
C Code Example
void WDT_off(void)
{
/* Reset WDT*/
_WDRC();
/* Write logical one to WDCE and WDE */
WDTCR |= (1<<WDCE) | (1<<WDE);
/* Turn off WDT */
WDTCR = 0x00;
}
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16. Interrupts
This section describes the specifics of the interrupt handling performed by the ATmega64A. For a general
explanation of the AVR interrupt handling, refer to Reset and Interrupt Handling.
Related Links
Reset and Interrupt Handling on page 27
16.1. Interrupt Vectors in ATmega64A
Table 16-1 Reset and Interrupt Vectors
Vector No. Program
Address(2)
Source Interrupt Definition
1 0x0000(1) RESET External Pin, Power-on Reset, Brown-out Reset, and
Watchdog Reset
2 0x0002 INT0 External Interrupt Request 0
3 0x0004 INT1 External Interrupt Request 1
4 0x0006 INT2 External Interrupt Request 2
5 0x0008 INT3 External Interrupt Request 3
6 0x000A INT4 External Interrupt Request 4
7 0x000C INT5 External Interrupt Request 5
8 0x000E INT6 External Interrupt Request 6
9 0x0010 INT7 External Interrupt Request 7
10 0x0012 TIMER2 COMP Timer/Counter2 Compare Match
11 0x0014 TIMER2 OVF Timer/Counter2 Overflow
12 0x0016 TIMER1 CAPT Timer/Counter1 Capture Event
13 0x0018 TIMER1 COMPA Timer/Counter1 Compare Match A
14 0x001A TIMER1 COMPB Timer/Counter1 Compare Match B
15 0x001C TIMER1 OVF Timer/Counter1 Overflow
16 0x001E TIMER0 COMP Timer/Counter0 Compare Match
17 0x0020 TIMER0 OVF Timer/Counter0 Overflow
18 0x0022 SPI, STC SPI Serial Transfer Complete
19 0x0024 USART0, RX USART0, Rx Complete
20 0x0026 USART0, UDRE USART0 Data Register Empty
21 0x0028 USART0, TX USART0, Tx Complete
22 0x002A ADC ADC Conversion Complete
23 0x002C EE READY EEPROM Ready
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Vector No. Program
Address(2)
Source Interrupt Definition
24 0x002E ANALOG COMP Analog Comparator
25 0x0030(3) TIMER1 COMPC Timer/Counter1 Compare Match C
26 0x0032(3) TIMER3 CAPT Timer/Counter3 Capture Even
27 0x0034(3) TIMER3 COMPA Timer/Counter3 Compare Match A
28 0x0036(3) TIMER3 COMPB Timer/Counter3 Compare Match B
29 0x0038(3) TIMER3 COMPC Timer/Counter3 Compare Match C
30 0x003A(3) TIMER3 OVF Timer/Counter3 Overflow
31 0x003C(3) USART1, RX USART1, Rx Complete
32 0x003E(3) USART1, UDRE USART1 Data Register Empty
33 0x0040(3) USART1, TX USART1, Tx Complete
34 0x0042(3) TWI Two-wire Serial Interface
35 0x0044(3) SPM READY Store Program Memory Ready
Note: 
1. When the BOOTRST fuse is programmed, the device will jump to the Boot Loader address at reset,
see Boot Loader Support – Read-While-Write Self-Programming.
2. When the IVSEL bit in MCUCR is set, interrupt vectors will be moved to the start of the Boot Flash
section. The address of each interrupt vector will then be address in this table added to the start
address of the boot Flash section.
3. The Interrupts on address 0x0030 - 0x0044 do not exist in ATmega103 compatibility mode.
The next table shows Reset and interrupt vectors placement for the various combinations of BOOTRST
and IVSEL settings. If the program never enables an interrupt source, the interrupt vectors are not used,
and regular program code can be placed at these locations. This is also the case if the Reset Vector is in
the Application section while the interrupt vectors are in the Boot section or vice versa.
Table 16-2 Reset and Interrupt Vectors Placement
BOOTRST(1) IVSEL Reset Address Interrupt Vectors Start Address
1 0 0x0000 0x0002
1 1 0x0000 Boot Reset Address + 0x0002
0 0 Boot Reset Address 0x0002
0 1 Boot Reset Address Boot Reset Address + 0x0002
Note:  1. The Boot Reset Address is shown in table Boot Size Configuration in the Boot Loader
Parameters section. For the BOOTRST Fuse “1” means unprogrammed while “0” means programmed.
The most typical and general program setup for the Reset and Interrupt Vector Addresses in ATmega64A
is:
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address Labels Code Comments
$0000 jmp RESET ; Reset Handler
$0002 jmp EXT_INT0 ; IRQ0 Handler
$0004 jmp EXT_INT1 ; IRQ1 Handler
$0006 jmp EXT_INT2 ; IRQ2 Handler
$0008 jmp EXT_INT3 ; IRQ3 Handler
$000A jmp EXT_INT4 ; IRQ4 Handler
$000C jmp EXT_INT5 ; IRQ5 Handler
$000E jmp EXT_INT6 ; IRQ6 Handler
$0010 jmp EXT_INT7 ; IRQ7 Handler
$0012 jmp TIM2_COMP ; Timer2 Compare
Handler
$0014 jmp TIM2_OVF ; Timer2 Overflow
Handler
$0016 jmp TIM1_CAPT ; Timer1 Capture
Handler
$0018 jmp TIM1_COMPA ; Timer1 CompareA
Handler
$001A jmp TIM1_COMPB ; Timer1 CompareB
Handler
$001C jmp TIM1_OVF ; Timer1 Overflow
Handler
$001E jmp TIM0_COMP ; Timer0 Compare
Handler
$0020 jmp TIM0_OVF ; Timer0 Overflow
Handler
$0022 jmp SPI_STC ; SPI Transfer
Complete Handler
$0024 jmp USART0_RXC ; USART0 RX
Complete Handler
$0026 jmp USART0_DRE ; USART0,UDR Empty
Handler
$0028 jmp USART0_TXC ; USART0 TX
Complete Handler
$002A jmp ADC ; ADC Conversion
Complete Handler
$002C jmp EE_RDY ; EEPROM Ready
Handler
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address Labels Code Comments
$002E jmp ANA_COMP ; Analog
Comparator Handler
$0030 jmp TIM1_COMPC ; Timer1 CompareC
Handler
$0032 jmp TIM3_CAPT ; Timer3 Capture
Handler
$0034 jmp TIM3_COMPA ; Timer3 CompareA
Handler
$0036 jmp TIM3_COMPB ; Timer3 CompareB
Handler
$0038 jmp TIM3_COMPC ; Timer3 CompareC
Handler
$003A jmp TIM3_OVF ; Timer3 Overflow
Handler
$003C jmp USART1_RXC ; USART1 RX
Complete Handler
$003E jmp USART1_DRE ; USART1,UDR Empty
Handler
$0040 jmp USART1_TXC ; USART1 TX
Complete Handler
$0042 jmp TWI ; Two-wire Serial
Interface
Interrupt Handler
$0044 jmp SPM_RDY ; SPM Ready
Handler
;
$0046 RESET: ldi r16, high(RAMEND) ; Main program
start
$0047 out SPH,r16 ; Set stack
pointer to top of
RAM
$0048 ldi r16, low(RAMEND)
$0049 out SPL,r16
$004A sei ; Enable
interrupts
$004B <instr> xxx
:. :. :. :. :.
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When the BOOTRST fuse is unprogrammed, the Boot section size set to 8 Kbytes and the IVSEL bit in
the MCUCR Register is set before any interrupts are enabled, the most typical and general program
setup for the Reset and Interrupt Vector Addresses is:
Adddress Labels Code Comments
$0000 RESET: ldi r16,high(RAMEND) ; Main program
start
$0001 out SPH,r16 ; Set stack
pointer to top of
RAM
$0002 RESET: ldi r16,low(RAMEND)
$0003 out SPL,r16
$0004 sei ; Enable
interrupts
$0005 <instr> xxx
;
.org $F002
$F002 jmp EXT_INT0 ; IRQ0 Handler
$F004 jmp EXT_INT1 ; IRQ1 Handler
:. :. :. ;
$F044 jmp SPM_RDY ; Store Program
Memory Ready
Handler
When the BOOTRST fuse is programmed and the Boot section size set to 8K bytes, the most typical and
general program setup for the Reset and Interrupt Vector Addresses is:
Address Labels Code Comments
.org $0002
$0002 jmp EXT_INT0 ; IRQ0 Handler
$0004 jmp EXT_INT1 ; IRQ1 Handler
:. :. :. ;
$0044 jmp SPM_RDY ; Store Program
Memory Handler
;
.org $F000
$F000 RESET: ldi r16,high(RAMEND) ; Main program
start
$F001 out SPH,r16 ; Set stack
pointer to top of
RAM
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Address Labels Code Comments
$F002 ldi r16,low(RAMEND)
$F003 out SPL,r16
$F004 sei ; Enable
interrupts
$F005 <instr> xxx
When the BOOTRST fuse is programmed, the Boot section size set to 8K bytes and the IVSEL bit in the
MCUCR Register is set before any interrupts are enabled, the most typical and general program setup for
the Reset and Interrupt Vector Addresses is:
Address Labels Code Comments
;
.org $F000
$F000 jmp RESET ; Reset handler
$F002 jmp EXT_INT0 ; IRQ0 Handler
$F004 jmp EXT_INT1 ; IRQ1 Handler
:. :. :. ;
$F044 jmp SPM_RDY ; Store Program
Memory Ready
Handler
$F046 RESET: ldi r16,high(RAMEND) ; Main program
start
$F047 out SPH,r16 ; Set Stack
Pointer to top of
RAM
$F048 ldi r16,low(RAMEND)
$F049 out SPL,r16
$F04A sei ; Enable
interrupts
$F04B <instr> XXX
Related Links
BTLDR - Boot Loader Support – Read-While-Write Self-Programming on page 366
16.1.1. Moving Interrupts Between Application and Boot Space
The General Interrupt Control Register controls the placement of the Interrupt Vector table.
16.2. Register Description
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16.2.1. MCUCR – MCU Control Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  MCUCR
Offset:  0x35
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x55
Bit 7 6 5 4 3 2 1 0
IVSEL IVCE
Access R/W R/W
Reset 0 0
Bit 1 – IVSEL: Interrupt Vector Select
When the IVSEL bit is cleared (zero), the interrupt vectors are placed at the start of the Flash memory.
When this bit is set (one), the interrupt vectors are moved to the beginning of the Boot Loader section of
the flash. The actual address of the start of the Boot Flash section is determined by the BOOTSZ fuses.
Refer to the section Boot Loader Support – Read-While-Write Self-Programming for details. To avoid
unintentional changes of interrupt vector tables, a special write procedure must be followed to change the
IVSEL bit:
1. Write the Interrupt Vector Change Enable (IVCE) bit to one.
2. Within four cycles, write the desired value to IVSEL while writing a zero to IVCE.
Interrupts will automatically be disabled while this sequence is executed. Interrupts are disabled in the
cycle IVCE is set, and they remain disabled until after the instruction following the write to IVSEL. If
IVSEL is not written, interrupts remain disabled for four cycles. The I-bit in the Status Register is
unaffected by the automatic disabling.
Note:  If interrupt vectors are placed in the Boot Loader section and Boot Lock bit BLB02 is programmed,
interrupts are disabled while executing from the Application section. If interrupt vectors are placed in the
Application section and Boot Lock bit BLB12 is programed, interrupts are disabled while executing from
the Boot Loader section. Refer to the section Boot Loader Support – Read-While-Write Self-Programming
for details on Boot Lock bits.
Bit 0 – IVCE: Interrupt Vector Change Enable
The IVCE bit must be written to logic one to enable change of the IVSEL bit. IVCE is cleared by hardware
four cycles after it is written or when IVSEL is written. Setting the IVCE bit will disable interrupts, as
explained in the IVSEL description above. See Code Example below.
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Assembly Code Example
Move_interrupts:
; Enable change of Interrupt Vectors
ldi r16, (1<<IVCE)
out MCUCR, r16
; Move interrupts to boot Flash section
ldi r16, (1<<IVSEL)
out MCUCR, r16
ret
C Code Example
void Move_interrupts(void)
{
/* Enable change of Interrupt Vectors */
MCUCR = (1<<IVCE);
/* Move interrupts to boot Flash section */
MCUCR = (1<<IVSEL);
}
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17. External Interrupts
The External Interrupts are triggered by the INT7:0 pins. Observe that, if enabled, the interrupts will
trigger even if the INT7:0 pins are configured as outputs. This feature provides a way of generating a
software interrupt. The External Interrupts can be triggered by a falling or rising edge or a low level. This
is set up as indicated in the specification for the External Interrupt Control Registers – EICRA (INT3:0)
and EICRB (INT7:4). When the external interrupt is enabled and is configured as level triggered, the
interrupt will trigger as long as the pin is held low. Note that recognition of falling or rising edge interrupts
on INT7:4 requires the presence of an I/O clock, described in Clock Systems and their Distribution. Low
level interrupts and the edge interrupt on INT3:0 are detected asynchronously. This implies that these
interrupts can be used for waking the part also from sleep modes other than Idle mode. The I/O clock is
halted in all sleep modes except Idle mode.
Note that if a level triggered interrupt is used for wake-up from Power-down mode, the changed level
must be held for some time to wake up the MCU. This makes the MCU less sensitive to noise. The
changed level is sampled twice by the Watchdog Oscillator clock. The period of the Watchdog Oscillator
is 1μs (nominal) at 5.0V and 25°C. The frequency of the Watchdog Oscillator is voltage dependent as
shown in the Electrical Characteristics. The MCU will wake up if the input has the required level during
this sampling or if it is held until the end of the start-up time. The start-up time is defined by the SUT fuses
as described in Clock Systems and their Distribution. If the level is sampled twice by the Watchdog
Oscillator clock but disappears before the end of the start-up time, the MCU will still wake up, but no
interrupt will be generated. The required level must be held long enough for the MCU to complete the
wake up to trigger the level interrupt.
Related Links
Clock Systems and their Distribution on page 52
Electrical Characteristics – TA = -40°C to 85°C on page 415
17.1. Register Description
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17.1.1. EICRA – External Interrupt Control Register A
This Register can not be reached in ATmega103 compatibility mode, but the initial value defines INT3:0
as low level interrupts, as in ATmega103.
Name:  EICRA
Offset:  0x6A
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
ISC31 ISC30 ISC21 ISC20 ISC11 ISC10 ISC01 ISC00
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:6 – ISC3n: External Interrupt 3 Sense Control Bits [n = 1:0]
The External Interrupts 3 - 0 are activated by the external pins INT3:0 if the SREG I-flag and the
corresponding interrupt mask in the EIMSK is set. The level and edges on the external pins that activate
the interrupts are defined in table Interrupt Sense Control below. Edges on INT3:INT0 are registered
asynchronously. Pulses on INT3:0 pins wider than the minimum pulse width given in table Asynchronous
External Interrupt Characteristics below will generate an interrupt. Shorter pulses are not guaranteed to
generate an interrupt. If low level interrupt is selected, the low level must be held until the completion of
the currently executing instruction to generate an interrupt. If enabled, a level triggered interrupt will
generate an interrupt request as long as the pin is held low. When changing the ISCn bit, an interrupt can
occur. Therefore, it is recommended to first disable INTn by clearing its Interrupt Enable bit in the EIMSK
Register. Then, the ISCn bit can be changed. Finally, the INTn interrupt flag should be cleared by writing
a logical one to its Interrupt Flag bit (INTFn) in the EIFR Register before the interrupt is re-enabled.
Table 17-1 Interrupt Sense Control(1)
ISCn1 ISCn0 Description
0 0 The low level of INTn generates an interrupt request.
0 1 Reserved.
1 0 The falling edge of INTn generates asynchronously an interrupt request.
1 1 The rising edge of INTn generates asynchronously an interrupt request.
Note:  1. n = 3, 2, 1 or 0. When changing the ISCn1/ISCn0 bits, the interrupt must be disabled by
clearing its Interrupt Enable bit in the EIMSK Register. Otherwise an interrupt can occur when the bits are
changed.
Table 17-2 Asynchronous External Interrupt Characteristics
Symbol Parameter Condition Min Typ Max Units
tINT Minimum pulse width for
asynchronous external interrupt
50 ns
Bits 5:4 – ISC2n: External Interrupt 2 Sense Control Bits [n = 1:0]
Refer to ISC3n bit description above.
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Bits 3:2 – ISC1n: External Interrupt 1 Sense Control Bits [n = 1:0]
Refer to ISC3n bit description above.
Bits 1:0 – ISC0n: External Interrupt 0 Sense Control Bits [n = 1:0]
Refer to ISC3n bit description above.
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17.1.2. EICRB – External Interrupt Control Register B
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
This Register can not be reached in ATmega103 compatibility mode, but the initial value defines INT3:0
as low level interrupts, as in ATmega103.
Name:  EICRB
Offset:  0x3A
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x5A
Bit 7 6 5 4 3 2 1 0
ISC71 ISC70 ISC61 ISC60 ISC51 ISC50 ISC41 ISC40
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:6 – ISC7n: External Interrupt 7 Sense Control Bits [n = 1:0]
The External Interrupts 7 - 4 are activated by the external pins INT7:4 if the SREG I-flag and the
corresponding interrupt mask in the EIMSK is set. The level and edges on the external pins that activate
the interrupts are defined in table Interrupt Sense Control below. The value on the INT7:4 pins are
sampled before detecting edges. If edge or toggle interrupt is selected, pulses that last longer than one
clock period will generate an interrupt. Shorter pulses are not guaranteed to generate an interrupt.
Observe that CPU clock frequency can be lower than the XTAL frequency if the XTAL divider is enabled.
If low level interrupt is selected, the low level must be held until the completion of the currently executing
instruction to generate an interrupt. If enabled, a level triggered interrupt will generate an interrupt request
as long as the pin is held low.
Table 17-3 Interrupt Sense Control(1)
ISCn1 ISCn0 Description
0 0 The low level of INTn generates an interrupt request.
0 1 Reserved.
1 0 The falling edge of INTn generates an interrupt request.
1 1 The rising edge of INTn generates an interrupt request.
Note:  1. n = 7, 6, 5 or 4. When changing the ISCn1/ISCn0 bits, the interrupt must be disabled by
clearing its Interrupt Enable bit in the EIMSK Register. Otherwise an interrupt can occur when the bits are
changed.
Bits 5:4 – ISC6n: External Interrupt 6 Sense Control Bits [n = 1:0]
Refer to ISC7n bit description above.
Bits 3:2 – ISC5n: External Interrupt 5 Sense Control Bits [n = 1:0]
Refer to ISC7n bit description above.
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Bits 1:0 – ISC4n: External Interrupt 4 Sense Control Bits [n = 1:0]
Refer to ISC7n bit description above.
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17.1.3. EIMSK – External Interrupt Mask Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  EIMSK
Offset:  0x39
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x59
Bit 7 6 5 4 3 2 1 0
INT7 INT6 INT5 INT4 INT3 INT2 INT1 INT0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – INTn: External Interrupt Request n Enable [n = 7:0]
When an INT7 – INT0 bit is written to one and the I-bit in the Status Register (SREG) is set (one), the
corresponding external pin interrupt is enabled. The Interrupt Sense Control bits in the External Interrupt
Control Registers – EICRA and EICRB – defines whether the external interrupt is activated on rising or
falling edge or level sensed. Activity on any of these pins will trigger an interrupt request even if the pin is
enabled as an output. This provides a way of generating a software interrupt.
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17.1.4. EIFR – External Interrupt Flag Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  EIFR
Offset:  0x38
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x58
Bit 7 6 5 4 3 2 1 0
INTF7 INTF6 INTF5 INTF4 INTF3 INTF2 INTF1 INTF0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – INTFn: External Interrupt Flags n [n = 7:0]
When an edge or logic change on the INT7:0 pin triggers an interrupt request, INTF7:0 becomes set
(one). If the I-bit in SREG and the corresponding interrupt enable bit, INT7:0 in EIMSK, are set (one), the
MCU will jump to the interrupt vector. The flag is cleared when the interrupt routine is executed.
Alternatively, the flag can be cleared by writing a logical one to it. These flags are always cleared when
INT7:0 are configured as level interrupt. Note that when entering sleep mode with the INT3:0 interrupts
disabled, the input buffers on these pins will be disabled. This may cause a logic change in internal
signals which will set the INTF3:0 flags. Refer to Digital Input Enable and Sleep Modes on page 96 for
more information.
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|________ AAAAA
18. I/O Ports
18.1. Overview
All AVR ports have true Read-Modify-Write functionality when used as general digital I/O ports. This
means that the direction of one port pin can be changed without unintentionally changing the direction of
any other pin with the SBI and CBI instructions. The same applies when changing drive value (if
configured as output) or enabling/disabling of pull-up resistors (if configured as input). Each output buffer
has symmetrical drive characteristics with both high sink and source capability. The pin driver is strong
enough to drive LED displays directly. All port pins have individually selectable pull-up resistors with a
supply-voltage invariant resistance. All I/O pins have protection diodes to both VCC and Ground as
indicated in the following figure. Refer to Electrical Characteristics – TA = -40°C to 85°C for a complete
list of parameters.
Figure 18-1 I/O Pin Equivalent Schematic
Cpin
Logic
Rpu
See Figure
"General Digital I/O" for
Details
Pxn
All registers and bit references in this section are written in general form. A lower case “x” represents the
numbering letter for the port, and a lower case “n” represents the bit number. However, when using the
register or bit defines in a program, the precise form must be used (i.e., PORTB3 for bit 3 in Port B, here
documented generally as PORTxn). The physical I/O Registers and bit locations are listed in Register
Description on page 111.
Three I/O memory address locations are allocated for each port, one each for the Data Register –
PORTx, Data Direction Register – DDRx, and the Port Input Pins – PINx. The Port Input Pins I/O location
is read only, while the Data Register and the Data Direction Register are read/write. In addition, the Pull-
up Disable – PUD bit in SFIOR disables the pull-up function for all pins in all ports when set.
Using the I/O port as General Digital I/O is described in Ports as General Digital I/O on page 93. Most
port pins are multiplexed with alternate functions for the peripheral features on the device. How each
alternate function interferes with the port pin is described in Alternate Port Functions on page 96. Refer
to the individual module sections for a full description of the alternate functions.
Note that enabling the alternate function of some of the port pins does not affect the use of the other pins
in the port as general digital I/O.
Related Links
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i
Electrical Characteristics – TA = -40°C to 85°C on page 415
18.2. Ports as General Digital I/O
The ports are bi-directional I/O ports with optional internal pull-ups. The following figure shows a
functional description of one I/O-port pin, here generically called Pxn.
Figure 18-2 General Digital I/O(1)
clk
RPx
RRx
RDx
WDx
PUD
SYNCHRONIZER
WDx: WRITE DDRx
RRx: READ PORTx REGISTER
RPx: READ PORTx PIN
PUD: PULLUP DISABLE
clkI/O: I/O CLOCK
RDx: READ DDRx
D
L
Q
Q
RESET
RESET
Q
QD
Q
Q D
CLR
PORTxn
Q
Q D
CLR
DDxn
PINxn
DATA BUS
SLEEP
SLEEP: SLEEP CONTROL
Pxn
I/O
WPx
WPx: WRITE PINx REGISTER
Note:  1. WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clkI/O, SLEEP,
and PUD are common to all ports
18.2.1. Configuring the Pin
Each port pin consists of three register bits: DDxn, PORTxn, and PINxn. As shown in Register
Description on page 111, the DDxn bits are accessed at the DDRx I/O address, the PORTxn bits at the
PORTx I/O address, and the PINxn bits at the PINx I/O address.
The DDxn bit in the DDRx Register selects the direction of this pin. If DDxn is written logic one, Pxn is
configured as an output pin. If DDxn is written logic zero, Pxn is configured as an input pin.
If PORTxn is written logic one when the pin is configured as an input pin, the pull-up resistor is activated.
To switch the pull-up resistor off, PORTxn has to be written logic zero or the pin has to be configured as
an output pin. The port pins are tri-stated when reset condition becomes active, even if no clocks are
running.
If PORTxn is written logic one when the pin is configured as an output pin, the port pin is driven high
(one). If PORTxn is written logic zero when the pin is configured as an output pin, the port pin is driven
low (zero).
When switching between tri-state ({DDxn, PORTxn} = 0b00) and output high ({DDxn, PORTxn} = 0b11),
an intermediate state with either pull-up enabled ({DDxn, PORTxn} = 0b01) or output low ({DDxn,
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PORTxn} = 0b10) must occur. Normally, the pull-up enabled state is fully acceptable, as a high-impedant
environment will not notice the difference between a strong high driver and a pull-up. If this is not the
case, the PUD bit in the SFIOR Register can be set to disable all pull-ups in all ports.
Switching between input with pull-up and output low generates the same problem. The user must use
either the tristate ({DDxn, PORTxn} = 0b00) or the output high state ({DDxn, PORTxn} = 0b11) as an
intermediate step.
The table below summarizes the control signals for the pin value.
Table 18-1 Port Pin Configurations
DDxn PORTxn PUD (in
SFIOR)
I/O Pull-up Comment
0 0 x Input No Tri-state (Hi-Z)
0 1 0 Input Yes Pxn will source current if
external pulled low.
0 1 1 Input No Tri-state (Hi-Z)
1 0 x Output No Output Low (Sink)
1 1 x Output No Output High (Source)
18.2.2. Reading the Pin Value
Independent of the setting of Data Direction bit DDxn, the port pin can be read through the PINxn
Register Bit. As shown in Figure 18-2 General Digital I/O(1) on page 93, the PINxn Register bit and the
preceding latch constitute a synchronizer. This is needed to avoid metastability if the physical pin changes
value near the edge of the internal clock, but it also introduces a delay. The next figure shows a timing
diagram of the synchronization when reading an externally applied pin value. The maximum and
minimum propagation delays are denoted tpd,max and tpd,min respectively.
Figure 18-3 Synchronization when Reading an Externally Applied Pin value
XXX in r17, PINx
0x00 0xFF
INSTRUCTIONS
SYNC LATCH
PINxn
r17
XXX
SYSTEM CLK
tpd, max
tpd, min
Consider the clock period starting shortly after the first falling edge of the system clock. The latch is
closed when the clock is low, and goes transparent when the clock is high, as indicated by the shaded
region of the “SYNC LATCH” signal. The signal value is latched when the system clock goes low. It is
clocked into the PINxn Register at the succeeding positive clock edge. As indicated by the two arrows
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tpd,max and tpd,min, a single signal transition on the pin will be delayed between ½ and 1-½ system clock
period depending upon the time of assertion.
When reading back a software assigned pin value, a nop instruction must be inserted as indicated in the
figure below. The out instruction sets the “SYNC LATCH” signal at the positive edge of the clock. In this
case, the delay tpd through the synchronizer is 1 system clock period.
Figure 18-4 Synchronization when Reading a Software Assigned Pin Value
out PORTx, r16 nop in r17, PINx
0xFF
0x00 0xFF
SYSTEM CLK
r16
INSTRUCTIONS
SYNC LATCH
PINxn
r17
tpd
The following code example shows how to set port B pins 0 and 1 high, 2 and 3 low, and define the port
pins from 4 to 7 as input with pull-ups assigned to port pins 6 and 7. The resulting pin values are read
back again, but as previously discussed, a nop instruction is included to be able to read back the value
recently assigned to some of the pins.
Assembly Code Example(1)
:.
; Define pull-ups and set outputs high
; Define directions for port pins
ldi r16,(1<<PB7)|(1<<PB6)|(1<<PB1)|(1<<PB0)
ldi r17,(1<<DDB3)|(1<<DDB2)|(1<<DDB1)|(1<<DDB0)
out PORTB,r16
out DDRB,r17
; Insert nop for synchronization
nop
; Read port pins
in r16,PINB
:.
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C Code Example(1)
unsigned char i;
:.
/* Define pull-ups and set outputs high */
/* Define directions for port pins */
PORTB = (1<<PB7)|(1<<PB6)|(1<<PB1)|(1<<PB0);
DDRB = (1<<DDB3)|(1<<DDB2)|(1<<DDB1)|(1<<DDB0);
/* Insert nop for synchronization*/
_NOP();
/* Read port pins */
i = PINB;
:.
Note:  1. For the assembly program, two temporary registers are used to minimize the time from pull-ups
are set on pins 0, 1, 6, and 7, until the direction bits are correctly set, defining bit 2 and 3 as low and
redefining bits 0 and 1 as strong high drivers.
18.2.3. Digital Input Enable and Sleep Modes
As shown in figure Figure 18-2 General Digital I/O(1) on page 93, the digital input signal can be clamped
to ground at the input of the Schmitt Trigger. The signal denoted SLEEP in the figure, is set by the MCU
Sleep Controller in Power-down mode, Power-save mode, and Standby mode to avoid high power
consumption if some input signals are left floating, or have an analog signal level close to VCC/2.
SLEEP is overridden for port pins enabled as external interrupt pins. If the external interrupt request is not
enabled, SLEEP is active also for these pins. SLEEP is also overridden by various other alternate
functions as described in Alternate Port Functions on page 96.
If a logic high level (“one”) is present on an Asynchronous External Interrupt pin configured as “Interrupt
on Rising Edge, Falling Edge, or Any Logic Change on Pin” while the external interrupt is not enabled, the
corresponding External Interrupt Flag will be set when resuming from the above mentioned sleep modes,
as the clamping in these sleep modes produces the requested logic change.
18.2.4. Unconnected Pins
If some pins are unused, it is recommended to ensure that these pins have a defined level. Even though
most of the digital inputs are disabled in the deep sleep modes as described above, floating inputs should
be avoided to reduce current consumption in all other modes where the digital inputs are enabled (Reset,
Active mode and Idle mode).
The simplest method to ensure a defined level of an unused pin, is to enable the internal pull-up. In this
case, the pull-up will be disabled during reset. If low power consumption during reset is important, it is
recommended to use an external pull-up or pull-down. Connecting unused pins directly to VCC or GND is
not recommended, since this may cause excessive currents if the pin is accidentally configured as an
output.
18.3. Alternate Port Functions
Most port pins have alternate functions in addition to being general digital I/Os. The following figure
shows how the port pin control signals from the simplified Figure 18-2 General Digital I/O(1) on page 93
can be overridden by alternate functions. The overriding signals may not be present in all port pins, but
the figure serves as a generic description applicable to all port pins in the AVR microcontroller family.
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E
Figure 18-5 Alternate Port Functions(1)
clk
RPx
RRx
RDx
WDx
PUD
SYNCHRONIZER
WDx: WRITE DDRx
RRx: READ PORTx REGISTER
RPx: READ PORTx PIN
PUD: PULLUP DISABLE
clkI/O: I/O CLOCK
RDx: READ DDRx
D
L
Q
Q
SET
CLR
0
1
0
1
0
1
DIxn
AIOxn
DIEOExn
PVOVxn
PVOExn
DDOVxn
DDOExn
PUOExn
PUOVxn
PUOExn: Pxn PULL-UP OVERRIDE ENABLE
PUOVxn: Pxn PULL-UP OVERRIDE VALUE
DDOExn: Pxn DATA DIRECTION OVERRIDE ENABLE
DDOVxn: Pxn DATA DIRECTION OVERRIDE VALUE
PVOExn: Pxn PORT VALUE OVERRIDE ENABLE
PVOVxn: Pxn PORT VALUE OVERRIDE VALUE
DIxn: DIGITAL INPUT PIN n ON PORTx
AIOxn: ANALOG INPUT/OUTPUT PIN n ON PORTx
RESET
RESET
Q
Q D
CLR
Q
Q D
CLR
Q
QD
CLR
PINxn
PORTxn
DDxn
DATA BUS
0
1
DIEOVxn
SLEEP
DIEOExn: Pxn DIGITAL INPUT-ENABLE OVERRIDE ENABLE
DIEOVxn: Pxn DIGITAL INPUT-ENABLE OVERRIDE VALUE
SLEEP: SLEEP CONTROL
Pxn
I/O
WPx: WRITE PINx
WPx
Note:  1. WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clkI/O, SLEEP,
and PUD are common to all ports. All other signals are unique for each pin.
The following table summarizes the function of the overriding signals. The pin and port indexes from the
figure above are not shown in the succeeding tables. The overriding signals are generated internally in
the modules having the alternate function.
Table 18-2 Generic Description of Overriding Signals for Alternate Functions
Signal Name Full Name Description
PUOE Pull-up Override
Enable
If this signal is set, the pull-up enable is controlled by the PUOV signal.
If this signal is cleared, the pull-up is enabled when {DDxn, PORTxn,
PUD} = 0b010.
PUOV Pull-up Override Value If PUOE is set, the pull-up is enabled/disabled when PUOV is set/
cleared, regardless of the setting of the DDxn, PORTxn, and PUD
Register bits.
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Signal Name Full Name Description
DDOE Data Direction
Override Enable
If this signal is set, the Output Driver Enable is controlled by the DDOV
signal. If this signal is cleared, the Output driver is enabled by the DDxn
Register bit.
DDOV Data Direction
Override Value
If DDOE is set, the Output Driver is enabled/disabled when DDOV is
set/cleared, regardless of the setting of the DDxn Register bit.
PVOE Port Value Override
Enable
If this signal is set and the Output Driver is enabled, the port value is
controlled by the PVOV signal. If PVOE is cleared, and the Output
Driver is enabled, the port Value is controlled by the PORTxn Register
bit.
PVOV Port Value Override
Value
If PVOE is set, the port value is set to PVOV, regardless of the setting of
the PORTxn Register bit.
DIEOE Digital Input Enable
Override Enable
If this bit is set, the Digital Input Enable is controlled by the DIEOV
signal. If this signal is cleared, the Digital Input Enable is determined by
MCU state (Normal mode, sleep mode).
DIEOV Digital Input Enable
Override Value
If DIEOE is set, the Digital Input is enabled/disabled when DIEOV is set/
cleared, regardless of the MCU state (Normal mode, sleep mode).
DI Digital Input This is the Digital Input to alternate functions. In the figure, the signal is
connected to the output of the Schmitt Trigger but before the
synchronizer. Unless the Digital Input is used as a clock source, the
module with the alternate function will use its own synchronizer.
AIO Analog Input/Output This is the Analog Input/output to/from alternate functions. The signal is
connected directly to the pad, and can be used bi-directionally.
The following subsections shortly describe the alternate functions for each port, and relate the overriding
signals to the alternate function. Refer to the alternate function description for further details.
18.3.1. Alternate Functions of Port A
The Port A has an alternate function as the address low byte and data lines for the External Memory
Interface.
Table 18-3 Port A Pins Alternate Functions
Port Pin Alternate Functions
PA7 AD7 (External memory interface address and data bit 7)
PA6 AD6 (External memory interface address and data bit 6)
PA5 AD5 (External memory interface address and data bit 5)
PA4 AD4 (External memory interface address and data bit 4)
PA3 AD3 (External memory interface address and data bit 3)
PA2 AD2 (External memory interface address and data bit 2)
PA1 AD1 (External memory interface address and data bit 1)
PA0 AD0 (External memory interface address and data bit 0)
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The two tables below relates the alternate functions of Port A to the overriding signals shown in the figure
in section Alternate Port Functions on page 96.
Table 18-4 Overriding Signals for Alternate Functions in PA7:PA4
Signal
Name
PA7/AD7 PA6/AD6 PA5/AD5 PA4/AD4
PUOE SRE SRE SRE SRE
PUOV ~(WR | ADA(1)) •
PORTA7 • PUD
~(WR | ADA(1)) •
PORTA6 • PUD
~(WR | ADA(1)) •
PORTA5 • PUD
~(WR | ADA(1)) •
PORTA4 • PUD
DDOE SRE SRE SRE SRE
DDOV WR | ADA WR | ADA WR | ADA WR | ADA
PVOE SRE SRE SRE SRE
PVOV A7 • ADA | D7
OUTPUT • WR
A6 • ADA | D6
OUTPUT • WR
A5 • ADA | D5
OUTPUT • WR
A4 • ADA | D4
OUTPUT • WR
DIEOE 0 0 0 0
DIEOV 0 0 0 0
DI D7 INPUT D6 INPUT D5 INPUT D4 INPUT
AIO – – – –
Note:  1. ADA is short for ADdress Active and represents the time when address is output. See External
Memory Interface for details.
Table 18-5 Overriding Signals for Alternate Functions in PA3:PA0
Signal
Name
PA3/AD3 PA2/AD2 PA1/AD1 PA0/AD0
PUOE SRE SRE SRE SRE
PUOV ~(WR | ADA(1)) •
PORTA3 • PUD
~(WR | ADA(1)) •
PORTA2 • PUD
~(WR | ADA(1)) •
PORTA1 • PUD
~(WR | ADA(1)) •
PORTA0 • PUD
DDOE SRE SRE SRE SRE
DDOV WR | ADA WR | ADA WR | ADA WR | ADA
PVOE SRE SRE SRE SRE
PVOV A3 • ADA | D3
OUTPUT • WR
A2 • ADA | D2
OUTPUT • WR
A1 • ADA | D1
OUTPUT • WR
A0 • ADA | D0
OUTPUT • WR
DIEOE 0 0 0 0
DIEOV 0 0 0 0
DI D3 INPUT D2 INPUT D1 INPUT D0 INPUT
AIO – – – –
Related Links
External Memory Interface on page 34
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18.3.2. Alternate Functions of Port B
The Port B pins with alternate functions are shown in the table below:
Table 18-6 Port B Pins Alternate Functions
Port Pin Alternate Functions
PB7 OC2/OC1C(1) (Output Compare and PWM Output for Timer/Counter2 or Output Compare and
PWM Output C for Timer/Counter1)
PB6 OC1B (Output Compare and PWM Output B for Timer/Counter1)
PB5 OC1A (Output Compare and PWM Output A for Timer/Counter1)
PB4 OC0 (Output Compare and PWM Output for Timer/Counter0)
PB3 MISO (SPI Bus Master Input/Slave Output)
PB2 MOSI (SPI Bus Master Output/Slave Input)
PB1 SCK (SPI Bus Serial Clock)
PB0 SS (SPI Slave Select input)
Note:  1. OC1C not applicable in ATmega103 compatibility mode.
The alternate pin configuration is as follows:
• OC2/OC1C – Port B, Bit 7
OC2, Output Compare Match output: The PB7 pin can serve as an external output for the Timer/Counter2
Output Compare. The pin has to be configured as an output (DDB7 set “one”) to serve this function. The
OC2 pin is also the output pin for the PWM mode timer function.
OC1C, Output Compare Match C output: The PB7 pin can serve as an external output for the Timer/
Counter1 Output Compare C. The pin has to be configured as an output (DDB7 set (one)) to serve this
function. The OC1C pin is also the output pin for the PWM mode timer function.
• OC1B – Port B, Bit 6
OC1B, Output Compare Match B output: The PB6 pin can serve as an external output for the Timer/
Counter1 Output Compare B. The pin has to be configured as an output (DDB6 set (one)) to serve this
function. The OC1B pin is also the output pin for the PWM mode timer function.
• OC1A – Port B, Bit 5
OC1A, Output Compare Match A output: The PB5 pin can serve as an external output for the Timer/
Counter1 Output Compare A. The pin has to be configured as an output (DDB5 set (one)) to serve this
function. The OC1A pin is also the output pin for the PWM mode timer function.
• OC0 – Port B, Bit 4
OC0, Output Compare Match output: The PB4 pin can serve as an external output for the Timer/Counter0
Output Compare. The pin has to be configured as an output (DDB4 set (one)) to serve this function. The
OC0 pin is also the output pin for the PWM mode timer function.
• MISO – Port B, Bit 3
MISO: Master Data input, Slave Data output pin for SPI channel. When the SPI is enabled as a master,
this pin is configured as an input regardless of the setting of DDB3. When the SPI is enabled as a slave,
the data direction of this pin is controlled by DDB3. When the pin is forced to be an input, the pull-up can
still be controlled by the PORTB3 bit.
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• MOSI – Port B, Bit 2
MOSI: SPI Master Data output, Slave Data input for SPI channel. When the SPI is enabled as a slave,
this pin is configured as an input regardless of the setting of DDB2. When the SPI is enabled as a master,
the data direction of this pin is controlled by DDB2. When the pin is forced to be an input, the pull-up can
still be controlled by the PORTB2 bit.
• SCK – Port B, Bit 1
SCK: Master Clock output, Slave Clock input pin for SPI channel. When the SPI is enabled as a slave,
this pin is configured as an input regardless of the setting of DDB1. When the SPI is enabled as a master,
the data direction of this pin is controlled by DDB1. When the pin is forced to be an input, the pull-up can
still be controlled by the PORTB1 bit.
• SS – Port B, Bit 0
SS: Slave Port Select input. When the SPI is enabled as a slave, this pin is configured as an input
regardless of the setting of DDB0. As a slave, the SPI is activated when this pin is driven low. When the
SPI is enabled as a master, the data direction of this pin is controlled by DDB0. When the pin is forced to
be an input, the pull-up can still be controlled by the PORTB0 bit.
The tables below relate the alternate functions of Port B to the overriding signals shown in the figure in
section Alternate Port Functions on page 96. SPI MSTR INPUT and SPI SLAVE OUTPUT constitute the
MISO signal, while MOSI is divided into SPI MSTR OUTPUT and SPI SLAVE INPUT.
Table 18-7 Overriding Signals for Alternate Functions in PB7:PB4
Signal
Name
PB7/OC2/OC1C PB6/OC1B PB5/OC1A PB4/OC0
PUOE 0 0 0 0
PUOV 0 0 0 0
DDOE 0 0 0 0
DDOV 0 0 0 0
PVOE OC2/OC1C ENABLE(1) OC1B ENABLE OC1A ENABLE OC0 ENABLE
PVOV OC2/OC1C(1) OC1B OC1A OC0B
DIEOE 0 0 0 0
DIEOV 0 0 0 0
DI – – – –
AIO – – – –
Note:  1. See Output Compare Modulator (OCM1C2) for details. OC1C does not exist in ATmega103
compatibility mode.
Table 18-8 Overriding Signals for Alternate Functions in PB3:PB0
Signal
Name
PB3/MISO PB2/MOSI PB1/SCK PB0/SS
PUOE SPE • MSTR SPE • MSTR SPE • MSTR SPE • MSTR
PUOV PORTB3 • PUD PORTB2 • PUD PORTB1 • PUD PORTB0 • PUD
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Signal
Name
PB3/MISO PB2/MOSI PB1/SCK PB0/SS
DDOE SPE • MSTR SPE • MSTR SPE • MSTR SPE • MSTR
DDOV 0 0 0 0
PVOE SPE • MSTR SPE • MSTR SPE • MSTR 0
PVOV SPI SLAVE OUTPUT SPI MSTR OUTPUT SCK OUTPUT 0
DIEOE 0 0 0 0
DIEOV 0 0 0 0
DI SPI MSTR INPUT SPI SLAVE INPUT SCK INPUT SPI SS
AIO – – – –
Related Links
Output Compare Modulator (OCM1C2) on page 233
18.3.3. Alternate Functions of Port C
In ATmega103 compatibility mode, Port C is output only. The Port C has an alternate function as the
address high byte for the External Memory Interface.
Table 18-9 Port C Pins Alternate Functions
Port Pin Alternate Function
PC7 A15
PC6 A14
PC5 A13
PC4 A12
PC3 A11
PC2 A10
PC1 A9
PC0 A8
The two following tables relate the alternate functions of Port C to the overriding signals shown in the
figure in section Alternate Port Functions on page 96.
The alternate pin configuration is as follows:
Table 18-10 Overriding Signals for Alternate Functions in PC7:PC4
Signal
Name
PC7/A15 PC6/A14 PC5/A13 PC4/A12
PUOE SRE • (XMM(1)<1) SRE • (XMM<2) SRE • (XMM<3) SRE • (XMM<4)
PUOV 0 0 0 0
DDOE SRE • (XMM<1) SRE • (XMM<2) SRE • (XMM<3) SRE • (XMM<4)
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Signal
Name
PC7/A15 PC6/A14 PC5/A13 PC4/A12
DDOV 1 1 1 1
PVOE SRE • (XMM<1) SRE • (XMM<2) SRE • (XMM<3) SRE • (XMM<4)
PVOV A15 A14 A13 A12
DIEOE 0 0 0 0
DIEOV 0 0 0 0
DI – – – –
AIO – – – –
Note:  1. XMM = 0 in ATmega103 compatibility mode.
Table 18-11 Overriding Signals for Alternate Functions in PC3:PC0(1)
Signal
Name
PC3/A11 PC2/A10 PC1/A9 PC0/A8
PUOE SRE • (XMM<5) SRE • (XMM<6) SRE • (XMM<7) SRE • (XMM<7)
PUOV 0 0 0 0
DDOE SRE • (XMM<5) SRE • (XMM<6) SRE • (XMM<7) SRE • (XMM<7)
DDOV 1 1 1 1
PVOE SRE • (XMM<5) SRE • (XMM<6) SRE • (XMM<7) SRE • (XMM<7)
PVOV A11 A10 A9 A8
DIEOE 0 0 0 0
DIEOV 0 0 0 0
DI – – – –
AIO – – – –
Note:  1. XMM = 0 in ATmega103 compatibility mode.
18.3.4. Alternate Functions of Port D
The Port D pins with alternate functions are shown in the table below:
Table 18-12 Port D Pins Alternate Functions
Port Pin Alternate Function
PD7 T2 (Timer/Counter2 Clock Input)
PD6 T1 (Timer/Counter1 Clock Input)
PD5 XCK1(1) (USART1 External Clock Input/Output)
PD4 ICP1 (Timer/Counter1 Input Capture Pin)
PD3 INT3/TXD1(1) (External Interrupt3 Input or UART1 Transmit Pin)
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Port Pin Alternate Function
PD2 INT2/RXD1(1) (External Interrupt2 Input or UART1 Receive Pin)
PD1 INT1/SDA(1) (External Interrupt1 Input or TWI Serial Data)
PD0 INT0/SCL(1) (External Interrupt0 Input or TWI Serial Clock)
Note:  1. XCK1, TXD1, RXD1, SDA, and SCL not applicable in ATmega103 compatibility mode.
The alternate pin configuration is as follows:
• T2 – Port D, Bit 7
T2, Timer/Counter2 counter source.
• T1 – Port D, Bit 6
T1, Timer/Counter1 counter source.
• XCK1 – Port D, Bit 5
XCK1, USART1 External clock. The Data Direction Register (DDD5) controls whether the clock is output
(DDD5 set) or input (DDD5 cleared). The XCK1 pin is active only when the USART1 operates in
Synchronous mode.
• ICP1 – Port D, Bit 4
ICP1 – Input Capture Pin1: The PD4 pin can act as an Input Capture Pin for Timer/Counter1.
• INT3/TXD1 – Port D, Bit 3
INT3, External Interrupt source 3: The PD3 pin can serve as an external interrupt source to the MCU.
TXD1, Transmit Data (Data output pin for the USART1). When the USART1 Transmitter is enabled, this
pin is configured as an output regardless of the value of DDD3.
• INT2/RXD1 – Port D, Bit 2
INT2, External Interrupt source 2. The PD2 pin can serve as an External Interrupt source to the MCU.
RXD1, Receive Data (Data input pin for the USART1). When the USART1 receiver is enabled this pin is
configured as an input regardless of the value of DDD2. When the USART forces this pin to be an input,
the pull-up can still be controlled by the PORTD2 bit.
•INT1/SDA – Port D, Bit 1
INT1, External Interrupt source 1. The PD1 pin can serve as an external interrupt source to the MCU.
SDA, Two-wire Serial Interface Data: When the TWEN bit in TWCR is set (one) to enable the Two-wire
Serial Interface, pin PD1 is disconnected from the port and becomes the Serial Data I/O pin for the Two-
wire Serial Interface. In this mode, there is a spike filter on the pin to suppress spikes shorter than 50 ns
on the input signal, and the pin is driven by an open drain driver with slew-rate limitation.
• INT0/SCL – Port D, Bit 0
INT0, External Interrupt source 0. The PD0 pin can serve as an external interrupt source to the MCU.
SCL, Two-wire Serial Interface Clock: When the TWEN bit in TWCR is set (one) to enable the Two-wire
Serial Interface, pin PD0 is disconnected from the port and becomes the Serial Clock I/O pin for the Two-
wire Serial Interface. In this mode, there is a spike filter on the pin to suppress spikes shorter than 50 ns
on the input signal, and the pin is driven by an open drain driver with slew-rate limitation.
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The tables below relate the alternate functions of Port D to the overriding signals shown in the figure in
section Alternate Port Functions on page 96.
Table 18-13 Overriding Signals for Alternate Functions PD7:PD4
Signal
Name
PD7/T2 PD6/T1 PD5/XCK1 PD4/ICP1
PUOE 0 0 0 0
PUOV 0 0 0 0
DDOE 0 0 0 0
DDOV 0 0 0 0
PVOE 0 0 UMSEL1 0
PVOV 0 0 XCK1 OUTPUT 0
DIEOE 0 0 0 0
DIEOV 0 0 0 0
DI T2 INPUT T1 INPUT XCK1 INPUT ICP1 INPUT
AIO – – – –
Table 18-14 Overriding Signals for Alternate Functions in PD3:PD0(1)
Signal
Name
PD3/INT3/TXD1 PD2/INT2/RXD1 PD1/INT1/SDA PD0/INT0/SCL
PUOE TXEN1 RXEN1 TWEN TWEN
PUOV 0 PORTD2 • PUD PORTD1 • PUD PORTD0 • PUD
DDOE TXEN1 RXEN1 TWEN TWEN
DDOV 1 0 SDA_OUT SCL_OUT
PVOE TXEN1 0 TWEN TWEN
PVOV TXD1 0 0 0
DIEOE INT3 ENABLE INT2 ENABLE INT1 ENABLE INT0 ENABLE
DIEOV 1 1 1 1
DI
INT3 INPUT
INT2 INPUT/RXD1 INT1 INPUT INT0 INPUT
AIO – – SDA INPUT SCL INPUT
Note:  1. When enabled, the Two-wire Serial Interface enables Slew-Rate controls on the output pins
PD0 and PD1. This is not shown in this table. In addition, spike filters are connected between the AIO
outputs shown in the port figure and the digital logic of the TWI module.
18.3.5. Alternate Functions of Port E
The Port E pins with alternate functions are shown in the table below:
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Table 18-15 Port E Pins Alternate Functions
Port Pin Alternate Function
PE7 INT7/ICP3(1) (External Interrupt 7 Input or Timer/Counter3 Input Capture Pin)
PE6 INT6/ T3(1) (External Interrupt 6 Input or Timer/Counter3 Clock Input)
PE5 INT5/OC3C(1) (External Interrupt 5 Input or Output Compare and PWM Output C for Timer/
Counter3)
PE4 INT4/OC3B(1) (External Interrupt4 Input or Output Compare and PWM Output B for Timer/
Counter3)
PE3 AIN1/OC3A (1) (Analog Comparator Negative Input or Output Compare and PWM Output A for
Timer/Counter3)
PE2 AIN0/XCK0(1) (Analog Comparator Positive Input or USART0 external clock input/output)
PE1 PDO/TXD0 (Programming Data Output or UART0 Transmit Pin)
PE0 PDI/RXD0 (Programming Data Input or UART0 Receive Pin)
Note:  1. ICP3, T3, OC3C, OC3B, OC3B, OC3A, and XCK0 not applicable in ATmega103 compatibility
mode.
• INT7/ICP3 – Port E, Bit 7
INT7, External Interrupt source 7: The PE7 pin can serve as an external interrupt source.
ICP3 – Input Capture Pin3: The PE7 pin can act as an Input Capture Pin for Timer/Counter3.
• INT6/T3 – Port E, Bit 6
INT6, External Interrupt source 6: The PE6 pin can serve as an external interrupt source.
T3, Timer/Counter3 counter source.
• INT5/OC3C – Port E, Bit 5
INT5, External Interrupt source 5: The PE5 pin can serve as an External Interrupt source.
OC3C, Output Compare Match C output: The PE5 pin can serve as an External output for the Timer/
Counter3 Output Compare C. The pin has to be configured as an output (DDE5 set “one”) to serve this
function. The OC3C pin is also the output pin for the PWM mode timer function.
• INT4/OC3B – Port E, Bit 4
INT4, External Interrupt source 4: The PE4 pin can serve as an External Interrupt source.
OC3B, Output Compare Match B output: The PE4 pin can serve as an External output for the Timer/
Counter3 Output Compare B. The pin has to be configured as an output (DDE4 set (one)) to serve this
function. The OC3B pin is also the output pin for the PWM mode timer function.
• AIN1/OC3A – Port E, Bit 3
AIN1 – Analog Comparator Negative input. This pin is directly connected to the negative input of the
Analog Comparator.
OC3A, Output Compare Match A output: The PE3 pin can serve as an External output for the Timer/
Counter3 Output Compare A. The pin has to be configured as an output (DDE3 set “one”) to serve this
function. The OC3A pin is also the output pin for the PWM mode timer function.
• AIN0/XCK0 – Port E, Bit 2
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AIN0 – Analog Comparator Positive input. This pin is directly connected to the positive input of the Analog
Comparator.
XCK0, USART0 External clock. The Data Direction Register (DDE2) controls whether the clock is output
(DDE2 set) or input (DDE2 cleared). The XCK0 pin is active only when the USART0 operates in
Synchronous mode.
• PDO/TXD0 – Port E, Bit 1
PDO, SPI Serial Programming Data Output. During Serial Program Downloading, this pin is used as data
output line for the ATmega64A.
TXD0, UART0 Transmit pin.
• PDI/RXD0 – Port E, Bit 0
PDI, SPI Serial Programming Data Input. During Serial Program Downloading, this pin is used as data
input line for the ATmega64A.
RXD0, USART0 Receive Pin. Receive Data (Data input pin for the USART0). When the USART0 receiver
is enabled this pin is configured as an input regardless of the value of DDRE0. When the USART0 forces
this pin to be an input, a logical one in PORTE0 will turn on the internal pull-up.
The tables below relates the alternate functions of Port E to the overriding signals shown in the figure in
section Alternate Port Functions on page 96.
Table 18-16 Overriding Signals for Alternate Functions PE7:PE4
Signal
Name
PE7/INT7/ICP3 PE6/INT6/T3 PE5/INT5/OC3C PE4/INT4/OC3B
PUOE 0 0 0 0
PUOV 0 0 0 0
DDOE 0 0 0 0
DDOV 0 0 0 0
PVOE 0 0 OC3C ENABLE OC3B ENABLE
PVOV 0 0 OC3C OC3B
DIEOE INT7 ENABLE INT6 ENABLE INT5 ENABLE INT4 ENABLE
DIEOV 1 1 1 1
DI INT7 INPUT/ICP3 INPUT INT7 INPUT/T3 INPUT INT5 INPUT INT4 INPUT
AIO – – – –
Table 18-17 Overriding Signals for Alternate Functions in PE3:PE0
Signal
Name
PE3/AIN1/OC3A PE2/AIN0/XCK0 PE1/PDO/TXD0 PE0/PDI/RXD0
PUOE 0 0 TXEN0 RXEN0
PUOV 0 0 0 PORTE0 • PUD
DDOE 0 0 TXEN0 RXEN0
DDOV 0 0 1 0
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Signal
Name
PE3/AIN1/OC3A PE2/AIN0/XCK0 PE1/PDO/TXD0 PE0/PDI/RXD0
PVOE OC3B ENABLE UMSEL0 TXEN0 0
PVOV OC3B XCK0 OUTPUT TXD0 0
DIEOE 0 0 0 0
DIEOV 0 0 0 0
DI 0 XCK0 INPUT – RXD0
AIO AIN1 INPUT AIN0 INPUT – –
18.3.6. Alternate Functions of Port F
The Port F pins with alternate functions are shown in the table below. If some Port F pins are configured
as outputs, it is essential that these do not switch when a conversion is in progress. This might corrupt the
result of the conversion. In ATmega103 compatibility mode Port F is input only. If the JTAG interface is
enabled, the pull-up resistors on pins PF7(TDI), PF5(TMS), and PF4(TCK) will be activated even if a
Reset occurs.
Table 18-18 Port F Pins Alternate Functions
Port Pin Alternate Function
PF7 ADC7/TDI (ADC input channel 7 or JTAG Test Data Input)
PF6 ADC6/TDO (ADC input channel 6 or JTAG Test Data Output)
PF5 ADC5/TMS (ADC input channel 5 or JTAG Test Mode Select)
PF4 ADC4/TCK (ADC input channel 4 or JTAG Test Clock)
PF3 ADC3 (ADC input channel 3)
PF2 ADC2 (ADC input channel 2)
PF1 ADC1 (ADC input channel 1)
PF0 ADC0 (ADC input channel 0)
• TDI, ADC7 – Port F, Bit 7
ADC7, Analog to Digital Converter, Channel 7.
TDI, JTAG Test Data In: Serial input data to be shifted in to the Instruction Register or Data Register
(scan chains). When the JTAG interface is enabled, this pin can not be used as an I/O pin.
• TDO, ADC6 – Port F, Bit 6
ADC6, Analog to Digital Converter, Channel 6.
TDO, JTAG Test Data Out: Serial output data from Instruction Register or Data Register. When the JTAG
interface is enabled, this pin can not be used as an I/O pin.
The TDO pin is tri-stated unless TAP states that shift out data are entered.
• TMS, ADC5 – Port F, Bit 5
ADC5, Analog to Digital Converter, Channel 5.
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TMS, JTAG Test Mode Select: This pin is used for navigating through the TAP-controller state machine.
When the JTAG interface is enabled, this pin can not be used as an I/O pin.
• TCK, ADC4 – Port F, Bit 4
ADC4, Analog to Digital Converter, Channel 4.
TCK, JTAG Test Clock: JTAG operation is synchronous to TCK. When the JTAG interface is enabled, this
pin can not be used as an I/O pin.
• ADC3 – ADC0 – Port F, Bit 3:0
Analog to Digital Converter, Channel 3:0.
Table 18-19 Overriding Signals for Alternate Functions PF7:PF4
Signal
Name
PF7/ADC7/TDI PF6/ADC6/TDO PF5/ADC5/TMS PF4/ADC4/TCK
PUOE JTAGEN JTAGEN JTAGEN JTAGEN
PUOV 1 0 1 1
DDOE JTAGEN JTAGEN JTAGEN JTAGEN
DDOV 0 SHIFT_IR + SHIFT_DR 0 0
PVOE 0 JTAGEN 0 0
PVOV 0 TDO 0 0
DIEOE JTAGEN JTAGEN JTAGEN JTAGEN
DIEOV 0 0 0 0
DI – – – –
AIO TDI/ADC7 INPUT ADC6 INPUT TMS/ADC5 INPUT TCKADC4 INPUT
Table 18-20 Overriding Signals for Alternate Functions in PF3:PF0
Signal
Name
PF3/ADC3 PF2/ADC2 PF1/ADC1 PF0/ADC0
PUOE 0 0 0 0
PUOV 0 0 0 0
DDOE 0 0 0 0
DDOV 0 0 0 0
PVOE 0 0 0 0
PVOV 0 0 0 0
DIEOE 0 0 0 0
DIEOV 0 0 0 0
DI – – – –
AIO ADC3 INPUT ADC2 INPUT ADC1 INPUT ADC0 INPUT
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18.3.7. Alternate Functions of Port G
In ATmega103 compatibility mode, only the alternate functions are the defaults for Port G, and Port G
cannot be used as General Digital Port Pins. The alternate pin configuration is as follows:
Table 18-21 Port G Pins Alternate Functions
Port Pin Alternate Function
PG4 TOSC1 (RTC Oscillator Timer/Counter0)
PG3 TOSC2 (RTC Oscillator Timer/Counter0)
PG2 ALE (Address Latch Enable to external memory)
PG1 RD (Read strobe to external memory)
PG0 WR (Write strobe to external memory)
• TOSC1 – Port G, Bit 4
TOSC1, Timer Oscillator pin 1: When the AS0 bit in ASSR is set (one) to enable asynchronous clocking
of Timer/Counter0, pin PG4 is disconnected from the port, and becomes the input of the inverting
Oscillator amplifier. In this mode, a Crystal Oscillator is connected to this pin, and the pin can not be used
as an I/O pin.
• TOSC2 – Port G, Bit 3
TOSC2, Timer Oscillator pin 2: When the AS0 bit in ASSR is set (one) to enable asynchronous clocking
of Timer/Counter0, pin PG3 is disconnected from the port, and becomes the inverting output of the
Oscillator amplifier. In this mode, a Crystal Oscillator is connected to this pin, and the pin can not be used
as an I/O pin.
• ALE – Port G, Bit 2
ALE is the external data memory Address Latch Enable signal.
• RD – Port G, Bit 1
RD is the external data memory read control strobe.
• WR – Port G, Bit 0
WR is the external data memory write control strobe.
The tables below relate the alternate functions of Port G to the overriding signals shown in the figure in
section Alternate Port Functions on page 96.
Table 18-22 Overriding Signals for Alternate Functions in PG4:PG1
Signal
Name
PG4/TOSC1 PG3/TOSC2 PG2/ALE PG1/RD
PUOE AS0 AS0 SRE SRE
PUOV 0 0 0 0
DDOE AS0 AS0 SRE SRE
DDOV 0 0 1 1
PVOE 0 0 SRE SRE
PVOV 0 0 ALE RD
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Signal
Name
PG4/TOSC1 PG3/TOSC2 PG2/ALE PG1/RD
DIEOE AS0 AS0 0 0
DIEOV 0 0 0 0
DI – – – –
AIO T/C0 OSC INPUT T/C0 OSC OUTPUT – –
Table 18-23 Overriding Signals for Alternate Functions in PG0
Signal
Name
PG0/WR
PUOE SRE
PUOV 0
DDOE SRE
DDOV 1
PVOE SRE
PVOV WR
DIEOE 0
DIEOV 0
DI –
AIO –
18.4. Register Description
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18.4.1. SFIOR – Special Function IO Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  SFIOR
Offset:  0x20
Reset:  0
Property:
 
When addressing I/O Registers as data space the offset address is 0x40
Bit 7 6 5 4 3 2 1 0
PUD
Access R/W
Reset 0
Bit 2 – PUD: Pull-up Disable
When this bit is written to one, the pull-ups in the I/O ports are disabled even if the DDxn and PORTxn
Registers are configured to enable the pull-ups ({DDxn, PORTxn} = 0b01). See Configuring the Pin on
page 93 for more details about this feature.
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18.4.2. PORTA – Port A Data Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  PORTA
Offset:  0x1B
Reset:  0
Property:
 
When addressing I/O Registers as data space the offset address is 0x3B
Bit 7 6 5 4 3 2 1 0
PORTA7 PORTA6 PORTA5 PORTA4 PORTA3 PORTA2 PORTA1 PORTA0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – PORTAn: Port A Data Register [n = 7:0]
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18.4.3. DDRA – Port A Data Direction Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  DDRA
Offset:  0x1A
Reset:  0
Property:
 
When addressing I/O Registers as data space the offset address is 0x3A
Bit 7 6 5 4 3 2 1 0
DDA7 DDA6 DDA5 DDA4 DDA3 DDA2 DDA1 DDA0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – DDAn: Port A Data Direction Register [n = 7:0]
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18.4.4. PINA – Port A Input Pins Address
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  PINA
Offset:  0x19
Reset:  0
Property:
 
When addressing I/O Registers as data space the offset address is 0x39
Bit 7 6 5 4 3 2 1 0
PINA7 PINA6 PINA5 PINA4 PINA3 PINA2 PINA1 PINA0
Access R R R R R R R R
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – PINAn: Port A Input Pins Address [n = 7:0]
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18.4.5. PORTB – The Port B Data Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  PORTB
Offset:  0x18
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x38
Bit 7 6 5 4 3 2 1 0
PORTB7 PORTB6 PORTB5 PORTB4 PORTB3 PORTB2 PORTB1 PORTB0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – PORTBn: Port B Data [n = 7:0]
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18.4.6. DDRB – The Port B Data Direction Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  DDRB
Offset:  0x17
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x37
Bit 7 6 5 4 3 2 1 0
DDB7 DDB6 DDB5 DDB4 DDB3 DDB2 DDB1 DDB0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – DDBn: Port B Data Direction [n = 7:0]
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18.4.7. PINB – The Port B Input Pins Address
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  PINB
Offset:  0x16
Reset:  N/A
Property:
 
When addressing I/O Registers as data space the offset address is 0x36
Bit 7 6 5 4 3 2 1 0
PINB7 PINB6 PINB5 PINB4 PINB3 PINB2 PINB1 PINB0
Access R R R R R R R R
Reset x x x x x x x x
Bits 7:0 – PINBn: Port B Input Pins Address [n = 7:0]
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18.4.8. PORTC – The Port C Data Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  PORTC
Offset:  0x15
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x35
Bit 7 6 5 4 3 2 1 0
PORTC7 PORTC6 PORTC5 PORTC4 PORTC3 PORTC2 PORTC1 PORTC0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – PORTCn: Port C Data [n = 7:0]
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18.4.9. DDRC – The Port C Data Direction Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  DDRC
Offset:  0x14
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x34
Bit 7 6 5 4 3 2 1 0
DDC7 DDC6 DDC5 DDC4 DDC3 DDC2 DDC1 DDC0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – DDCn: Port C Data Direction [n = 7:0]
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18.4.10. PINC – The Port C Input Pins Address
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
In ATmega103 compatibility mode, DDRC and PINC Registers are initialized to being Push-Pull Zero
Output. The port pins assumes their initial value, even if the clock is not running. Note that the DDRC and
PINC Registers are available in ATmega103 compatibility mode, and should not be used for 100% back-
ward compatibility.
Name:  PINC
Offset:  0x13
Reset:  N/A
Property:
 
When addressing I/O Registers as data space the offset address is 0x33
Bit 7 6 5 4 3 2 1 0
PINC7 PINC6 PINC5 PINC4 PINC3 PINC2 PINC1 PINC0
Access R R R R R R R R
Reset 0 x x x x x x x
Bits 7:0 – PINCn: Port C Input Pins Address [n = 7:0]
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18.4.11. PORTD – The Port D Data Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  PORTD
Offset:  0x12
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x32
Bit 7 6 5 4 3 2 1 0
PORTD7 PORTD6 PORTD5 PORTD4 PORTD3 PORTD2 PORTD1 PORTD0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – PORTDn: Port D Data [n = 7:0]
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18.4.12. DDRD – The Port D Data Direction Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  DDRD
Offset:  0x11
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x31
Bit 7 6 5 4 3 2 1 0
DDD7 DDD6 DDD5 DDD4 DDD3 DDD2 DDD1 DDD0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – DDDn: Port D Data Direction [n = 7:0]
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18.4.13. PIND – The Port D Input Pins Address
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  PIND
Offset:  0x10
Reset:  N/A
Property:
 
When addressing I/O Registers as data space the offset address is 0x30
Bit 7 6 5 4 3 2 1 0
PIND7 PIND6 PIND5 PIND4 PIND3 PIND2 PIND1 PIND0
Access R R R R R R R R
Reset x x x x x x x x
Bits 7:0 – PINDn: Port D Input Pins Address [n = 7:0]
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18.4.14. PORTE – The Port E Data Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  PORTE
Offset:  0x03
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x23
Bit 7 6 5 4 3 2 1 0
PORTE7 PORTE6 PORTE5 PORTE4 PORTE3 PORTE2 PORTE1 PORTE0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – PORTEn: Port E Data [n = 7:0]
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18.4.15. DDRE – The Port E Data Direction Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  DDRE
Offset:  0x02
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x22
Bit 7 6 5 4 3 2 1 0
DDRE7 DDRE6 DDRE5 DDRE4 DDRE3 DDRE2 DDRE1 DDRE0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – DDREn: Port E Data Direction [n = 7:0]
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18.4.16. PINE – The Port E Input Pins Address
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  PINE
Offset:  0x01
Reset:  N/A
Property:
 
When addressing I/O Registers as data space the offset address is 0x21
Bit 7 6 5 4 3 2 1 0
PINE7 PINE6 PINE5 PINE4 PINE3 PINE2 PINE1 PINE0
Access R R R R R R R R
Reset x x x x x x x x
Bits 7:0 – PINEn: Port E Input Pins Address [n = 7:0]
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18.4.17. PORTF – The Port F Data Register
Name:  PORTF
Offset:  0x62
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
PORTF7 PORTF6 PORTF5 PORTF4 PORTF3 PORTF2 PORTF1 PORTF0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – PORTFn: Port F Data [n = 7:0]
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18.4.18. DDRF – The Port F Data Direction Register
Name:  DDRF
Offset:  0x61
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
DDRF7 DDRF6 DDRF5 DDRF4 DDRF3 DDRF2 DDRF1 DDRF0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – DDRFn: Port F Data Direction [n = 7:0]
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18.4.19. PINF – The Port F Input Pins Address
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Note:  PORTF and DDRF Registers are not available in ATmega103 compatibility mode where Port F
serves as digital input only.
Name:  PINF
Offset:  0x00
Reset:  N/A
Property:
 
When addressing I/O Registers as data space the offset address is 0x20
Bit 7 6 5 4 3 2 1 0
PINF7 PINF6 PINF5 PINF4 PINF3 PINF2 PINF1 PINF0
Access R R R R R R R R
Reset x x x x x x x x
Bits 7:0 – PINFn: Port F Input Pins Address [n = 7:0]
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18.4.20. PORTG – The Port G Data Register
Name:  PORTG
Offset:  0x65
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
PORTG4 PORTG3 PORTG2 PORTG1 PORTG0
Access R/W R/W R/W R/W R/W
Reset 0 0 0 0 0
Bits 4:0 – PORTGn: Port G Data [n = 4:0]
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18.4.21. DDRG – The Port G Data Direction Register
Name:  DDRG
Offset:  0x64
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
DDRG4 DDRG3 DDRG2 DDRG1 DDRG0
Access R/W R/W R/W R/W R/W
Reset 0 0 0 0 0
Bits 4:0 – DDRGn: Port G Data Direction [n = 4:0]
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18.4.22. PING – The Port G Input Pins Address
Note:  PORTG and DDRG Registers are not available in ATmega103 compatibility mode where Port G
serves as digital input only.
Name:  PING
Offset:  0x63
Reset:  N/A
Property:
 
–
Bit 7 6 5 4 3 2 1 0
PING4 PING3 PING2 PING1 PING0
Access R R R R R
Reset x x x x x
Bits 4:0 – PINGn: Port G Input Pins Address [n = 4:0]
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19. Timer/Counter3, Timer/Counter2, and Timer/Counter1 Prescalers
19.1. Overview
Timer/Counter3, Timer/Counter2, and Timer/Counter1 share the same prescaler module, but the Timer/
Counters can have different prescaler settings. The description below applies to Timer/Counter3, Timer/
Counter2, and Timer/Counter1.
19.2. Internal Clock Source
The Timer/Counter can be clocked directly by the system clock (by setting the CSn2:0 = 1). This provides
the fastest operation, with a maximum Timer/Counter clock frequency equal to system clock frequency
(fCLK_I/O). Alternatively, one of four taps from the prescaler can be used as a clock source. The prescaled
clock has a frequency of either fCLK_I/O/8, fCLK_I/O/64, fCLK_I/O/256, or fCLK_I/O/1024.
19.3. Prescaler Reset
The prescaler is free running (i.e., operates independently of the clock select logic of the Timer/Counter)
and it is shared by Timer/Counter3, Timer/Counter2, and Timer/Counter1. Since the prescaler is not
affected by the Timer/Counter’s clock select, the state of the prescaler will have implications for situations
where a prescaled clock is used. One example of prescaling artifacts occurs when the timer is enabled
and clocked by the prescaler (6 > CSn2:0 > 1). The number of system clock cycles from when the timer is
enabled to the first count occurs can be from 1 to N+1 system clock cycles, where N equals the prescaler
divisor (8, 64, 256, or 1024).
It is possible to use the prescaler reset for synchronizing the Timer/Counter to program execution.
However, care must be taken if the other Timer/Counter that shares the same prescaler also uses
prescaling. A prescaler reset will affect the prescaler period for all Timer/Counters it is connected to.
19.4. External Clock Source
An external clock source applied to the T3/T2/T1 pin can be used as Timer/Counter clock (clkT3/clkT2/
clkT1). The T3/T2/T1 pin is sampled once every system clock cycle by the pin synchronization logic. The
synchronized (sampled) signal is then passed through the edge detector. The figure below shows a
functional equivalent block diagram of the T3/T2/T1 synchronization and edge detector logic. The
registers are clocked at the positive edge of the internal system clock (clkI/O). The latch is transparent in
the high period of the internal system clock.
The edge detector generates one clkT3/clkT2/clkT1 pulse for each positive (CSn2:0 = 7) or negative
(CSn2:0 = 6) edge it detects.
Figure 19-1 T3/T2/T1 Pin Sampling
Tn_s ync
(To Clock
Select Logic)
Edge De te ctorSynchronization
D QD Q
LE
D Q
Tn
clkI/O
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SOURCE (2|an Atmel
The synchronization and edge detector logic introduces a delay of 2.5 to 3.5 system clock cycles from an
edge has been applied to the T3/T2/T1 pin to the counter is updated.
Enabling and disabling of the clock input must be done when T3/T2/T1 has been stable for at least one
system clock cycle, otherwise it is a risk that a false Timer/Counter clock pulse is generated.
Each half period of the external clock applied must be longer than one system clock cycle to ensure
correct sampling. The external clock must be guaranteed to have less than half the system clock
frequency (fExtClk < fclk_I/O/2) given a 50/50% duty cycle. Since the edge detector uses sampling, the
maximum frequency of an external clock it can detect is half the sampling frequency (Nyquist sampling
theorem). However, due to variation of the system clock frequency and duty cycle caused by Oscillator
source (crystal, resonator, and capacitors) tolerances, it is recommended that maximum frequency of an
external clock source is less than fclk_I/O/2.5.
An external clock source can not be prescaled.
Figure 19-2 Prescaler for Timer/Counter3, Timer/Counter2, and Timer/Counter1(1)
CSn0
CSn1
CSn2
Synchronization
10-BIT T/C PRESCALER
Tn
clkI/O
PSR10
Clear
CK/8
CK/256
CK/64
CK/1024
OFF
TIMER/COUNTERn CLOCK
SOURCE clk Tn
Note:  1. The synchronization logic on the input pins (T3/T2/T1) is shown in figure T3/T2/T1 Pin
Sampling in this section.
19.5. Register Description
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19.5.1. SFIOR – Special Function IO Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  SFIOR
Offset:  0x20
Reset:  0
Property:
 
When addressing I/O Registers as data space the offset address is 0x40
Bit 7 6 5 4 3 2 1 0
TSM PSR321
Access R/W R/W
Reset 0 0
Bit 7 – TSM: Timer/Counter Synchronization Mode
Writing the TSM bit to one activates the Timer/Counter Synchronization mode. In this mode, the value
that is written to the PSR0 and PSR321 bits is kept, hence keeping the corresponding prescaler reset
signals asserted. This ensures that the corresponding Timer/Counters are halted and can be configured
to the same value without the risk of one of them advancing during configuration. When the TSM bit is
written to zero, the PSR0 and PSR321 bits are cleared by hardware, and the Timer/Counters start
counting simultaneously.
Bit 0 – PSR321: Prescaler Reset Timer/Counter3, Timer/Counter2, and Timer/Counter1
When this bit is one, the Timer/Counter3, Timer/Counter1, and Timer/Counter2 prescaler will be reset.
This bit is normally cleared immediately by hardware, except if the TSM bit is set. Note that Timer/
Counter3, Timer/Counter1, and Timer/Counter2 share the same prescaler and a reset of this prescaler
will affect all three timers.
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20. 16-bit Timer/Counter (Timer/Counter1 and Timer/Counter3)
20.1. Features
•True 16-bit Design (i.e., allows 16-bit PWM)
•Three independent Output Compare Units
•Double Buffered Output Compare Registers
•One Input Capture Unit
•Input Capture Noise Canceler
•Clear Timer on Compare Match (Auto Reload)
•Glitch-free, Phase Correct Pulse Width Modulator (PWM)
•Variable PWM Period
•Frequency Generator
•External Event Counter
•Ten Independent Interrupt Sources (TOV1, OCF1A, OCF1B, OCF1C, ICF1, TOV3, OCF3A,
OCF3B, OCF3C, and ICF3)
20.1.1. Restrictions in ATmega103 Compatibility Mode
Note that in ATmega103 compatibility mode, only one 16-bit Timer/Counter is available (Timer/Counter1).
Also note that in ATmega103 compatibility mode, the Timer/Counter1 has two Compare Registers
(Compare A and Compare B) only.
20.2. Overview
The 16-bit Timer/Counter unit allows accurate program execution timing (event management), wave
generation, and signal timing measurement. Most register and bit references in this document are written
in general form. A lower case “n” replaces the Timer/Counter number, and a lower case “x” replaces the
Output Compare unit channel. However, when using the register or bit defines in a program, the precise
form must be used i.e., TCNT1 for accessing Timer/Counter1 counter value and so on.
A simplified block diagram of the 16-bit Timer/Counter is shown below. For the actual placement of I/O
pins, refer to Pin Configurations. CPU accessible I/O Registers, including I/O bits and I/O pins, are shown
in bold. The device-specific I/O Register and bit locations are listed in the Register Description on page
157.
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Figure 20-1 16-bit Timer/Counter Block Diagram(1)
Clock Select
Timer/Counter
DATA BUS
OCRnA
OCRnB
ICRn
=
TCNTn
Waveform
Generation
Waveform
Generation
OCnA
OCnB
Noise
Canceler
ICPn
=
Fixed
TOP
Values
Edge
Detector
Control Logic
= 0
TOP BOTTOM
Count
Clear
Direction
TOVn
(Int.Req.)
OCFnA
(Int.Req.)
OCFnB
(Int.Req.)
ICFn (Int.Req.)
TCCRnA TCCRnB TCCRnC
( From Analog
Comparator Ouput )
Tn
Edge
Detector
( From Prescaler )
clkTn
=
=Waveform
Generation OCnC
OCFnC
(Int.Req.)
OCRnC
Note:  1. Refer to Pin Configurations, table Port B Pins Alternate Functions in Alternate Functions of Port
B, and Port E Pins Alternate Functions in Alternate Functions of Port E for Timer/Counter1 and 3 pin
placement and description.
Related Links
Pin Configurations on page 14
Alternate Functions of Port B on page 100
Alternate Functions of Port E on page 105
20.2.1. Registers
The Timer/Counter (TCNTn), Output Compare Registers (OCRnA/B/C), and Input Capture Register
(ICRn) are all 16-bit registers. Special procedures must be followed when accessing the 16-bit registers.
These procedures are described in the section Accessing 16-bit Registers on page 140. The Timer/
Counter Control Registers (TCCRnA/B/C) are 8-bit registers and have no CPU access restrictions.
Interrupt requests (abbreviated to Int.Req. in the figure) signals are all visible in the Timer Interrupt Flag
Register (TIFR) and Extended Timer Interrupt Flag Register (ETIFR). All interrupts are individually
masked with the Timer Interrupt Mask Register (TIMSK) and Extended Timer Interrupt Mask Register
(ETIMSK). (E)TIFR and (E)TIMSK are not shown in the figure since these registers are shared by other
timer units.
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The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source on the Tn
pin. The Clock Select logic block controls which clock source and edge the Timer/Counter uses to
increment (or decrement) its value. The Timer/Counter is inactive when no clock source is selected. The
output from the clock select logic is referred to as the timer clock (clkTn).
The double buffered Output Compare Registers (OCRnA/B/C) are compared with the Timer/Counter
value at all time. The result of the compare can be used by the waveform generator to generate a PWM
or variable frequency output on the Output Compare Pin (OCnA/B/C). See Output Compare Units on
page 146. The Compare Match event will also set the Compare Match Flag (OCFnA/B/C) which can be
used to generate an Output Compare interrupt request.
The Input Capture Register can capture the Timer/Counter value at a given external (edge triggered)
event on either the Input Capture Pin (ICPn) or on the Analog Comparator pins (see Analog Comparator).
The Input Capture unit includes a digital filtering unit (Noise Canceler) for reducing the chance of
capturing noise spikes.
The TOP value, or maximum Timer/Counter value, can in some modes of operation be defined by either
the OCRnA Register, the ICRn Register, or by a set of fixed values. When using OCRnA as TOP value in
a PWM mode, the OCRnA Register can not be used for generating a PWM output. However, the TOP
value will in this case be double buffered allowing the TOP value to be changed in run time. If a fixed TOP
value is required, the ICRn Register can be used as an alternative, freeing the OCRnA to be used as
PWM output.
Related Links
Analog Comparator on page 306
20.2.2. Definitions
The following definitions are used extensively throughout the document:
Table 20-1 Definitions
BOTTOM The counter reaches the BOTTOM when it becomes 0x0000.
MAX The counter reaches its MAXimum when it becomes 0xFFFF (decimal 65535).
TOP The counter reaches the TOP when it becomes equal to the highest value in the count
sequence. The TOP value can be assigned to be one of the fixed values: 0x00FF, 0x01FF, or
0x03FF, or to the value stored in the OCRnA or ICRn Register. The assignment is dependent
of the mode of operation.
20.2.3. Compatibility
The 16-bit Timer/Counter has been updated and improved from previous versions of the 16-bit AVR
Timer/Counter. This 16-bit Timer/Counter is fully compatible with the earlier version regarding:
•All 16-bit Timer/Counter related I/O Register address locations, including Timer Interrupt Registers.
•Bit locations inside all 16-bit Timer/Counter Registers, including Timer Interrupt Registers.
•Interrupt Vectors.
The following control bits have changed name, but have same functionality and register location:
•PWMn0 is changed to WGMn0.
•PWMn1 is changed to WGMn1.
•CTCn is changed to WGMn2.
The following registers are added to the 16-bit Timer/Counter:
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•Timer/Counter Control Register C (TCCRnC).
•Output Compare Register C, OCRnCH and OCRnCL, combined OCRnC.
The following bits are added to the 16-bit Timer/Counter Control Registers:
•COM1C1:0 are added to TCCR1A.
•FOCnA, FOCnB, and FOCnC are added in the new TCCRnC Register.
•WGMn3 is added to TCCRnB.
Interrupt flag and mask bits for output compare unit C are added.
The 16-bit Timer/Counter has improvements that will affect the compatibility in some special cases.
20.3. Accessing 16-bit Registers
The TCNTn, OCRnA/B/C, and ICRn are 16-bit registers that can be accessed by the AVR CPU via the 8-
bit data bus. A 16-bit register must be byte accessed using two read or write operations. The 16-bit timer
has a single 8-bit register for temporary storing of the High byte of the 16-bit access. The same temporary
register is shared between all 16-bit registers within the 16-bit timer. Accessing the Low byte triggers the
16-bit read or write operation. When the Low byte of a 16-bit register is written by the CPU, the High byte
stored in the temporary register, and the Low byte written are both copied into the 16-bit register in the
same clock cycle. When the Low byte of a 16-bit register is read by the CPU, the High byte of the 16-bit
register is copied into the temporary register in the same clock cycle as the Low byte is read.
Not all 16-bit accesses uses the temporary register for the High byte. Reading the OCRnA/B/C 16-bit
registers does not involve using the temporary register.
To do a 16-bit write, the High byte must be written before the Low byte. For a 16-bit read, the Low byte
must be read before the High byte.
The following code examples show how to access the 16-bit Timer Registers assuming that no interrupts
updates the temporary register. The same principle can be used directly for accessing the OCRnA/B/C
and ICRn Registers. Note that when using “C”, the compiler handles the 16-bit access.
Assembly Code Example(1)
:.
; Set TCNTn to 0x01FF
ldi r17,0x01
ldi r16,0xFF
out TCNTnH,r17
out TCNTnL,r16
; Read TCNTn into r17:r16
in r16,TCNTnL
in r17,TCNTnH
:.
C Code Example(1)
unsigned int i;
:.
/* Set TCNTn to 0x01FF */
TCNTn = 0x1FF;
/* Read TCNTn into i */
i = TCNTn;
:.
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Note:  1. See About Code Examples.
The assembly code example returns the TCNTn value in the r17:r16 Register pair.
It is important to notice that accessing 16-bit registers are atomic operations. If an interrupt occurs
between the two instructions accessing the 16-bit register, and the interrupt code updates the temporary
register by accessing the same or any other of the 16-bit Timer Registers, then the result of the access
outside the interrupt will be corrupted. Therefore, when both the main code and the interrupt code update
the temporary register, the main code must disable the interrupts during the 16-bit access.
The following code examples show how to do an atomic read of the TCNTn Register contents. Reading
any of the OCRnA/B/C or ICRn Registers can be done by using the same principle.
Asesmbly Code Example(1)
TIM16_ReadTCNTn:
; Save global interrupt flag
in r18,SREG
; Disable interrupts
cli
; Read TCNTn into r17:r16
in r16,TCNTnL
in r17,TCNTnH
; Restore global interrupt flag
out SREG,r18
ret
C Code Example(1)
unsigned int TIM16_ReadTCNTn( void )
{
unsigned char sreg;
unsigned int i;
/* Save global interrupt flag */
sreg = SREG;
/* Disable interrupts */
_CLI();
/* Read TCNTn into i */
i = TCNTn;
/* Restore global interrupt flag */
SREG = sreg;
return i;
}
Note:  1. See About Code Examples.
The assembly code example returns the TCNTn value in the r17:r16 Register pair.
The following code examples show how to do an atomic write of the TCNTn Register contents. Writing
any of the OCRnA/B/C or ICRn Registers can be done by using the same principle.
Assembly Code Example(1)
TIM16_WriteTCNTn:
; Save global interrupt flag
in r18,SREG
; Disable interrupts
cli
; Set TCNTn to r17:r16
out TCNTnH,r17
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out TCNTnL,r16
; Restore global interrupt flag
out SREG,r18
ret
C Code Example(1)
void TIM16_WriteTCNTn( unsigned int i )
{
unsigned char sreg;
unsigned int i;
/* Save global interrupt flag */
sreg = SREG;
/* Disable interrupts */
_CLI();
/* Set TCNTn to i */
TCNTn = i;
/* Restore global interrupt flag */
SREG = sreg;
}
Note:  1. See About Code Examples.
The assembly code example requires that the r17:r16 Register pair contains the value to be written to
TCNTn.
Related Links
About Code Examples on page 20
20.3.1. Reusing the Temporary High Byte Register
If writing to more than one 16-bit register where the High byte is the same for all registers written, then the
High byte only needs to be written once. However, note that the same rule of atomic operation described
previously also applies in this case.
20.4. Timer/Counter Clock Sources
The Timer/Counter can be clocked by an internal or an external clock source. The clock source is
selected by the clock select logic which is controlled by the clock select (CSn2:0) bits located in the
Timer/Counter Control Register B (TCCRnB). For details on clock sources and prescaler, see Timer/
Counter3, Timer/Counter2, and Timer/Counter1 Prescalers.
Related Links
Timer/Counter3, Timer/Counter2, and Timer/Counter1 Prescalers on page 134
20.5. Counter Unit
The main part of the 16-bit Timer/Counter is the programmable 16-bit bi-directional counter unit. The
figure below shows a block diagram of the counter and its surroundings.
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Figure 20-2 Counter Unit Block Diagram
TEMP (8-bit)
DATA BUS (8-bit)
TCNTn (16-bit Counter)
TCNTnH (8-bit) TCNTnL (8-bit) Control Logic
Count
Clear
Direction
TOVn
(Int.Req.)
Clock Select
TOP BOTTOM
Tn
Edge
Detector
( From Prescaler )
clkTn
Signal description (internal signals):
count Increment or decrement TCNTn by 1.
direction Select between increment and decrement.
clear Clear TCNTn (set all bits to zero).
clkTn Timer/Counter clock.
TOP Signalize that TCNTn has reached maximum value.
BOTTOM Signalize that TCNTn has reached minimum value (zero).
The 16-bit counter is mapped into two 8-bit I/O memory locations: counter high (TCNTnH) containing the
upper eight bits of the counter, and Counter Low (TCNTnL) containing the lower eight bits. The TCNTnH
Register can only be indirectly accessed by the CPU. When the CPU does an access to the TCNTnH I/O
location, the CPU accesses the High byte temporary register (TEMP). The temporary register is updated
with the TCNTnH value when the TCNTnL is read, and TCNTnH is updated with the temporary register
value when TCNTnL is written. This allows the CPU to read or write the entire 16-bit counter value within
one clock cycle via the 8-bit data bus. It is important to notice that there are special cases of writing to the
TCNTn Register when the counter is counting that will give unpredictable results. The special cases are
described in the sections where they are of importance.
Depending on the mode of operation used, the counter is cleared, incremented, or decremented at each
timer clock (clkTn). The clkTn can be generated from an external or internal clock source, selected by the
clock select bits (CSn2:0). When no clock source is selected (CSn2:0 = 0) the timer is stopped. However,
the TCNTn value can be accessed by the CPU, independent of whether clkTn is present or not. A CPU
write overrides (has priority over) all counter clear or count operations.
The counting sequence is determined by the setting of the Waveform Generation mode bits (WGMn3:0)
located in the Timer/Counter Control Registers A and B (TCCRnA and TCCRnB). There are close
connections between how the counter behaves (counts) and how waveforms are generated on the Output
Compare Outputs OCnx. For more details about advanced counting sequences and waveform
generation, refer to Modes of Operation on page 148.
The Timer/Counter Overflow (TOVn) flag is set according to the mode of operation selected by the
WGMn3:0 bits. TOVn can be used for generating a CPU interrupt.
20.6. Input Capture Unit
The Timer/Counter incorporates an Input Capture unit that can capture external events and give them a
timestamp indicating time of occurrence. The external signal indicating an event, or multiple events, can
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be applied via the ICPn pin or alternatively, for the Timer/Counter1 only, via the Analog Comparator unit.
The time-stamps can then be used to calculate frequency, duty-cycle, and other features of the signal
applied. Alternatively the time-stamps can be used for creating a log of the events.
The Input Capture unit is illustrated by the block diagram below. The elements of the block diagram that
are not directly a part of the Input Capture unit are gray shaded. The small “n” in register and bit names
indicates the Timer/Counter number.
Figure 20-3 Input Capture Unit Block Diagram
ICFn (Int.Req.)
Analog
Comparator
WRITE ICRn (16-bit Register)
ICRnH (8-bit)
Noise
Canceler
ICPn
Edge
Detector
TEMP (8-bit)
DATA BUS (8-bit)
ICRnL (8-bit)
TCNTn (16-bit Counter)
TCNTnH (8-bit) TCNTnL (8-bit)
ACIC* ICNC ICES
ACO*
Note:  The Analog Comparator Output (ACO) can only trigger the Timer/Counter1 ICP – not Timer/
Counter3.
When a change of the logic level (an event) occurs on the Input Capture Pin (ICPn), alternatively on the
Analog Comparator Output (ACO), and this change confirms to the setting of the edge detector, a capture
will be triggered. When a capture is triggered, the 16-bit value of the counter (TCNTn) is written to the
Input Capture Register (ICRn). The Input Capture Flag (ICFn) is set at the same system clock as the
TCNTn value is copied into ICRn Register. If enabled (TICIEn = 1), the Input Capture Flag generates an
Input Capture interrupt. The ICFn Flag is automatically cleared when the interrupt is executed.
Alternatively the ICFn Flag can be cleared by software by writing a logical one to its I/O bit location.
Reading the 16-bit value in the Input Capture Register (ICRn) is done by first reading the Low byte
(ICRnL) and then the High byte (ICRnH). When the Low byte is read the High byte is copied into the High
byte temporary register (TEMP). When the CPU reads the ICRnH I/O location it will access the TEMP
Register.
The ICRn Register can only be written when using a Waveform Generation mode that utilizes the ICRn
Register for defining the counter’s TOP value. In these cases the Waveform Generation mode
(WGMn3:0) bits must be set before the TOP value can be written to the ICRn Register. When writing the
ICRn Register the High byte must be written to the ICRnH I/O location before the Low byte is written to
ICRnL.
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For more information on how to access the 16-bit registers refer to Accessing 16-bit Registers on page
140.
20.6.1. Input Capture Pin Source
The main trigger source for the Input Capture unit is the Input Capture Pin (ICPn). Timer/Counter 1 can
alternatively use the Analog Comparator Output as trigger source for the Input Capture unit. The Analog
Comparator is selected as trigger source by setting the Analog Comparator Input Capture (ACIC) bit in
the Analog Comparator Control and Status Register (ACSR). Be aware that changing trigger source can
trigger a capture. The Input Capture Flag must therefore be cleared after the change.
Both the Input Capture Pin (ICPn) and the Analog Comparator Output (ACO) inputs are sampled using
the same technique as for the Tn pin (see figure Tn Pin Sampling in section External Clock Source). The
edge detector is also identical. However, when the noise canceler is enabled, additional logic is inserted
before the edge detector, which increases the delay by four system clock cycles. Note that the input of the
noise canceler and edge detector is always enabled unless the Timer/Counter is set in a Waveform
Generation mode that uses ICRn to define TOP.
An Input Capture can be triggered by software by controlling the port of the ICPn pin.
Related Links
External Clock Source on page 134
20.6.2. Noise Canceler
The noise canceler improves noise immunity by using a simple digital filtering scheme. The noise
canceler input is monitored over four samples, and all four must be equal for changing the output that in
turn is used by the edge detector.
The noise canceler is enabled by setting the Input Capture Noise Canceler (ICNCn) bit in Timer/Counter
Control Register B (TCCRnB). When enabled the noise canceler introduces additional four system clock
cycles of delay from a change applied to the input, to the update of the ICRn Register. The noise canceler
uses the system clock and is therefore not affected by the prescaler.
20.6.3. Using the Input Capture Unit
The main challenge when using the Input Capture unit is to assign enough processor capacity for
handling the incoming events. The time between two events is critical. If the processor has not read the
captured value in the ICRn Register before the next event occurs, the ICRn will be overwritten with a new
value. In this case the result of the capture will be incorrect.
When using the Input Capture interrupt, the ICRn Register should be read as early in the interrupt handler
routine as possible. Even though the Input Capture interrupt has relatively high priority, the maximum
interrupt response time is dependent on the maximum number of clock cycles it takes to handle any of
the other interrupt requests.
Using the Input Capture unit in any mode of operation when the TOP value (resolution) is actively
changed during operation, is not recommended.
Measurement of an external signal’s duty cycle requires that the trigger edge is changed after each
capture. Changing the edge sensing must be done as early as possible after the ICRn Register has been
read. After a change of the edge, the Input Capture Flag (ICFn) must be cleared by software (writing a
logical one to the I/O bit location). For measuring frequency only, the clearing of the ICFn Flag is not
required (if an interrupt handler is used).
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20.7. Output Compare Units
The 16-bit comparator continuously compares TCNTn with the Output Compare Register (OCRnx). If
TCNT equals OCRnx the comparator signals a match. A match will set the Output Compare Flag
(OCFnx) at the next timer clock cycle. If enabled (OCIEnx = 1), the Output Compare Flag generates an
Output Compare interrupt. The OCFnx Flag is automatically cleared when the interrupt is executed.
Alternatively the OCFnx Flag can be cleared by software by writing a logical one to its I/O bit location. The
waveform generator uses the match signal to generate an output according to operating mode set by the
Waveform Generation mode (WGMn3:0) bits and Compare Output mode (COMnx1:0) bits. The TOP and
BOTTOM signals are used by the waveform generator for handling the special cases of the extreme
values in some modes of operation (Refer to Modes of Operation on page 148.)
A special feature of Output Compare unit A allows it to define the Timer/Counter TOP value (i.e. counter
resolution). In addition to the counter resolution, the TOP value defines the period time for waveforms
generated by the waveform generator.
The figure below shows a block diagram of the Output Compare unit. The small “n” in the register and bit
names indicates the device number (n = n for Timer/Counter n), and the “x” indicates Output Compare
unit (A/B/C). The elements of the block diagram that are not directly a part of the Output Compare unit are
gray shaded.
Figure 20-4 Output Compare Unit, Block Diagram
OCFnx (Int.Req.)
= (16-bit Comparator )
OCRnx Buffer (16-bit Register)
OCRnxH Buf. (8-bit)
OCnx
TEMP (8-bit)
DATA BUS (8-bit)
OCRnxL Buf. (8-bit)
TCNTn (16-bit Counter)
TCNTnH (8-bit) TCNTnL (8-bit)
COMnx1:0WGMn3:0
OCRnx (16-bit Register)
OCRnxH (8-bit) OCRnxL (8-bit)
Waveform Generator
TOP
BOTTOM
The OCRnx Register is double buffered when using any of the twelve Pulse Width Modulation (PWM)
modes. For the normal and Clear Timer on Compare (CTC) modes of operation, the double buffering is
disabled. The double buffering synchronizes the update of the OCRnx Compare Register to either TOP or
BOTTOM of the counting sequence. The synchronization prevents the occurrence of odd-length, non-
symmetrical PWM pulses, thereby making the output glitch-free.
The OCRnx Register access may seem complex, but this is not case. When the double buffering is
enabled, the CPU has access to the OCRnx Buffer Register, and if double buffering is disabled the CPU
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will access the OCRnx directly. The content of the OCR1x (Buffer or Compare) Register is only changed
by a write operation (the Timer/Counter does not update this register automatically as the TCNTn and
ICRn Register). Therefore OCRnx is not read via the High byte temporary register (TEMP). However, it is
a good practice to read the Low byte first as when accessing other 16-bit registers. Writing the OCRnx
Registers must be done via the TEMP Register since the compare of all 16-bit is done continuously. The
High byte (OCRnxH) has to be written first. When the High byte I/O location is written by the CPU, the
TEMP Register will be updated by the value written. Then when the Low byte (OCRnxL) is written to the
lower eight bits, the High byte will be copied into the upper 8-bits of either the OCRnx buffer or OCRnx
Compare Register in the same system clock cycle.
For more information of how to access the 16-bit registers refer to Accessing 16-bit Registers on page
140.
20.7.1. Force Output Compare
In non-PWM Waveform Generation modes, the match output of the comparator can be forced by writing a
one to the Force Output Compare (FOCnx) bit. Forcing Compare Match will not set the OCFnx Flag or
reload/clear the timer, but the OCnx pin will be updated as if a real Compare Match had occurred (the
COMn1:0 bits settings define whether the OCnx pin is set, cleared or toggled).
20.7.2. Compare Match Blocking by TCNTn Write
All CPU writes to the TCNTn Register will block any Compare Match that occurs in the next timer clock
cycle, even when the timer is stopped. This feature allows OCRnx to be initialized to the same value as
TCNTn without triggering an interrupt when the Timer/Counter clock is enabled.
20.7.3. Using the Output Compare Unit
Since writing TCNTn in any mode of operation will block all compare matches for one timer clock cycle,
there are risks involved when changing TCNTn when using any of the Output Compare channels,
independent of whether the Timer/Counter is running or not. If the value written to TCNTn equals the
OCRnx value, the Compare Match will be missed, resulting in incorrect waveform generation. Do not
write the TCNTn equal to TOP in PWM modes with variable TOP values. The Compare Match for the
TOP will be ignored and the counter will continue to 0xFFFF. Similarly, do not write the TCNTn value
equal to BOTTOM when the counter is downcounting.
The setup of the OCnx should be performed before setting the Data Direction Register for the port pin to
output. The easiest way of setting the OCnx value is to use the Force Output Compare (FOCnx) strobe
bits in Normal mode. The OCnx Register keeps its value even when changing between Waveform
Generation modes.
Be aware that the COMnx1:0 bits are not double buffered together with the compare value. Changing the
COMnx1:0 bits will take effect immediately.
20.8. Compare Match Output Unit
The Compare Output mode (COMnx1:0) bits have two functions. The waveform generator uses the
COMnx1:0 bits for defining the Output Compare (OCnx) state at the next Compare Match. Secondly the
COMnx1:0 bits control the OCnx pin output source. The figure below shows a simplified schematic of the
logic affected by the COMnx1:0 bit setting. The I/O Registers, I/O bits, and I/O pins in the figure are
shown in bold. Only the parts of the general I/O Port Control Registers (DDR and PORT) that are affected
by the COMnx1:0 bits are shown. When referring to the OCnx state, the reference is for the internal OCnx
Register, not the OCnx pin. If a System Reset occur, the OCnx Register is reset to “0”.
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Figure 20-5 Compare Match Output Unit, Schematic
PORT
DDR
D Q
D Q
OCnx
Pin
OCnx
D Q
Waveform
Generator
COMnx1
COMnx0
0
1
DATA BUS
FOCnx
clkI/O
The general I/O port function is overridden by the Output Compare (OCnx) from the waveform generator
if either of the COMnx1:0 bits are set. However, the OCnx pin direction (input or output) is still controlled
by the Data Direction Register (DDR) for the port pin. The Data Direction Register bit for the OCnx pin
(DDR_OCnx) must be set as output before the OCnx value is visible on the pin. The port override function
is generally independent of the Waveform Generation mode, but there are some exceptions. Refer to
tables Table 20-2 Compare Output Mode, non-PWM on page 159, Table 20-3 Compare Output Mode,
Fast PWM on page 160 and Table 20-4 Compare Output Mode, Phase Correct and Phase and
Frequency Correct PWM on page 160 for details.
The design of the Output Compare Pin logic allows initialization of the OCnx state before the output is
enabled. Note that some COMnx1:0 bit settings are reserved for certain modes of operation. See
Register Description on page 157.
The COMnx1:0 bits have no effect on the Input Capture unit.
20.8.1. Compare Output Mode and Waveform Generation
The waveform generator uses the COMnx1:0 bits differently in normal, CTC, and PWM modes. For all
modes, setting the COMnx1:0 = 0 tells the waveform generator that no action on the OCnx Register is to
be performed on the next Compare Match. For compare output actions in the non-PWM modes refer to
Table 20-2 Compare Output Mode, non-PWM on page 159. For fast PWM mode refer to Table 20-3 
Compare Output Mode, Fast PWM on page 160, and for phase correct and phase and frequency correct
PWM refer to Table 20-4 Compare Output Mode, Phase Correct and Phase and Frequency Correct PWM
on page 160.
A change of the COMnx1:0 bits state will have effect at the first Compare Match after the bits are written.
For nonPWM modes, the action can be forced to have immediate effect by using the FOCnx strobe bits.
20.9. Modes of Operation
The mode of operation (i.e., the behavior of the Timer/Counter and the Output Compare pins) is defined
by the combination of the Waveform Generation mode (WGMn3:0) and Compare Output mode
(COMnx1:0) bits. The Compare Output mode bits do not affect the counting sequence, while the
Waveform Generation mode bits do. The COMnx1:0 bits control whether the PWM output generated
should be inverted or not (inverted or non-inverted PWM). For non-PWM modes the COMnx1:0 bits
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control whether the output should be set, cleared or toggle at a Compare Match. See Compare Match
Output Unit on page 147.
For detailed timing information refer to Timer/Counter Timing Diagrams on page 156.
20.9.1. Normal Mode
The simplest mode of operation is the Normal mode (WGMn3:0 = 0). In this mode the counting direction
is always up (incrementing), and no counter clear is performed. The counter simply overruns when it
passes its maximum 16-bit value (MAX = 0xFFFF) and then restarts from the BOTTOM (0x0000). In
normal operation the Timer/Counter Overflow Flag (TOVn) will be set in the same timer clock cycle as the
TCNTn becomes zero. The TOVn Flag in this case behaves like a 17th bit, except that it is only set, not
cleared. However, combined with the timer overflow interrupt that automatically clears the TOVn Flag, the
timer resolution can be increased by software. There are no special cases to consider in the Normal
mode, a new counter value can be written anytime.
The Input Capture unit is easy to use in Normal mode. However, observe that the maximum interval
between the external events must not exceed the resolution of the counter. If the interval between events
are too long, the timer overflow interrupt or the prescaler must be used to extend the resolution for the
capture unit.
The Output Compare units can be used to generate interrupts at some given time. Using the Output
Compare to generate waveforms in Normal mode is not recommended, since this will occupy too much of
the CPU time.
20.9.2. Clear Timer on Compare Match (CTC) Mode
In Clear Timer on Compare or CTC mode (WGMn3:0 = 4 or 12), the OCRnA or ICRn Register are used to
manipulate the counter resolution. In CTC mode the counter is cleared to zero when the counter value
(TCNTn) matches either the OCRnA (WGMn3:0 = 4) or the ICRn (WGMn3:0 = 12). The OCRnA or ICRn
define the top value for the counter, hence also its resolution. This mode allows greater control of the
Compare Match output frequency. It also simplifies the operation of counting external events.
The timing diagram for the CTC mode is shown below. The counter value (TCNTn) increases until a
Compare Match occurs with either OCRnA or ICRn, and then counter (TCNTn) is cleared.
Figure 20-6 CTC Mode, Timing Diagram
TCNTn
OCnA
(Toggle)
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
1 4
Period 2 3
(COMnA1:0 = 1)
An interrupt can be generated at each time the counter value reaches the TOP value by either using the
OCFnA or ICFn Flag according to the register used to define the TOP value. If the interrupt is enabled,
the interrupt handler routine can be used for updating the TOP value. However, changing the TOP to a
value close to BOTTOM when the counter is running with none or a low prescaler value must be done
with care since the CTC mode does not have the double buffering feature. If the new value written to
OCRnA or ICRn is lower than the current value of TCNTn, the counter will miss the Compare Match. The
counter will then have to count to its maximum value (0xFFFF) and wrap around starting at 0x0000 before
the Compare Match can occur. In many cases this feature is not desirable. An alternative will then be to
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use the fast PWM mode using OCRnA for defining TOP (WGMn3:0 = 15) since the OCRnA then will be
double buffered.
For generating a waveform output in CTC mode, the OCnA output can be set to toggle its logical level on
each Compare Match by setting the Compare Output mode bits to toggle mode (COMnA1:0 = 1). The
OCnA value will not be visible on the port pin unless the data direction for the pin is set to output
(DDR_OCnA = 1). The waveform generated will have a maximum frequency of fOCnA = fclk_I/O/2 when
OCRnA is set to zero (0x0000). The waveform frequency is defined by the following equation:
OCnA =clk_I/O
2   1 + OCRnA
N represents the prescaler factor (1, 8, 64, 256, or 1024).
As for the Normal mode of operation, the Timer Counter TOVn Flag is set in the same timer clock cycle
that the counter counts from MAX to 0x0000.
20.9.3. Fast PWM Mode
The fast Pulse Width Modulation or fast PWM mode (WGMn3:0 = 5, 6, 7, 14, or 15) provides a high
frequency PWM waveform generation option. The fast PWM differs from the other PWM options by its
single-slope operation. The counter counts from BOTTOM to TOP then restarts from BOTTOM. In non-
inverting Compare Output mode, the Output Compare (OCnx) is cleared on the Compare Match between
TCNTn and OCRnx, and set at BOTTOM. In inverting Compare Output mode output is set on Compare
Match and cleared at BOTTOM. Due to the singleslope operation, the operating frequency of the fast
PWM mode can be twice as high as the phase correct and phase and frequency correct PWM modes that
use dual-slope operation. This high frequency makes the fast PWM mode well suited for power
regulation, rectification, and DAC applications. High frequency allows physically small sized external
components (coils, capacitors), hence reduces total system cost.
The PWM resolution for fast PWM can be fixed to 8-, 9-, or 10-bit, or defined by either ICRn or OCRnA.
The minimum resolution allowed is 2-bit (ICRn or OCRnA set to 0x0003), and the maximum resolution is
16-bit (ICRn or OCRnA set to MAX). The PWM resolution in bits can be calculated by using the following
equation:
FPWM =log TOP+1
log 2
In fast PWM mode the counter is incremented until the counter value matches either one of the fixed
values 0x00FF, 0x01FF, or 0x03FF (WGMn3:0 = 5, 6, or 7), the value in ICRn (WGMn3:0 = 14), or the
value in OCRnA (WGMn3:0 = 15). The counter is then cleared at the following timer clock cycle. The
timing diagram for the fast PWM mode is shown in the figure below. The figure shows fast PWM mode
when OCRnA or ICRn is used to define TOP. The TCNTn value is in the timing diagram shown as a
histogram for illustrating the single-slope operation. The diagram includes non-inverted and inverted
PWM outputs. The small horizontal line marks on the TCNTn slopes represent compare matches
between OCRnx and TCNTn. The OCnx Interrupt Flag will be set when a Compare Match occurs.
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Figure 20-7 Fast PWM Mode, Timing Diagram
TCNTn
OCRnx/TOP Update and
TOVn Interrupt Flag Set and
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
1 7
Period 2 3 4 5 6 8
OCnx
OCnx
(COMnx1:0 = 2)
(COMnx1:0 = 3)
The Timer/Counter Overflow Flag (TOVn) is set each time the counter reaches TOP. In addition the OCnA
or ICFn Flag is set at the same timer clock cycle as TOVn is set when either OCRnA or ICRn is used for
defining the TOP value. If one of the interrupts are enabled, the interrupt handler routine can be used for
updating the TOP and compare values.
When changing the TOP value the program must ensure that the new TOP value is higher or equal to the
value of all of the Compare Registers. If the TOP value is lower than any of the Compare Registers, a
Compare Match will never occur between the TCNTn and the OCRnx. Note that when using fixed TOP
values the unused bits are masked to zero when any of the OCRnx Registers are written.
The procedure for updating ICRn differs from updating OCRnA when used for defining the TOP value.
The ICRn Register is not double buffered. This means that if ICRn is changed to a low value when the
counter is running with none or a low prescaler value, there is a risk that the new ICRn value written is
lower than the current value of TCNTn. The result will then be that the counter will miss the Compare
Match at the TOP value. The counter will then have to count to the MAX value (0xFFFF) and wrap around
starting at 0x0000 before the Compare Match can occur. The OCRnA Register, however, is double
buffered. This feature allows the OCRnA I/O location to be written anytime. When the OCRnA I/O location
is written the value written will be put into the OCRnA Buffer Register. The OCRnA Compare Register will
then be updated with the value in the Buffer Register at the next timer clock cycle the TCNTn matches
TOP. The update is done at the same timer clock cycle as the TCNTn is cleared and the TOVn Flag is
set.
Using the ICRn Register for defining TOP works well when using fixed TOP values. By using ICRn, the
OCRnA Register is free to be used for generating a PWM output on OCnA. However, if the base PWM
frequency is actively changed (by changing the TOP value), using the OCRnA as TOP is clearly a better
choice due to its double buffer feature.
In fast PWM mode, the compare units allow generation of PWM waveforms on the OCnx pins. Setting the
COMnx1:0 bits to 2 will produce a non-inverted PWM and an inverted PWM output can be generated by
setting the COMnx1:0 to 3. Refer to Table 20-3 Compare Output Mode, Fast PWM on page 160. The
actual OCnx value will only be visible on the port pin if the data direction for the port pin is set as output
(DDR_OCnx). The PWM waveform is generated by setting (or clearing) the OCnx Register at the
Compare Match between OCRnx and TCNTn, and clearing (or setting) the OCnx Register at the timer
clock cycle the counter is cleared (changes from TOP to BOTTOM).
The PWM frequency for the output can be calculated by the following equation:
OCnxPWM =clk_I/O
  1 + TOP
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N represents the prescale divider (1, 8, 64, 256, or 1024).
The extreme values for the OCRnx Register represents special cases when generating a PWM waveform
output in the fast PWM mode. If the OCRnx is set equal to BOTTOM (0x0000) the output will be a narrow
spike for each TOP+1 timer clock cycle. Setting the OCRnx equal to TOP will result in a constant high or
low output (depending on the polarity of the output set by the COMnx1:0 bits.)
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by setting OCnA
to toggle its logical level on each Compare Match (COMnA1:0 = 1). This applies only if OCRnA is used to
define the TOP value (WGMn3:0 = 15). The waveform generated will have a maximum frequency of fOCnA
= fclk_I/O/2 when OCRnA is set to zero (0x0000). This feature is similar to the OCnA toggle in CTC mode,
except the double buffer feature of the Output Compare unit is enabled in the fast PWM mode.
20.9.4. Phase Correct PWM Mode
The phase correct Pulse Width Modulation or phase correct PWM mode (WGMn3:0 = 1, 2, 3, 10, or 11)
provides a high resolution phase correct PWM waveform generation option. The phase correct PWM
mode is, like the phase and frequency correct PWM mode, based on a dual-slope operation. The counter
counts repeatedly from BOTTOM (0x0000) to TOP and then from TOP to BOTTOM. In non-inverting
Compare Output mode, the Output Compare (OCnx) is cleared on the Compare Match between TCNTn
and OCRnx while upcounting, and set on the Compare Match while downcounting. In inverting Output
Compare mode, the operation is inverted. The dual-slope operation has lower maximum operation
frequency than single slope operation. However, due to the symmetric feature of the dual-slope PWM
modes, these modes are preferred for motor control applications.
The PWM resolution for the phase correct PWM mode can be fixed to 8-, 9-, or 10-bit, or defined by either
ICRn or OCRnA. The minimum resolution allowed is 2-bit (ICRn or OCRnA set to 0x0003), and the
maximum resolution is 16-bit (ICRn or OCRnA set to MAX). The PWM resolution in bits can be calculated
by using the following equation:
PCPWM =log TOP+1
log 2
In phase correct PWM mode the counter is incremented until the counter value matches either one of the
fixed values 0x00FF, 0x01FF, or 0x03FF (WGMn3:0 = 1, 2, or 3), the value in ICRn (WGMn3:0 = 10), or
the value in OCRnA (WGMn3:0 = 11). The counter has then reached the TOP and changes the count
direction. The TCNTn value will be equal to TOP for one timer clock cycle. The timing diagram for the
phase correct PWM mode is shown in the figure below. The figure shows phase correct PWM mode when
OCRnA or ICRn is used to define TOP. The TCNTn value is in the timing diagram shown as a histogram
for illustrating the dual-slope operation. The diagram includes non-inverted and inverted PWM outputs.
The small horizontal line marks on the TCNTn slopes represent compare matches between OCRnx and
TCNTn. The OCnx Interrupt Flag will be set when a Compare Match occurs.
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Figure 20-8 Phase Correct PWM Mode, Timing Diagram
OCRnx/TOP Update and
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
1 2 3 4
TOVn Interrupt Flag Set
(Interrupt on Bottom)
TCNTn
Period
OCnx
OCnx
(COMnx1:0 = 2)
(COMnx1:0 = 3)
The Timer/Counter Overflow Flag (TOVn) is set each time the counter reaches BOTTOM. When either
OCRnA or ICRn is used for defining the TOP value, the OCnA or ICFn Flag is set accordingly at the same
timer clock cycle as the OCRnx Registers are updated with the double buffer value (at TOP). The
Interrupt Flags can be used to generate an interrupt each time the counter reaches the TOP or BOTTOM
value.
When changing the TOP value the program must ensure that the new TOP value is higher or equal to the
value of all of the Compare Registers. If the TOP value is lower than any of the Compare Registers, a
Compare Match will never occur between the TCNTn and the OCRnx. Note that when using fixed TOP
values, the unused bits are masked to zero when any of the OCRnx Registers are written. As the third
period shown in the timing diagram above illustrates, changing the TOP actively while the Timer/Counter
is running in the Phase Correct mode can result in an unsymmetrical output. The reason for this can be
found in the time of update of the OCRnx Register. Since the OCRnx update occurs at TOP, the PWM
period starts and ends at TOP. This implies that the length of the falling slope is determined by the
previous TOP value, while the length of the rising slope is determined by the new TOP value. When these
two values differ the two slopes of the period will differ in length. The difference in length gives the
unsymmetrical result on the output.
It is recommended to use the Phase and Frequency Correct mode instead of the Phase Correct mode
when changing the TOP value while the Timer/Counter is running. When using a static TOP value there
are practically no differences between the two modes of operation.
In phase correct PWM mode, the compare units allow generation of PWM waveforms on the OCnx pins.
Setting the COMnx1:0 bits to 2 will produce a non-inverted PWM and an inverted PWM output can be
generated by setting the COMnx1:0 to 3. Refer to Table 20-4 Compare Output Mode, Phase Correct and
Phase and Frequency Correct PWM on page 160. The actual OCnx value will only be visible on the port
pin if the data direction for the port pin is set as output (DDR_OCnx). The PWM waveform is generated by
setting (or clearing) the OCnx Register at the Compare Match between OCRnx and TCNTn when the
counter increments, and clearing (or setting) the OCnx Register at Compare Match between OCRnx and
TCNTn when the counter decrements. The PWM frequency for the output when using phase correct
PWM can be calculated by the following equation:
OCnxPCPWM =clk_I/O
2   TOP
N variable represents the prescale divider (1, 8, 64, 256, or 1024).
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The extreme values for the OCRnx Register represent special cases when generating a PWM waveform
output in the phase correct PWM mode. If the OCRnx is set equal to BOTTOM the output will be
continuously low and if set equal to TOP the output will be continuously high for non-inverted PWM mode.
For inverted PWM the output will have the opposite logic values.
If OCRnA is used to define the TOP value (WGMn3:0 = 11) and COMnA1:0 = 1, the OCnA output will
toggle with a 50% duty cycle.
20.9.5. Phase and Frequency Correct PWM Mode
The phase and frequency correct Pulse Width Modulation, or phase and frequency correct PWM mode
(WGMn3:0 = 8 or 9) provides a high resolution phase and frequency correct PWM waveform generation
option. The phase and frequency correct PWM mode is, like the phase correct PWM mode, based on a
dual-slope operation. The counter counts repeatedly from BOTTOM (0x0000) to TOP and then from TOP
to BOTTOM. In non-inverting Compare Output mode, the Output Compare (OCnx) is cleared on the
Compare Match between TCNTn and OCRnx while upcounting, and set on the Compare Match while
downcounting. In inverting Compare Output mode, the operation is inverted. The dual-slope operation
gives a lower maximum operation frequency compared to the single-slope operation. However, due to the
symmetric feature of the dual-slope PWM modes, these modes are preferred for motor control
applications.
The main difference between the phase correct, and the phase and frequency correct PWM mode is the
time the OCRnx Register is updated by the OCRnx Buffer Register, (refer to Figure 20-8 Phase Correct
PWM Mode, Timing Diagram on page 153 and the timing diagram below).
The PWM resolution for the phase and frequency correct PWM mode can be defined by either ICRn or
OCRnA. The minimum resolution allowed is 2-bit (ICRn or OCRnA set to 0x0003), and the maximum
resolution is 16-bit (ICRn or OCRnA set to MAX). The PWM resolution in bits can be calculated using the
following equation:
PFCPWM =log TOP+1
log 2
In phase and frequency correct PWM mode the counter is incremented until the counter value matches
either the value in ICRn (WGMn3:0 = 8), or the value in OCRnA (WGMn3:0 = 9). The counter has then
reached the TOP and changes the count direction. The TCNTn value will be equal to TOP for one timer
clock cycle. The timing diagram for the phase correct and frequency correct PWM mode is shown on
timing diagram below. The figure shows phase and frequency correct PWM mode when OCRnA or ICRn
is used to define TOP. The TCNTn value is in the timing diagram shown as a histogram for illustrating the
dual-slope operation. The diagram includes non-inverted and inverted PWM outputs. The small horizontal
line marks on the TCNTn slopes represent compare matches between OCRnx and TCNTn. The OCnx
Interrupt Flag will be set when a Compare Match occurs.
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Figure 20-9 Phase and Frequency Correct PWM Mode, Timing Diagram
OCRnx/TOP Updateand
TOVn Interrupt Flag Set
(Interrupt on Bottom)
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
1 2 3 4
TCNTn
Period
OCnx
OCnx
(COMnx1:0 = 2)
(COMnx1:0 = 3)
The Timer/Counter Overflow Flag (TOVn) is set at the same timer clock cycle as the OCRnx Registers
are updated with the double buffer value (at BOTTOM). When either OCRnA or ICRn is used for defining
the TOP value, the OCnA or ICFn Flag set when TCNTn has reached TOP. The Interrupt Flags can then
be used to generate an interrupt each time the counter reaches the TOP or BOTTOM value.
When changing the TOP value the program must ensure that the new TOP value is higher or equal to the
value of all of the Compare Registers. If the TOP value is lower than any of the Compare Registers, a
Compare Match will never occur between the TCNTn and the OCRnx.
As the timing diagram above shows the output generated is, in contrast to the Phase Correct mode,
symmetrical in all periods. Since the OCRnx Registers are updated at BOTTOM, the length of the rising
and the falling slopes will always be equal. This gives symmetrical output pulses and is therefore
frequency correct.
Using the ICRn Register for defining TOP works well when using fixed TOP values. By using ICRn, the
OCRnA Register is free to be used for generating a PWM output on OCnA. However, if the base PWM
frequency is actively changed by changing the TOP value, using the OCRnA as TOP is clearly a better
choice due to its double buffer feature.
In phase and frequency correct PWM mode, the compare units allow generation of PWM waveforms on
the OCnx pins. Setting the COMnx1:0 bits to 2 will produce a non-inverted PWM and an inverted PWM
output can be generated by setting the COMnx1:0 to 3. Refer to Table 20-4 Compare Output Mode,
Phase Correct and Phase and Frequency Correct PWM on page 160. The actual OCnx value will only be
visible on the port pin if the data direction for the port pin is set as output (DDR_OCnx). The PWM
waveform is generated by setting (or clearing) the OCnx Register at the Compare Match between OCRnx
and TCNTn when the counter increments, and clearing (or setting) the OCnx Register at Compare Match
between OCRnx and TCNTn when the counter decrements. The PWM frequency for the output when
using phase and frequency correct PWM can be calculated by the following equation:
OCnxPFCPWM =clk_I/O
2   TOP
The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).
The extreme values for the OCRnx Register represents special cases when generating a PWM waveform
output in the phase correct PWM mode. If the OCRnx is set equal to BOTTOM the output will be
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continuously low and if set equal to TOP the output will be set to high for non-inverted PWM mode. For
inverted PWM the output will have the opposite logic values.
If OCnA is used to define the TOP value (WGMn3:0 = 9) and COMnA1:0 = 1, the OCnA output will toggle
with a 50% duty cycle.
20.10. Timer/Counter Timing Diagrams
The Timer/Counter is a synchronous design and the timer clock (clkTn) is therefore shown as a clock
enable signal in the following figures. The figures include information on when Interrupt Flags are set, and
when the OCRnx Register is updated with the OCRnx buffer value (only for modes utilizing double
buffering). The next figure shows a timing diagram for the setting of OCFnx.
Figure 20-10 Timer/Counter Timing Diagram, Setting of OCFnx, no Prescaling
clkTn
(clkI/O/1)
OCFnx
clkI/O
OCRnx
TCNTn
OCRnx Value
OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2
The next figure shows the same timing data, but with the prescaler enabled.
Figure 20-11 Timer/Counter Timing Diagram, Setting of OCFnx, with Prescaler (fclk_I/O/8)
OCFnx
OCRnx
TCNTn
OCRnx Value
OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2
clkI/O
clkTn
(clkI/O/8)
The next figure shows the count sequence close to TOP in various modes. When using phase and
frequency correct PWM mode the OCRnx Register is updated at BOTTOM. The timing diagrams will be
the same, but TOP should be replaced by BOTTOM, TOP-1 by BOTTOM+1 and so on. The same
renaming applies for modes that set the TOVn Flag at BOTTOM.
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Figure 20-12 Timer/Counter Timing Diagram, no Prescaling.
TOVn (FPWM)
and ICF n (if used
as TOP)
OCRnx
(Update at TOP)
TCNTn
(CTC and FPWM)
TCNTn
(PC and PFC PWM) TOP - 1 TOP TOP - 1 TOP - 2
Old OCRnx Value New OCRnx Value
TOP - 1 TOP BOTTOM BOTTOM + 1
clkTn
(clkI/O/1)
clkI/O
The next figure shows the same timing data, but with the prescaler enabled.
Figure 20-13 Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)
TOVn (FPWM)
and ICF n (if used
as TOP)
OCRnx
(Update at TOP)
TCNTn
(CTC and FPWM)
TCNTn
(PC and PFC PWM) TOP - 1 TOP TOP - 1 TOP - 2
Old OCRnx Value New OCRnx Value
TOP - 1 TOP BOTTOM BOTTOM + 1
clk
I/O
clk
Tn
(clk
I/O
/8)
20.11. Register Description
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20.11.1. TCCR1A – Timer/Counter1 Control Register A
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  TCCR1A
Offset:  0x2F
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x4F
Bit 7 6 5 4 3 2 1 0
COM1A1 COM1A0 COM1B1 COM1B0 COM1C1 COM1C0 WGM11 WGM10
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:6 – COM1An: Compare Output Mode for Channel A [n = 1:0]
Bits 5:4 – COM1Bn: Compare Output Mode for Channel B [n = 1:0]
Bits 3:2 – COM1Cn: Compare Output Mode for Channel C [n = 1:0]
Bits 1:0 – WGM1n: Waveform Generation Mode [n = 1:0]
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20.11.2. TCCR3A – Timer/Counter3 Control Register A
Name:  TCCR3A
Offset:  0x8B
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x4F
Bit 7 6 5 4 3 2 1 0
COM3A1 COM3A0 COM3B1 COM3B0 COM3C1 COM3C0 WGM11 WGM10
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:6 – COM3An: Compare Output Mode for Channel A [n = 1:0]
Bits 5:4 – COM3Bn: Compare Output Mode for Channel B [n = 1:0]
Bits 3:2 – COM3Cn: Compare Output Mode for Channel C [n = 1:0]
The COMnA1:0, COMnB1:0, and COMnC1:0 control the output compare pins (OCnA, OCnB, and OCnC
respectively) behavior. If one or both of the COMnA1:0 bits are written to one, the OCnA output overrides
the normal port functionality of the I/O pin it is connected to. If one or both of the COMnB1:0 bits are
written to one, the OCnB output overrides the normal port functionality of the I/O pin it is connected to. If
one or both of the COMnC1:0 bits are written to one, the OCnC output overrides the normal port
functionality of the I/O pin it is connected to. However, note that the Data Direction Register (DDR) bit
corresponding to the OCnA, OCnB or OCnC pin must be set in order to enable the output driver.
When the OCnA, OCnB or OCnC is connected to the pin, the function of the COMnx1:0 bits is dependent
of the WGMn3:0 bits setting. The table below shows the COMnx1:0 bit functionality when the WGMn3:0
bits are set to a normal or a CTC mode (non-PWM).
Table 20-2 Compare Output Mode, non-PWM
COMnA1/COMnB1/
COMnC1
COMnA0/COMnB0/
COMnC0
Description
0 0 Normal port operation, OCnA/OCnB/OCnC disconnected.
0 1 Toggle OCnA/OCnB/OCnC on compare match.
1 0 Clear OCnA/OCnB/OCnC on compare match (set output to
low level).
1 1 Set OCnA/OCnB/OCnC on compare match (set output to
high level).
The next table shows the COMnx1:0 bit functionality when the WGMn3:0 bits are set to the fast PWM
mode.
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Table 20-3 Compare Output Mode, Fast PWM
COMnA1/
COMnB1/
COMnC1
COMnA0/
COMnB0/
COMnC0
Description
0 0 Normal port operation, OCnA/OCnB/OCnC disconnected.
0 1 WGMn3:0 = 15: Toggle OCnA on Compare Match, OCnB/OCnC
disconnected (normal port operation). For all other WGMn
settings, normal port operation, OCnA/OCnB/OCnC disconnected.
1 0 Clear OCnA/OCnB/OCnC on compare match, set OCnA/OCnB/
OCnC at BOTTOM, (non-inverting mode)
1 1 Set OCnA/OCnB/OCnC on compare match, clear OCnA/OCnB/
OCnC at BOTTOM, (inverting mode)
Note:  1. A special case occurs when OCRnA/OCRnB/OCRnC equals TOP and COMnA1/COMnB1/
COMnC1 is set. In this case the compare match is ignored, but the set or clear is done at BOTTOM.
Refer to Fast PWM Mode on page 150 for details.
The table below shows the COMnx1:0 bit functionality when the WGMn3:0 bits are set to the phase
correct and frequency correct PWM mode.
Table 20-4 Compare Output Mode, Phase Correct and Phase and Frequency Correct PWM
COMnA1/
COMnB1/
COMnC1
COMnA0/
COMnB0/
COMnC0
Description
0 0 Normal port operation, OCnA/OCnB/OCnC disconnected.
0 1 WGMn3:0 = 9 or 11: Toggle OCnA on Compare Match, OCnB/
OCnC disconnected (normal port operation). For all other WGMn
settings, normal port operation, OCnA/OCnB/OCnC disconnected.
1 0 Clear OCnA/OCnB/OCnC on compare match when up-counting.
Set OCnA/OCnB/OCnC on compare match when downcounting.
1 1 Set OCnA/OCnB/OCnC on compare match when up-counting.
Clear OCnA/OCnB/OCnC on compare match when downcounting.
Note:  1. A special case occurs when OCRnA/OCRnB/OCRnC equals TOP and COMnA1/COMnB1/
COMnC1 is set. Refer to Phase Correct PWM Mode on page 152 for details.
Bits 1:0 – WGM1n: Waveform Generation Mode [n = 1:0]
Combined with the WGMn3:2 bits found in the TCCRnB Register, these bits control the counting
sequence of the counter, the source for maximum (TOP) counter value, and what type of waveform
generation to be used, refer to the table below. Modes of operation supported by the Timer/Counter unit
are: Normal mode (counter), Clear Timer on Compare match (CTC) mode, and three types of Pulse Width
Modulation (PWM) modes. (Refer to Modes of Operation on page 148).
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Table 20-5 Waveform Generation Mode Bit Description
Mode WGMn3 WGMn2
(CTCn)
WGMn1
(PWMn1)
WGMn0
(PWMn0)
Timer/Counter
Mode of Operation(1)
TOP Update of
OCRnx at
TOVn Flag
Set on
0 0 0 0 0 Normal 0xFFFF Immediate MAX
1 0 0 0 1 PWM, Phase Correct, 8-bit 0x00FF TOP BOTTOM
2 0 0 1 0 PWM, Phase Correct, 9-bit 0x01FF TOP BOTTOM
3 0 0 1 1 PWM, Phase Correct, 10-bit 0x03FF TOP BOTTOM
4 0 1 0 0 CTC OCRnA Immediate MAX
5 0 1 0 1 Fast PWM, 8-bit 0x00FF BOTTOM TOP
6 0 1 1 0 Fast PWM, 9-bit 0x01FF BOTTOM TOP
7 0 1 1 1 Fast PWM, 10-bit 0x03FF BOTTOM TOP
8 1 0 0 0 PWM, Phase and Frequency
Correct
ICRn BOTTOM BOTTOM
9 1 0 0 1 PWM, Phase and Frequency
Correct
OCRnA BOTTOM BOTTOM
10 1 0 1 0 PWM, Phase Correct ICRn TOP BOTTOM
11 1 0 1 1 PWM, Phase Correct OCRnA TOP BOTTOM
12 1 1 0 0 CTC ICRn Immediate MAX
13 1 1 0 1 Reserved - - -
14 1 1 1 0 Fast PWM ICRn BOTTOM TOP
15 1 1 1 1 Fast PWM OCRnA BOTTOM TOP
Note: 
1. The CTCn and PWMn1:0 bit definition names are obsolete. Use the WGMn2:0 definitions.
However, the functionality and location of these bits are compatible with previous versions of the
timer.
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20.11.3. TCCR1B – Timer/Counter1 Control Register B
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  TCCR1B
Offset:  0x2E
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x4E
Bit 7 6 5 4 3 2 1 0
ICNC1 ICES1 WGM13 WGM12 CS12 CS11 CS10
Access R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0
Bit 7 – ICNC1: Input Capture Noise Canceler
Bit 6 – ICES1: Input Capture Edge Select
Bit 4 – WGM13: Waveform Generation Mode
Bit 3 – WGM12: Waveform Generation Mode
Bits 2:0 – CS1n: Clock Select [n = 0:2]
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20.11.4. TCCR3B – Timer/Counter3 Control Register B
Name:  TCCR3B
Offset:  0x8A
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
ICNC3 ICES3 WGM33 WGM32 CS32 CS31 CS30
Access R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0
Bit 7 – ICNC3: Input Capture Noise Canceler
Setting this bit (to one) activates the Input Capture Noise Canceler. When the Noise Canceler is
activated, the input from the Input Capture Pin (ICPn) is filtered. The filter function requires four
successive equal valued samples of the ICPn pin for changing its output. The Input Capture is therefore
delayed by four Oscillator cycles when the noise canceler is enabled.
Bit 6 – ICES3: Input Capture Edge Select
This bit selects which edge on the Input Capture Pin (ICPn) that is used to trigger a capture event. When
the ICESn bit is written to zero, a falling (negative) edge is used as trigger, and when the ICESn bit is
written to one, a rising (positive) edge will trigger the capture.
When a capture is triggered according to the ICESn setting, the counter value is copied into the Input
Capture Register (ICRn). The event will also set the Input Capture Flag (ICFn), and this can be used to
cause an Input Capture Interrupt, if this interrupt is enabled.
When the ICRn is used as TOP value (see description of the WGMn3:0 bits located in the TCCRnA and
the TCCRnB Register), the ICPn is disconnected and consequently the Input Capture function is
disabled.
Bit 4 – WGM33: Waveform Generation Mode
Refer to TCCR3A.
Bit 3 – WGM32: Waveform Generation Mode
Refer to TCCR3A.
Bits 2:0 – CS3n: Clock Select [n = 0:2]
The three Clock Select bits select the clock source to be used by the Timer/Counter. Refer to Figure
20-10 Timer/Counter Timing Diagram, Setting of OCFnx, no Prescaling on page 156 and Figure 20-11 
Timer/Counter Timing Diagram, Setting of OCFnx, with Prescaler (fclk_I/O/8) on page 156.
Table 20-6 Clock Select Bit Description
CA12 CA11 CS10 Description
0 0 0 No clock source (Timer/Counter stopped).
0 0 1 clkI/O/1 (No prescaling)
0 1 0 clkI/O/8 (From prescaler)
0 1 1 clkI/O/64 (From prescaler)
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CA12 CA11 CS10 Description
1 0 0 clkI/O/256 (From prescaler)
1 0 1 clkI/O/1024 (From prescaler)
1 1 0 External clock source on Tn pin. Clock on falling edge.
1 1 1 External clock source on Tn pin. Clock on rising edge.
If external pin modes are used for the Timer/Countern, transitions on the Tn pin will clock the counter
even if the pin is configured as an output. This feature allows software control of the counting.
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20.11.5. TCCR1C – Timer/Counter1 Control Register C
Name:  TCCR1C
Offset:  0x7A
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
FOC1A FOC1B FOC1C
Access W W W
Reset 0 0 0
Bit 7 – FOC1A: Force Output Compare for channel A
Bit 6 – FOC1B: Force Output Compare for channel B
Bit 5 – FOC1C: Force Output Compare for channel C
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20.11.6. TCCR3C – Timer/Counter3 Control Register C
Name:  TCCR3C
Offset:  0x8C
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
FOC3A FOC3B FOC3C
Access W W W
Reset 0 0 0
Bit 7 – FOC3A: Force Output Compare for channel A
Bit 6 – FOC3B: Force Output Compare for channel B
Bit 5 – FOC3C: Force Output Compare for channel C
The FOCnA/FOCnB/FOCnC bits are only active when the WGMn3:0 bits specifies a non-PWM mode.
When writing a logical one to the FOCnA/FOCnB/FOCnC bit, an immediate compare match is forced on
the waveform generation unit. The OCnA/OCnB/OCnC output is changed according to its COMnx1:0 bits
setting. Note that the FOCnA/FOCnB/FOCnC bits are implemented as strobes. Therefore it is the value
present in the COMnx1:0 bits that determine the effect of the forced compare.
A FOCnA/FOCnB/FOCnC strobe will not generate any interrupt nor will it clear the timer in Clear Timer on
Compare Match (CTC) mode using OCRnA as TOP.
The FOCnA/FOCnB/FOCnB bits are always read as zero.
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20.11.7. TCNT1L – Timer/Counter1 Low byte
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  TCNT1L
Offset:  0x2C
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x4C
Bit 7 6 5 4 3 2 1 0
TCNT1L[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – TCNT1L[7:0]: Timer/Counter 1 Low byte
Refer to TCNT3H.
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20.11.8. TCNT1H – Timer/Counter1 High byte
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  TCNT1H
Offset:  0x2D
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x4D
Bit 7 6 5 4 3 2 1 0
TCNT1H[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – TCNT1H[7:0]: Timer/Counter 1 High byte
Refer to TCNT3H.
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20.11.9. TCNT3L – Timer/Counter3 Low byte
Name:  TCNT3L
Offset:  0x88
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
TCNT3L[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – TCNT3L[7:0]: Timer/Counter 3 Low byte
Refer to TCNT3H.
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20.11.10. TCNT3H – Timer/Counter3 High byte
Name:  TCNT3H
Offset:  0x89
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
TCNT1H[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – TCNT1H[7:0]: Timer/Counter 1 High byte
The two Timer/Counter I/O locations (TCNTnH and TCNTnL, combined TCNTn) give direct access, both
for read and for write operations, to the Timer/Counter unit 16-bit counter. To ensure that both the high
and low bytes are read and written simultaneously when the CPU accesses these registers, the access is
performed using an 8-bit temporary High Byte Register (TEMP). This Temporary Register is shared by all
the other 16-bit registers. Refer to Accessing 16-bit Registers for details.
Modifying the counter (TCNTn) while the counter is running introduces a risk of missing a compare match
between TCNTn and one of the OCRnx Registers.
Writing to the TCNTn Register blocks (removes) the compare match on the following timer clock for all
compare units.
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20.11.11. OCR1AL – Output Compare Register 1 A Low byte
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  OCR1AL
Offset:  0x2A
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x4A
Bit 7 6 5 4 3 2 1 0
OCR1AL[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – OCR1AL[7:0]: Output Compare 1 A Low byte
Refer to OCR3CH on page 182.
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20.11.12. OCR1AH – Output Compare Register 1 A High byte
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  OCR1AH
Offset:  0x2B
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x4B
Bit 7 6 5 4 3 2 1 0
OCR1AH[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – OCR1AH[7:0]: Output Compare 1 A High byte
Refer to OCR3CH on page 182.
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20.11.13. OCR1BL – Output Compare Register 1 B Low byte
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  OCR1BL
Offset:  0x28
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x48
Bit 7 6 5 4 3 2 1 0
OCR1BL[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – OCR1BL[7:0]: Output Compare 1 B Low byte
Refer to OCR3CH on page 182.
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20.11.14. OCR1BH – Output Compare Register 1 B High byte
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  OCR1BH
Offset:  0x29
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x49
Bit 7 6 5 4 3 2 1 0
OCR1BH[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – OCR1BH[7:0]: Output Compare 1 B High byte
Refer to OCR3CH on page 182.
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20.11.15. OCR1CL – Output Compare Register 1 C Low byte
Name:  OCR1CL
Offset:  0x78
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
OCR1CL[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – OCR1CL[7:0]: Output Compare 1 C Low byte
Refer to OCR3CH on page 182.
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20.11.16. OCR1CH – Output Compare Register 1 C High byte
Name:  OCR1CH
Offset:  0x79
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
OCR1CH[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – OCR1CH[7:0]: Output Compare 1 C High byte
Refer to OCR3CH on page 182.
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20.11.17. OCR3AL – Output Compare Register 3 A Low byte
Name:  OCR3AL
Offset:  0x86
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
OCR3AL[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – OCR3AL[7:0]: Output Compare 3 A Low byte
Refer to OCR3CH on page 182.
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20.11.18. OCR3AH – Output Compare Register 3 A High byte
Name:  OCR3AH
Offset:  0x87
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
OCR1AH[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – OCR1AH[7:0]: Output Compare 3 A High byte
Refer to OCR3CH on page 182.
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20.11.19. OCR3BL – Output Compare Register 3 B Low byte
Name:  OCR3BL
Offset:  0x84
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
OCR3BL[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – OCR3BL[7:0]: Output Compare 3 B Low byte
Refer to OCR3CH on page 182.
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20.11.20. OCR3BH – Output Compare Register 3 B High byte
Name:  OCR3BH
Offset:  0x85
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
OCR3BH[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – OCR3BH[7:0]: Output Compare 3 B High byte
Refer to OCR3CH on page 182.
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20.11.21. OCR3CL – Output Compare Register 3 C Low byte
Name:  OCR3CL
Offset:  0x82
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
OCR3CL[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – OCR3CL[7:0]: Output Compare 3 C Low byte
Refer to OCR3CH on page 182.
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20.11.22. OCR3CH – Output Compare Register 3 C High byte
Name:  OCR3CH
Offset:  0x83
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
OCR3CH[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – OCR3CH[7:0]: Output Compare 3 C High byte
The Input Capture is updated with the counter (TCNTn) value each time an event occurs on the ICPn pin
(or optionally on the Analog Comparator Output for Timer/Counter1). The Input Capture can be used for
defining the counter TOP value.
The Input Capture Register is 16-bit in size. To ensure that both the high and low bytes are read
simultaneously when the CPU accesses these registers, the access is performed using an 8-bit
temporary High Byte Register (TEMP). This Temporary Register is shared by all the other 16-bit registers.
Refer to Accessing 16-bit Registers on page 140 for details.
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20.11.23. ICR1L – Input Capture Register 1 Low byte
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  ICR1L
Offset:  0x26
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x46
Bit 7 6 5 4 3 2 1 0
ICR1L[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – ICR1L[7:0]: Input Capture 1 Low byte
Refer to ICR3H on page 186.
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20.11.24. ICR1H – Input Capture Register 1 High byte
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  ICR1H
Offset:  0x27
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x47
Bit 7 6 5 4 3 2 1 0
ICR1H[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – ICR1H[7:0]: Input Capture 1 High byte
Refer to ICR3H on page 186.
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20.11.25. ICR3L – Input Capture Register 3 Low byte
Name:  ICR3L
Offset:  0x80
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
ICR3L[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – ICR3L[7:0]: Input Capture 3 Low byte
Refer to ICR3H on page 186.
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20.11.26. ICR3H – Input Capture Register 3 High byte
Name:  ICR3H
Offset:  0x81
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
ICR3H[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – ICR3H[7:0]: Input Capture 3 High byte
The Input Capture is updated with the counter (TCNTn) value each time an event occurs on the ICPn pin
(or optionally on the Analog Comparator Output for Timer/Counter1). The Input Capture can be used for
defining the counter TOP value.
The Input Capture Register is 16-bit in size. To ensure that both the high and low bytes are read
simultaneously when the CPU accesses these registers, the access is performed using an 8-bit
temporary High Byte Register (TEMP). This Temporary Register is shared by all the other 16-bit registers.
Refer to Accessing 16-bit Registers on page 140 for details.
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20.11.27. TIMSK – Timer/Counter Interrupt Mask Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Note:  1. This register contains interrupt control bits for several Timer/Counters, but only Timer1 bits are
described in this section. The remaining bits are described in their respective timer sections.
Name:  TIMSK
Offset:  0x37
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x57
Bit 7 6 5 4 3 2 1 0
TICIE1 OCIE1A OCIE1B TOIE1
Access R/W R/W R/W R/W
Reset 0 0 0 0
Bit 5 – TICIE1: Timer/Counter1, Input Capture Interrupt Enable
When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally enabled), the
Timer/Counter1 Input Capture interrupt is enabled. The corresponding Interrupt Vector (refer to Interrupts
on page 77) is executed when the ICF1 Flag, located in TIFR, is set.
Bit 4 – OCIE1A: Timer/Counter1, Output Compare A Match Interrupt Enable
When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally enabled), the
Timer/Counter1 Output Compare A match interrupt is enabled. The corresponding Interrupt Vector (refer
to Interrupts on page 77) is executed when the OCF1A Flag, located in TIFR, is set.
Bit 3 – OCIE1B: Timer/Counter1, Output Compare B Match Interrupt Enable
When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally enabled), the
Timer/Counter1 Output Compare B match interrupt is enabled. The corresponding Interrupt Vector(refer
to Interrupts on page 77) is executed when the OCF1B Flag, located in TIFR, is set.
Bit 2 – TOIE1: Timer/Counter1, Overflow Interrupt Enable
When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally enabled), the
Timer/Counter1 Overflow Interrupt is enabled. The corresponding Interrupt Vector (refer to Interrupts on
page 77) is executed when the TOV1 Flag, located in TIFR, is set.
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20.11.28. ETIMSK – Extended Timer/Counter Interrupt Mask Register
Note:  1. This register is not available in ATmega103 compatibility mode.
Name:  ETIMSK
Offset:  0x7D
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
TICIE3 OCIE3A OCIE3B TOIE3 OCIE3C OCIE1C
Access R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0
Bit 5 – TICIE3: Timer/Counter3, Input Capture Interrupt Enable
When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally enabled), the
Timer/Counter3 Input Capture Interrupt is enabled. The corresponding interrupt vector (refer to Interrupts
on page 77) is executed when the ICF3 flag, located in ETIFR, is set.
Bit 4 – OCIE3A: Timer/Counter3, Output Compare A Match Interrupt Enable
When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally enabled), the
Timer/Counter3 Output Compare A Match Interrupt is enabled. The corresponding interrupt vector (refer
to Interrupts on page 77) is executed when the OCF3A flag, located in ETIFR, is set.
Bit 3 – OCIE3B: Timer/Counter3, Output Compare B Match Interrupt Enable
When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally enabled), the
Timer/Counter3 Output Compare B Match Interrupt is enabled. The corresponding interrupt vector (refer
to Interrupts on page 77) is executed when the OCF3B flag, located in ETIFR, is set.
Bit 2 – TOIE3: Timer/Counter3, Overflow Interrupt Enable
When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally enabled), the
Timer/Counter3 Overflow Interrupt is enabled. The corresponding interrupt vector (refer to Interrupts on
page 77) is executed when the TOV3 flag, located in ETIFR, is set.
Bit 1 – OCIE3C: Timer/Counter3, Output Compare C Match Interrupt Enable
When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally enabled), the
Timer/Counter3 Output Compare C Match Interrupt is enabled. The corresponding interrupt vector (refer
to Interrupts on page 77) is executed when the OCF3C flag, located in ETIFR, is set.
Bit 0 – OCIE1C: Timer/Counter1, Output Compare C Match Interrupt Enable
When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally enabled), the
Timer/Counter1 Output Compare C Match Interrupt is enabled. The corresponding interrupt vector (refer
to Interrupts on page 77) is executed when the OCF1C flag, located in ETIFR, is set.
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20.11.29. TIFR – Timer/Counter Interrupt Flag Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Note:  1. This register contains flag bits for several Timer/Counters, but only Timer1 bits are described in
this section. The remaining bits are described in their respective timer sections.
Name:  TIFR
Offset:  0x36
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x56
Bit 7 6 5 4 3 2 1 0
ICF1 OCF1A OCF1B TOV1
Access R/W R/W R/W R/W
Reset 0 0 0 0
Bit 5 – ICF1: Timer/Counter1, Input Capture Flag
This flag is set when a capture event occurs on the ICP1 pin. When the Input Capture Register (ICR1) is
set by the WGMn3:0 to be used as the TOP value, the ICF1 Flag is set when the counter reaches the
TOP value.
ICF1 is automatically cleared when the Input Capture Interrupt Vector is executed. Alternatively, ICF1 can
be cleared by writing a logic one to its bit location.
Bit 4 – OCF1A: Timer/Counter1, Output Compare A Match Flag
This flag is set in the timer clock cycle after the counter (TCNT1) value matches the Output Compare
Register A (OCR1A).
Note that a Forced Output Compare (FOC1A) strobe will not set the OCF1A Flag.
OCF1A is automatically cleared when the Output Compare Match A Interrupt Vector is executed.
Alternatively, OCF1A can be cleared by writing a logic one to its bit location.
Bit 3 – OCF1B: Timer/Counter1, Output Compare B Match Flag
This flag is set in the timer clock cycle after the counter (TCNT1) value matches the Output Compare
Register B (OCR1B).
Note that a Forced Output Compare (FOC1B) strobe will not set the OCF1B Flag.
OCF1B is automatically cleared when the Output Compare Match B Interrupt Vector is executed.
Alternatively, OCF1B can be cleared by writing a logic one to its bit location.
Bit 2 – TOV1: Timer/Counter1, Overflow Flag
The setting of this flag is dependent of the WGMn3:0 bits setting. In Normal and CTC modes, the TOV1
Flag is set when the timer overflows. Refer to Table 20-5 Waveform Generation Mode Bit Description on
page 161 for the TOV1 Flag behavior when using another WGMn3:0 bit setting.
•TOV1 is automatically cleared when the Timer/Counter1 Overflow Interrupt Vector is
executed. Alternatively, TOV1 can be cleared by writing a logic one to its bit location.
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20.11.30. ETIFR – Extended Timer/Counter Interrupt Flag Register
Name:  ETIFR
Offset:  0x7C
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
ICF3 OCF3A OCF3B TOV3 OCF3C OCF1C
Access R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0
Bit 5 – ICF3: Timer/Counter3, Input Capture Flag
This flag is set when a capture event occurs on the ICP3 pin. When the Input Capture Register (ICR3) is
set by the WGM3:0 to be used as the TOP value, the ICF3 flag is set when the counter reaches the TOP
value.
ICF3 is automatically cleared when the Input Capture 3 interrupt vector is executed. Alternatively, ICF3
can be cleared by writing a logic one to its bit location.
Bit 4 – OCF3A: Timer/Counter3, Output Compare A Match Flag
This flag is set in the timer clock cycle after the counter (TCNT3) value matches the Output Compare
Register A (OCR3A).
Note that a forced output compare (FOC3A) strobe will not set the OCF3A flag.
OCF3A is automatically cleared when the Output Compare Match 3 A interrupt vector is executed.
Alternatively, OCF3A can be cleared by writing a logic one to its bit location.
Bit 3 – OCF3B: Timer/Counter3, Output Compare B Match Flag
This flag is set in the timer clock cycle after the counter (TCNT3) value matches the Output Compare
Register B (OCR3B).
Note that a forced output compare (FOC3B) strobe will not set the OCF3B flag.
OCF3B is automatically cleared when the Output Compare Match 3 B interrupt vector is executed.
Alternatively, OCF3B can be cleared by writing a logic one to its bit location.
Bit 2 – TOV3: Timer/Counter3, Overflow Flag
The setting of this flag is dependent of the WGM3:0 bits setting. In normal and CTC modes, the TOV3
flag is set when the timer overflows. Refer to Table 22-2 Waveform Generation Mode Bit Description on
page 226 for the TOV3 flag behavior when using another WGM3:0 bit setting.
TOV3 is automatically cleared when the Timer/Counter3 Overflow interrupt vector is executed.
Alternatively, TOV3 can be cleared by writing a logic one to its bit location.
Bit 1 – OCF3C: Timer/Counter3, Output Compare C Match Flag
This flag is set in the timer clock cycle after the counter (TCNT3) value matches the Output Compare
Register C (OCR3C).
Note that a forced output compare (FOC3C) strobe will not set the OCF3C flag.
OCF3C is automatically cleared when the Output Compare Match 3 C interrupt vector is executed.
Alternatively, OCF3C can be cleared by writing a logic one to its bit location.
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Bit 0 – OCF1C: Timer/Counter1, Output Compare C Match Flag
This flag is set in the timer clock cycle after the counter (TCNT1) value matches the Output Compare
Register C (OCR1C).
Note that a forced output compare (FOC1C) strobe will not set the OCF1C flag.
OCF1C is automatically cleared when the Output Compare Match 1 C interrupt vector is executed.
Alternatively, OCF1C can be cleared by writing a logic one to its bit location.
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21. 8-bit Timer/Counter0 with PWM and Asynchronous Operation
21.1. Features
• Single Channel Counter
• Clear Timer on Compare Match (Auto Reload)
• Glitch-free, phase Correct Pulse Width Modulator (PWM)
• Frequency Generator
• 10-bit Clock Prescaler
• Overflow and Compare Match Interrupt Sources (TOV0 and OCF0)
• Allows Clocking from External 32kHz Watch Crystal Independent of the I/O Clock
21.2. Overview
Timer/Counter0 is a general purpose, single channel, 8-bit Timer/Counter module. A simplified block
diagram of the 8-bit Timer/Counter is shown in the figure below. For the actual placement of I/O pins, refer
to Pin Configurations. CPU accessible I/O Registers, including I/O bits and I/O pins, are shown in bold.
The device-specific I/O Register and bit locations are listed in the Register Description on page 205.
Figure 21-1 8-bit Timer/Counter Block Diagram
Timer/Counte r
DATA BUS
=
TCNTn
Waveform
Gene ra tion OCn
= 0
Control Logic
= 0xFF
TOPBOTTOM
count
clea r
direction
TOVn
(Int. Req.)
OCn
(Int. Req.)
Synchro nization Unit
OCRn
TCCRn
ASS Rn
Sta tus Flags
clkI/O
clkAS Y
Synchronize d Sta tus Fla gs
asynchronous Mode
Se le ct (ASn)
TOSC1
T/C
Oscillator
TOSC2
Pre sca ler
clkTn
clkI/O
Related Links
Pin Configurations on page 14
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21.2.1. Registers
The Timer/Counter (TCNT0) and Output Compare Register (OCR0) are 8-bit registers. Interrupt request
(shorten as Int.Req.) signals are all visible in the Timer Interrupt Flag Register (TIFR). All interrupts are
individually masked with the Timer Interrupt Mask Register (TIMSK). TIFR and TIMSK are not shown in
the figure since these registers are shared by other timer units.
The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source on the T0
pin. The Clock Select logic block controls which clock source and edge the Timer/Counter uses to
increment (or decrement) its value. The Timer/Counter is inactive when no clock source is selected. The
output from the clock select logic is referred to as the timer clock (clkT0).
The double buffered Output Compare Register (OCR0) is compared with the Timer/Counter value at all
times. The result of the compare can be used by the waveform generator to generate a PWM or variable
frequency output on the Output Compare Pin (OC0). Refer to Output Compare Unit on page 194 for
details. The Compare Match event will also set the Compare Flag (OCF0) which can be used to generate
an Output Compare interrupt request.
21.2.2. Definitions
Many register and bit references in this document are written in general form. A lower case “n” replaces
the Timer/Counter number, in this case 0. However, when using the register or bit defines in a program,
the precise form must be used (i.e., TCNT0 for accessing Timer/Counter0 counter value and so on).
The definitions in the following table are also used extensively throughout the document.
Table 21-1 Definitions
BOTTOM The counter reaches the BOTTOM when it becomes zero (0x00).
MAX The counter reaches its MAXimum when it becomes 0xFF (decimal 255).
TOP The counter reaches the TOP when it becomes equal to the highest value in the
count sequence. The TOP value can be assigned to be the fixed value 0xFF (MAX)
or the value stored in the OCR0 Register. The assignment is dependent on the
mode of operation.
21.3. Timer/Counter Clock Sources
The Timer/Counter can be clocked by an internal synchronous or an external asynchronous clock source.
The clock source clkT0 is by default equal to the MCU clock, clkI/O. When the AS0 bit in the ASSR
Register is written to logic one, the clock source is taken from the Timer/Counter Oscillator connected to
TOSC1 and TOSC2. For details on asynchronous operation, refer to Asynchronous Operation of the
Timer/Counter on page 203. For details on clock sources and prescaler, refer to Timer/Counter Prescaler
on page 204.
21.4. Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. The following
figure shows a block diagram of the counter and its surrounding environment.
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Figure 21-2 Counter Unit Block Diagram
DATA BUS
TCNTn Control Logic
count
TOVn
(Int. Req.)
TOPBOTTOM
direction
clear
TOS C1
T/C
Oscillator
TOS C2
Pres ca le r
clkI/O
clk Tn
Signal description (internal signals):
count Increment or decrement TCNT0 by 1.
direction Selects between increment and decrement.
clear Clear TCNT0 (set all bits to zero).
clkT0 Timer/Counter clock.
TOP Signalizes that TCNT0 has reached maximum value.
BOTTOM Signalizes that TCNT0 has reached minimum value (zero).
Depending on the mode of operation used, the counter is cleared, incremented, or decremented at each
timer clock (clkT0). clkT0 can be generated from an external or internal clock source, selected by the clock
select bits (CS02:0). When no clock source is selected (CS02:0 = 0) the timer is stopped. However, the
TCNT0 value can be accessed by the CPU, regardless of whether clkT0 is present or not. A CPU write
overrides (has priority over) all counter clear or count operations.
The counting sequence is determined by the setting of the WGM01 and WGM00 bits located in the Timer/
Counter Control Register (TCCR0). There are close connections between how the counter behaves
(counts) and how waveforms are generated on the Output Compare Output OC0. For more details about
advanced counting sequences and waveform generation, refer to Modes of Operation on page 197 .
The Timer/Counter Overflow (TOV0) Flag is set according to the mode of operation selected by the
WGM01:0 bits. TOV0 can be used for generating a CPU interrupt.
21.5. Output Compare Unit
The 8-bit comparator continuously compares TCNT0 with the Output Compare Register (OCR0).
Whenever TCNT0 equals OCR0, the comparator signals a match. A match will set the Output Compare
Flag (OCF0) at the next timer clock cycle. If enabled (OCIE0 = 1), the Output Compare Flag generates an
Output Compare interrupt. The OCF0 Flag is automatically cleared when the interrupt is executed.
Alternatively, the OCF0 Flag can be cleared by software by writing a logical one to its I/O bit location. The
waveform generator uses the match signal to generate an output according to operating mode set by the
WGM01:0 bits and Compare Output mode (COM01:0) bits. The max and bottom signals are used by the
waveform generator for handling the special cases of the extreme values in some modes of operation
(refer to Modes of Operation on page 197).
The following figure shows a block diagram of the Output Compare unit.
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Figure 21-3 Output Compare Unit, Block Diagram
OCFn (Int. Re q.)
=(8-bit Comparator )
OCRn
OCxy
DATA BUS
TCNTn
WGMn1:0
Waveform Ge nerator
TOP
FOCn
COMn1:0
BOTTOM
The OCR0 Register is double buffered when using any of the Pulse Width Modulation (PWM) modes. For
the normal and Clear Timer on Compare (CTC) modes of operation, the double buffering is disabled. The
double buffering synchronizes the update of the OCR0 Compare Register to either top or bottom of the
counting sequence. The synchronization prevents the occurrence of odd-length, non-symmetrical PWM
pulses, thereby making the output glitch-free.
The OCR0 Register access may seem complex, but this is not case. When the double buffering is
enabled, the CPU has access to the OCR0 Buffer Register, and if double buffering is disabled the CPU
will access the OCR0 directly.
21.5.1. Force Output Compare
In non-PWM Waveform Generation modes, the match output of the comparator can be forced by writing a
one to the Force Output Compare (FOC0) bit. Forcing Compare Match will not set the OCF0 Flag or
reload/clear the timer, but the OC0 pin will be updated as if a real Compare Match had occurred (the
COM01:0 bits settings define whether the OC0 pin is set, cleared or toggled).
21.5.2. Compare Match Blocking by TCNT0 Write
All CPU write operations to the TCNT0 Register will block any Compare Match that occurs in the next
timer clock cycle, even when the timer is stopped. This feature allows OCR0 to be initialized to the same
value as TCNT0 without triggering an interrupt when the Timer/Counter clock is enabled.
21.5.3. Using the Output Compare Unit
Since writing TCNT0 in any mode of operation will block all compare matches for one timer clock cycle,
there are risks involved when changing TCNT0 when using the Output Compare channel, independently
of whether the Timer/Counter is running or not. If the value written to TCNT0 equals the OCR0 value, the
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Compare Match will be missed, resulting in incorrect waveform generation. Similarly, do not write the
TCNT0 value equal to BOTTOM when the counter is downcounting.
The setup of the OC0 should be performed before setting the Data Direction Register for the port pin to
output. The easiest way of setting the OC0 value is to use the Force Output Compare (FOC0) strobe bit
in Normal mode. The OC0 Register keeps its value even when changing between waveform generation
modes.
Be aware that the COM01:0 bits are not double buffered together with the compare value. Changing the
COM01:0 bits will take effect immediately.
21.6. Compare Match Output Unit
The Compare Output mode (COM01:0) bits have two functions. The waveform generator uses the
COM01:0 bits for defining the Output Compare (OC0) state at the next Compare Match. Also, the
COM01:0 bits control the OC0 pin output source. The figure below shows a simplified schematic of the
logic affected by the COM01:0 bit setting. The I/O Registers, I/O bits, and I/O pins in the figure are shown
in bold. Only the parts of the general I/O Port Control Registers (DDR and PORT) that are affected by the
COM01:0 bits are shown. When referring to the OC0 state, the reference is for the internal OC0 Register,
not the OC0 pin.
Figure 21-4 Compare Match Output Unit, Schematic
PORT
DDR
D Q
D Q
OCn
Pin
OCn
D Q
Waveform
Gene rator
COMn1
COMn0
0
1
DATABUS
FOCn
clkI/O
The general I/O port function is overridden by the Output Compare (OC0) from the waveform generator if
either of the COM01:0 bits are set. However, the OC0 pin direction (input or output) is still controlled by
the Data Direction Register (DDR) for the port pin. The Data Direction Register bit for the OC0 pin
(DDR_OC0) must be set as output before the OC0 value is visible on the pin. The port override function is
independent of the Waveform Generation mode.
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The design of the Output Compare Pin logic allows initialization of the OC0 state before the output is
enabled. Note that some COM01:0 bit settings are reserved for certain modes of operation. See Register
Description.
21.6.1. Compare Output Mode and Waveform Generation
The Waveform Generator uses the COM01:0 bits differently in normal, CTC, and PWM modes. For all
modes, setting the COM01:0 = 0 tells the waveform generator that no action on the OC0 Register is to be
performed on the next Compare Match. For compare output actions in the non-PWM modes refer to Table
21-3 Compare Output Mode, Non-PWM Mode on page 206. For fast PWM mode, refer to Table 21-4 
Compare Output Mode, Fast PWM Mode(1) on page 207, and for phase correct PWM refer to Table
21-5 Compare Output Mode, Phase Correct PWM Mode(1) on page 207.
A change of the COM01:0 bits state will have effect at the first Compare Match after the bits are written.
For non-PWM modes, the action can be forced to have immediate effect by using the FOC0 strobe bits.
21.7. Modes of Operation
The mode of operation (i.e., the behavior of the Timer/Counter and the Output Compare pins) is defined
by the combination of the Waveform Generation mode (WGM01:0) and Compare Output mode
(COM01:0) bits. The Compare Output mode bits do not affect the counting sequence, while the Waveform
Generation mode bits do. The COM01:0 bits control whether the PWM output generated should be
inverted or not (inverted or non-inverted PWM). For non-PWM modes the COM01:0 bits control whether
the output should be set, cleared, or toggled at a Compare Match (refer to Compare Match Output Unit on
page 196).
For detailed timing information refer to Timer/Counter Timing Diagrams on page 201.
21.7.1. Normal Mode
The simplest mode of operation is the Normal mode (WGM01:0 = 0). In this mode the counting direction
is always up (incrementing), and no counter clear is performed. The counter simply overruns when it
passes its maximum 8-bit value (TOP = 0xFF) and then restarts from the bottom (0x00). In normal
operation the Timer/Counter Overflow Flag (TOV0) will be set in the same timer clock cycle as the TCNT0
becomes zero. The TOV0 Flag in this case behaves like a ninth bit, except that it is only set, not cleared.
However, combined with the timer overflow interrupt that automatically clears the TOV0 Flag, the timer
resolution can be increased by software. There are no special cases to consider in the Normal mode, a
new counter value can be written anytime.
The Output Compare unit can be used to generate interrupts at some given time. Using the Output
Compare to generate waveforms in Normal mode is not recommended, since this will occupy too much of
the CPU time.
21.7.2. Clear Timer on Compare Match (CTC) Mode
In Clear Timer on Compare or CTC mode (WGM01:0 = 2), the OCR0 Register is used to manipulate the
counter resolution. In CTC mode the counter is cleared to zero when the counter value (TCNT0) matches
the OCR0. The OCR0 defines the top value for the counter, hence also its resolution. This mode allows
greater control of the Compare Match output frequency. It also simplifies the operation of counting
external events.
The timing diagram for the CTC mode is shown in the figure below. The counter value (TCNT0) increases
until a Compare Match occurs between TCNT0 and OCR0, and then counter (TCNT0) is cleared.
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Figure 21-5 CTC Mode, Timing Diagram
TCNTn
OCn
(Toggle)
OCn Inte rrupt Flag Se t
1 4
Pe riod 2 3
(COMn1:0 = 1)
An interrupt can be generated each time the counter value reaches the TOP value by using the OCF0
Flag. If the interrupt is enabled, the interrupt handler routine can be used for updating the TOP value.
However, changing the TOP to a value close to BOTTOM when the counter is running with none or a low
prescaler value must be done with care since the CTC mode does not have the double buffering feature.
If the new value written to OCR0 is lower than the current value of TCNT0, the counter will miss the
Compare Match. The counter will then have to count to its maximum value (0xFF) and wrap around
starting at 0x00 before the Compare Match can occur.
For generating a waveform output in CTC mode, the OC0 output can be set to toggle its logical level on
each Compare Match by setting the Compare Output mode bits to toggle mode (COM01:0 = 1). The OC0
value will not be visible on the port pin unless the data direction for the pin is set to output. The waveform
generated will have a maximum frequency of fOC0 = fclk_I/O/2 when OCR0 is set to zero (0x00). The
waveform frequency is defined by the following equation:
OCn =clk_I/O
2   1 + OCRn
The N variable represents the prescaler factor (1, 8, 32, 64, 128, 256, or 1024).
As for the Normal mode of operation, the TOV0 Flag is set in the same timer clock cycle that the counter
counts from MAX to 0x00.
21.7.3. Fast PWM Mode
The fast Pulse Width Modulation or fast PWM mode (WGM01:0 = 3) provides a high frequency PWM
waveform generation option. The fast PWM differs from the other PWM option by its single-slope
operation. The counter counts from BOTTOM to MAX then restarts from BOTTOM. In non-inverting
Compare Output mode, the Output Compare (OC0) is cleared on the Compare Match between TCNT0
and OCR0, and set at BOTTOM. In inverting Compare Output mode, the output is set on Compare Match
and cleared at BOTTOM. Due to the single-slope operation, the operating frequency of the fast PWM
mode can be twice as high as the phase correct PWM mode that uses dual-slope operation. This high
frequency makes the fast PWM mode well suited for power regulation, rectification, and DAC
applications. High frequency allows physically small sized external components (coils, capacitors), and
therefore reduces total system cost.
In fast PWM mode, the counter is incremented until the counter value matches the MAX value. The
counter is then cleared at the following timer clock cycle. The timing diagram for the fast PWM mode is
shown in the following figure. The TCNT0 value is in the timing diagram shown as a histogram for
illustrating the single-slope operation. The diagram includes non-inverted and inverted PWM outputs. The
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small horizontal line marks on the TCNT0 slopes represent compare matches between OCR0 and
TCNT0.
Figure 21-6 Fast PWM Mode, Timing Diagram
TCNTn
OCRn Upda te
and
TOVn Interrupt Fla g Se t
1
Pe riod 2 3
OCn
OCn
(COMn1:0 = 2)
(COMn1:0 = 3)
OCRn Inte rrupt Flag Se t
4 5 6 7
The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches MAX. If the interrupt is
enabled, the interrupt handler routine can be used for updating the compare value.
In fast PWM mode, the compare unit allows generation of PWM waveforms on the OC0 pin. Setting the
COM01:0 bits to 2 will produce a non-inverted PWM and an inverted PWM output can be generated by
setting the COM01:0 to 3. The actual OC0 value will only be visible on the port pin if the data direction for
the port pin is set as output. The PWM waveform is generated by setting (or clearing) the OC0 Register at
the Compare Match between OCR0 and TCNT0, and clearing (or setting) the OC0 Register at the timer
clock cycle the counter is cleared (changes from MAX to BOTTOM).
The PWM frequency for the output can be calculated by the following equation:
OCnPWM =clk_I/O
  256
The N variable represents the prescaler factor (1, 8, 32, 64, 128, 256, or 1024).
The extreme values for the OCR0 Register represent special cases when generating a PWM waveform
output in the fast PWM mode. If the OCR0 is set equal to BOTTOM, the output will be a narrow spike for
each MAX+1 timer clock cycle. Setting the OCR0 equal to MAX will result in a constantly high or low
output (depending on the polarity of the output set by the COM01:0 bits.)
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by setting OC0 to
toggle its logical level on each Compare Match (COM01:0 = 1). The waveform generated will have a
maximum frequency of foc0 = fclk_I/O/2 when OCR0 is set to zero. This feature is similar to the OC0 toggle
in CTC mode, except the double buffer feature of the Output Compare unit is enabled in the fast PWM
mode.
Related Links
TCCR0 on page 206
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21.7.4. Phase Correct PWM Mode
The phase correct PWM mode (WGM01:0 = 1) provides a high resolution phase correct PWM waveform
generation option. The phase correct PWM mode is based on a dual-slope operation. The counter counts
repeatedly from BOTTOM to MAX and then from MAX to BOTTOM. In non-inverting Compare Output
mode, the Output Compare (OC0) is cleared on the Compare Match between TCNT0 and OCR0 while
upcounting, and set on the Compare Match while downcounting. In inverting Output Compare mode, the
operation is inverted. The dual-slope operation has lower maximum operation frequency than single slope
operation. However, due to the symmetric feature of the dual-slope PWM modes, these modes are
preferred for motor control applications.
The PWM resolution for the phase correct PWM mode is fixed to eight bits. In phase correct PWM mode
the counter is incremented until the counter value matches MAX. When the counter reaches MAX, it
changes the count direction. The TCNT0 value will be equal to MAX for one timer clock cycle. The timing
diagram for the phase correct PWM mode is shown on the following figure. The TCNT0 value is in the
timing diagram shown as a histogram for illustrating the dual-slope operation. The diagram includes non-
inverted and inverted PWM outputs. The small horizontal line marks on the TCNT0 slopes represent
compare matches between OCR0 and TCNT0.
Figure 21-7 Phase Correct PWM Mode, Timing Diagram
TOVn Interrupt Flag S e t
OCn Interrupt Flag S e t
1 2 3
TCNTn
Pe riod
OCn
OCn
(COMn1:0 = 2)
(COMn1:0 = 3)
OCRn Update
The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches BOTTOM. The Interrupt
Flag can be used to generate an interrupt each time the counter reaches the BOTTOM value.
In phase correct PWM mode, the compare unit allows generation of PWM waveforms on the OC0 pin.
Setting the COM01:0 bits to 2 will produce a non-inverted PWM. An inverted PWM output can be
generated by setting the COM01:0 to 3 (refer to table Compare Output Mode, Phase Correct PWM
Mode). The actual OC0 value will only be visible on the port pin if the data direction for the port pin is set
as output. The PWM waveform is generated by clearing (or setting) the OC0 Register at the Compare
Match between OCR0 and TCNT0 when the counter increments, and setting (or clearing) the OC0
Register at Compare Match between OCR0 and TCNT0 when the counter decrements. The PWM
frequency for the output when using phase correct PWM can be calculated by the following equation:
OCnPCPWM =clk_I/O
  510
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The N variable represents the prescaler factor (1, 8, 32, 64, 128, 256, or 1024).
The extreme values for the OCR0 Register represent special cases when generating a PWM waveform
output in the phase correct PWM mode. If the OCR0 is set equal to BOTTOM, the output will be
continuously low and if set equal to MAX the output will be continuously high for non-inverted PWM
mode. For inverted PWM the output will have the opposite logic values.
At the very start of period 2 in the timing diagram above OCn has a transition from high to low even
though there is no Compare Match. The point of this transition is to guarantee symmetry around
BOTTOM. There are two cases that give a transition without Compare Match:
• OCR0 changes its value from MAX, like in the timing diagram above. When the OCR0 value is MAX the
OCn pin value is the same as the result of a down-counting Compare Match. To ensure symmetry around
BOTTOM the OCn value at MAX must correspond to the result of an up-counting Compare Match.
• The timer starts counting from a value higher than the one in OCR0, and for that reason misses the
Compare Match and hence the OCn change that would have happened on the way up.
21.8. Timer/Counter Timing Diagrams
The following figures show the Timer/Counter in Synchronous mode, and the timer clock (clkT0) is
therefore shown as a clock enable signal. In Asynchronous mode, clkI/O should be replaced by the Timer/
Counter Oscillator clock. The figures include information on when Interrupt Flags are set. The following
figure contains timing data for basic Timer/Counter operation. The figure shows the count sequence close
to the MAX value in all modes other than phase correct PWM mode.
Figure 21-8 Timer/Counter Timing Diagram, no Prescaling
clkTn
(clkI/O/1)
TOVn
clkI/O
TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1
The next figure shows the same timing data, but with the prescaler enabled.
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Figure 21-9 Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)
TOVn
TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1
clkI/O
clkTn
(clkI/O/8)
The next figure shows the setting of OCF0 in all modes except CTC mode.
Figure 21-10 Timer/Counter Timing Diagram, Setting of OCF0, with Prescaler (fclk_I/O/8)
OCFn
OCRn
TCNTn
OCRn Value
OCRn - 1 OCRn OCRn + 1 OCRn + 2
clkI/O
clkTn
(clkI/O/8)
The figure below shows the setting of OCF0 and the clearing of TCNT0 in CTC mode.
Figure 21-11 Timer/Counter Timing Diagram, Clear Timer on Compare Match Mode, with Prescaler (fclk_I/O/8)
OCFn
OCRn
TCNTn
(CTC)
TOP
TOP - 1 TOP BOTTOM BOTTOM + 1
clkI/O
clkTn
(clkI/O/8)
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21.9. Asynchronous Operation of the Timer/Counter
21.9.1. Asynchronous Operation of Timer/Counter0
When Timer/Counter0 operates asynchronously, some considerations must be taken.
•Warning: When switching between asynchronous and synchronous clocking of Timer/Counter0, the
Timer Registers TCNT0, OCR0, and TCCR0 might be corrupted. A safe procedure for switching
clock source is:
1. Disable the Timer/Counter0 interrupts by clearing OCIE0 and TOIE0.
2. Select clock source by setting AS0 as appropriate.
3. Write new values to TCNT0, OCR0, and TCCR0.
4. To switch to asynchronous operation: Wait for TCN0UB, OCR0UB, and TCR0UB.
5. Clear the Timer/Counter0 Interrupt Flags.
6. Enable interrupts, if needed.
•The Oscillator is optimized for use with a 32.768kHz watch crystal. Applying an external clock to the
TOSC1 pin may result in incorrect Timer/Counter0 operation. The CPU main clock frequency must
be more than four times the Oscillator frequency.
•When writing to one of the registers TCNT0, OCR0, or TCCR0, the value is transferred to a
temporary register, and latched after two positive edges on TOSC1. The user should not write a
new value before the contents of the temporary register have been transferred to its destination.
Each of the three mentioned registers have their individual temporary register, which means that
e.g. writing to TCNT0 does not disturb an OCR0 write in progress. To detect that a transfer to the
destination register has taken place, the Asynchronous Status Register – ASSR has been
implemented.
•When entering Power-save mode after having written to TCNT0, OCR0, or TCCR0, the user must
wait until the written register has been updated if Timer/Counter0 is used to wake up the device.
Otherwise, the MCU will enter sleep mode before the changes are effective. This is particularly
important if the Output Compare0 interrupt is used to wake up the device, since the Output
Compare function is disabled during writing to OCR0 or TCNT0. If the write cycle is not finished,
and the MCU enters sleep mode before the OCR0UB bit returns to zero, the device will never
receive a Compare Match interrupt, and the MCU will not wake up.
•If Timer/Counter0 is used to wake the device up from Power-save or Extended Standby mode,
precautions must be taken if the user wants to re-enter one of these modes: The interrupt logic
needs one TOSC1 cycle to be reset. If the time between wake-up and re-entering sleep mode is
less than one TOSC1 cycle, the interrupt will not occur, and the device will fail to wake up. If the
user is in doubt whether the time before re-entering Power-save or Extended Standby mode is
sufficient, the following algorithm can be used to ensure that one TOSC1 cycle has elapsed:
1. Write a value to TCCR0, TCNT0, or OCR0.
2. Wait until the corresponding Update Busy Flag in ASSR returns to zero.
3. Enter Power-save or Extended Standby mode.
•When the asynchronous operation is selected, the 32.768kHz Oscillator for Timer/Counter0 is
always running, except in Power-down and Standby modes. After a Power-up Reset or Wake-up
from Power-down or Standby mode, the user should be aware of the fact that this Oscillator might
take as long as one second to stabilize. The user is advised to wait for at least one second before
using Timer/Counter0 after Power-up or Wake-up from Power-down or Standby mode. The
contents of all Timer/Counter0 Registers must be considered lost after a wake-up from Power-down
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or Standby mode due to unstable clock signal upon start-up, no matter whether the Oscillator is in
use or a clock signal is applied to the TOSC1 pin.
•Description of wake up from Power-save or Extended Standby mode when the timer is clocked
asynchronously: When the interrupt condition is met, the wake up process is started on the
following cycle of the timer clock, that is, the timer is always advanced by at least one before the
processor can read the counter value. After wake-up, the MCU is halted for four cycles, it executes
the interrupt routine, and resumes execution from the instruction following SLEEP.
•Reading of the TCNT0 Register shortly after wake-up from Power-save may give an incorrect
result. Since TCNT0 is clocked on the asynchronous TOSC clock, reading TCNT0 must be done
through a register synchronized to the internal I/O clock domain. Synchronization takes place for
every rising TOSC1 edge. When waking up from Power-save mode, and the I/O clock (clkI/O) again
becomes active, TCNT0 will read as the previous value (before entering sleep) until the next rising
TOSC1 edge. The phase of the TOSC clock after waking up from Power-save mode is essentially
unpredictable, as it depends on the wake-up time. The recommended procedure for reading
TCNT0 is thus as follows:
1. Write any value to either of the registers OCR0 or TCCR0.
2. Wait for the corresponding Update Busy Flag to be cleared.
3. Read TCNT0.
•During asynchronous operation, the synchronization of the Interrupt Flags for the asynchronous
timer takes three processor cycles plus one timer cycle. The timer is therefore advanced by at least
one before the processor can read the timer value causing the setting of the Interrupt Flag. The
Output Compare Pin is changed on the timer clock and is not synchronized to the processor clock.
21.10. Timer/Counter Prescaler
Figure 21-12 Prescaler for Timer/Counter0
10-BIT T/C PRESCALER
TIMER/COUNTER2 CLOCK SOURCE
clkI/O clkT2S
TOS C1
AS2
CS 20
CS 21
CS 22
clkT2S
/8
clkT2S
/64
clkT2S
/128
clkT2S
/1024
clkT2S
/256
clkT2S
/32
0
PS R2
Clea r
clkT2
The clock source for Timer/Counter0 is named clkT0S. clkT0S is by default connected to the main system
clock clkI/O. By setting the AS0 bit in ASSR, Timer/Counter0 is asynchronously clocked from the TOSC1
pin. This enables use of Timer/Counter0 as a Real Time Counter (RTC). When AS0 is set, pins TOSC1
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and TOSC2 are disconnected from Port C. A crystal can then be connected between the TOSC1 and
TOSC2 pins to serve as an independent clock source for Timer/Counter0. The Oscillator is optimized for
use with a 32.768kHz crystal. Applying an external clock source to TOSC1 is not recommended.
For Timer/Counter0, the possible prescaled selections are: clkT0S/8, clkT0S/32, clkT0S/64, clkT0S/128,
clkT0S/256, and clkT0S/1024. Additionally, clkT0S as well as 0 (stop) may be selected. Setting the PSR0 bit
in SFIOR resets the prescaler. This allows the user to operate with a predictable prescaler.
21.11. Register Description
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21.11.1. TCCR0 – Timer/Counter Control Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  TCCR0
Offset:  0x33
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x53
Bit 7 6 5 4 3 2 1 0
FOC0 WGM01 COM01 COM00 WGM00 CS02 CS01 CS00
Access W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bit 7 – FOC0: Force Output Compare
The FOC0 bit is only active when the WGM bits specify a non-PWM mode. However, for ensuring
compatibility with future devices, this bit must be set to zero when TCCR0 is written when operating in
PWM mode. When writing a logical one to the FOC0 bit, an immediate Compare Match is forced on the
waveform generation unit. The OC0 output is changed according to its COM01:0 bits setting. Note that
the FOC0 bit is implemented as a strobe. Therefore it is the value present in the COM01:0 bits that
determines the effect of the forced compare.
A FOC0 strobe will not generate any interrupt, nor will it clear the timer in CTC mode using OCR0 as
TOP.
The FOC0 bit is always read as zero.
Bits 5:4 – COM0n: Compare Match Output Mode [n = 1:0]
These bits control the Output Compare Pin (OC0) behavior. If one or both of the COM01:0 bits are set,
the OC0 output overrides the normal port functionality of the I/O pin it is connected to. However, note that
the Data Direction Register (DDR) bit corresponding to OC0 pin must be set in order to enable the output
driver.
When OC0 is connected to the pin, the function of the COM01:0 bits depends on the WGM01:0 bit
setting. The following table shows the COM01:0 bit functionality when the WGM01:0 bits are set to a
normal or CTC mode (non-PWM).
Table 21-3 Compare Output Mode, Non-PWM Mode
COM01 COM00 Description
0 0 Normal port operation, OC0 disconnected.
0 1 Toggle OC0 on Compare Match
1 0 Clear OC0 on Compare Match
1 1 Set OC0 on Compare Match
The next table shows the COM01:0 bit functionality when the WGM01:0 bits are set to fast PWM mode.
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Table 21-4 Compare Output Mode, Fast PWM Mode(1)
COM01 COM00 Description
0 0 Normal port operation, OC0 disconnected.
0 1 Reserved
1 0 Clear OC0 on Compare Match, set OC0 at BOTTOM,
(non-inverting mode)
1 1 Set OC0 on Compare Match, clear OC0 at BOTTOM,
(inverting mode)
Note:  1. A special case occurs when OCR0 equals TOP and COM01 is set. In this case, the Compare
Match is ignored, but the set or clear is done at BOTTOM. Refer to Fast PWM Mode on page 198 for
more details.
The table below shows the COM01:0 bit functionality when the WGM01:0 bits are set to phase correct
PWM mode.
Table 21-5 Compare Output Mode, Phase Correct PWM Mode(1)
COM01 COM00 Description
0 0 Normal port operation, OC0 disconnected.
0 1 Reserved
1 0 Clear OC0 on Compare Match when up-counting. Set OC0 on Compare Match when
downcounting.
1 1 Set OC0 on Compare Match when up-counting. Clear OC0 on Compare Match when
downcounting.
Note:  1. A special case occurs when OCR0 equals TOP and COM01 is set. In this case, the Compare
Match is ignored, but the set or clear is done at TOP. Refer to Phase Correct PWM Mode on page 200 for
more details.
Bits 2:0 – CS0n: Clock Select [n = 2:0]
The three Clock Select bits select the clock source to be used by the Timer/Counter.
Table 21-6 Clock Select Bit Description
CS02 CS01 CS00 Description
0 0 0 No clock source (Timer/Counter stopped).
0 0 1 clkI/O/ (No prescaling)
0 1 0 clkI/O/8 (From prescaler)
0 1 1 clkI/O/32 (From prescaler)
1 0 0 clkI/O/64 (From prescaler)
1 0 1 clkI/O/128 (From prescaler)
1 1 0 clkI/O/256 (From prescaler)
1 1 1 clkI/O/1024 (From prescaler)
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If external pin modes are used for the Timer/Counter2, transitions on the T2 pin will clock the counter
even if the pin is configured as an output. This feature allows software control of the counting.
Bits 6,3 – WGM0n: Waveform Generation Mode [n=0:1]
These bits control the counting sequence of the counter, the source for the maximum (TOP) counter
value, and what type of waveform generation to be used. Modes of operation supported by the Timer/
Counter unit are: Normal mode, Clear Timer on Compare Match (CTC) mode, and two types of Pulse
Width Modulation (PWM) modes. See table below and Modes of Operation.
Table 21-2 Waveform Generation Mode Bit Description
Mode WGM01
(CTC0)
WGM00
(PWM0)
Timer/Counter Mode of Operation(1) TOP Update of
OCR0
TOV0 Flag
Set
0 0 0 Normal 0xFF Immediate MAX
1 0 1 PWM, Phase Correct 0xFF TOP BOTTOM
2 1 0 CTC OCR0 Immediate MAX
3 1 1 Fast PWM 0xFF BOTTOM MAX
Note:  1. The CTC0 and PWM0 bit definition names are now obsolete. Use the WGM01:0 definitions.
However, the functionality and location of these bits are compatible with previous versions of the timer.
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21.11.2. TCNT0 – Timer/Counter Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
The Timer/Counter Register gives direct access, both for read and write operations, to the Timer/Counter
unit 8-bit counter. Writing to the TCNT0 Register blocks (removes) the Compare Match on the following
timer clock. Modifying the counter (TCNT0) while the counter is running, introduces a risk of missing a
Compare Match between TCNT0 and the OCR0 Register.
Name:  TCNT0
Offset:  0x32
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x52
Bit 7 6 5 4 3 2 1 0
TCNT0[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – TCNT0[7:0]
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21.11.3. OCR0 – Output Compare Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
The Output Compare Register contains an 8-bit value that is continuously compared with the counter
value (TCNT0). A match can be used to generate an Output Compare interrupt, or to generate a
waveform output on the OC0 pin.
Name:  OCR0
Offset:  0x31
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x51
Bit 7 6 5 4 3 2 1 0
OCR0[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – OCR0[7:0]
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21.11.4. ASSR – Asynchronous Status Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  ASSR
Offset:  0x30
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x50
Bit 7 6 5 4 3 2 1 0
AS0 TCN0UB OCR0UB TCR0UB
Access R/W R R R
Reset 0 0 0 0
Bit 3 – AS0: Asynchronous Timer/Counter0
When AS0 is written to zero, Timer/Counter0 is clocked from the I/O clock, clkI/O. When AS0 is written to
one, Timer/Counter0 is clocked from a crystal Oscillator connected to the Timer Oscillator 1 (TOSC1) pin.
When the value of AS0 is changed, the contents of TCNT0, OCR0, and TCCR0 might be corrupted.
Bit 2 – TCN0UB: Timer/Counter0 Update Busy
When Timer/Counter0 operates asynchronously and TCNT0 is written, this bit becomes set. When
TCNT0 has been updated from the temporary storage register, this bit is cleared by hardware. A logical
zero in this bit indicates that TCNT0 is ready to be updated with a new value.
Bit 1 – OCR0UB: Output Compare Register0 Update Busy
When Timer/Counter0 operates asynchronously and OCR0 is written, this bit becomes set. When OCR0
has been updated from the temporary storage register, this bit is cleared by hardware. A logical zero in
this bit indicates that OCR0 is ready to be updated with a new value.
Bit 0 – TCR0UB: Timer/Counter Control Register0 Update Busy
When Timer/Counter0 operates asynchronously and TCCR0 is written, this bit becomes set. When
TCCR0 has been updated from the temporary storage register, this bit is cleared by hardware. A logical
zero in this bit indicates that TCCR0 is ready to be updated with a new value.
If a write is performed to any of the three Timer/Counter0 Registers while its update busy flag is set, the
updated value might get corrupted and cause an unintentional interrupt to occur.
The mechanisms for reading TCNT0, OCR0, and TCCR0 are different. When reading TCNT0, the actual
timer value is read. When reading OCR0 or TCCR0, the value in the temporary storage register is read.
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21.11.5. TIMSK – Timer/Counter Interrupt Mask Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  TIMSK
Offset:  0x37
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x57
Bit 7 6 5 4 3 2 1 0
OCIE0 TOIE0
Access R/W R/W
Reset 0 0
Bit 1 – OCIE0: Timer/Counter0 Output Compare Match Interrupt Enable
When the OCIE0 bit is written to one and the I-bit in the Status Register is set (one), the Timer/Counter0
Compare Match interrupt is enabled. The corresponding interrupt is executed if a Compare Match in
Timer/Counter0 occurs (i.e., when the OCF0 bit is set in the Timer/Counter Interrupt Flag Register –
TIFR).
Bit 0 – TOIE0: Timer/Counter0 Overflow Interrupt Enable
When the TOIE0 bit is written to one and the I-bit in the Status Register is set (one), the Timer/Counter0
Overflow interrupt is enabled. The corresponding interrupt is executed if an overflow in Timer/Counter0
occurs (i.e., when the TOV0 bit is set in the Timer/Counter Interrupt Flag Register – TIFR).
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21.11.6. TIFR – Timer/Counter Interrupt Flag Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  TIFR
Offset:  0x36
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x56
Bit 7 6 5 4 3 2 1 0
OCF0 TOV0
Access R/W R/W
Reset 0 0
Bit 1 – OCF0: Output Compare Flag 0
The OCF0 bit is set (one) when a Compare Match occurs between the Timer/Counter0 and the data in
OCR0 – Output Compare Register0. OCF0 is cleared by hardware when executing the corresponding
interrupt Handling Vector. Alternatively, OCF0 is cleared by writing a logic one to the flag. When the I-bit
in SREG, OCIE0 (Timer/Counter0 Compare Match Interrupt Enable), and OCF0 are set (one), the Timer/
Counter0 Compare Match Interrupt is executed.
Bit 0 – TOV0: Timer/Counter0 Overflow Flag
The TOV0 bit is set (one) when an overflow occurs in Timer/Counter0. TOV0 is cleared by hardware
when executing the corresponding interrupt Handling Vector. Alternatively, TOV0 is cleared by writing a
logic one to the flag. When the SREG I-bit, TOIE0 (Timer/Counter0 Overflow Interrupt Enable), and TOV0
are set (one), the Timer/Counter0 Overflow interrupt is executed. In PWM mode, this bit is set when
Timer/Counter0 changes counting direction at 0x00.
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21.11.7. SFIOR – Special Function IO Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  SFIOR
Offset:  0x20
Reset:  0
Property:
 
When addressing I/O Registers as data space the offset address is 0x40
Bit 7 6 5 4 3 2 1 0
TSM PSR0
Access R/W R/W
Reset 0 0
Bit 7 – TSM: Timer/Counter Synchronization Mode
Writing the TSM bit to one activates the Timer/Counter Synchronization mode. In this mode, the value
that is written to the PSR0 and PSR321 bits is kept, hence keeping the corresponding prescaler reset
signals asserted. This ensures that the corresponding Timer/Counters are halted and can be configured
to the same value without the risk of one of them advancing during configuration. When the TSM bit is
written to zero, the PSR0 and PSR321 bits are cleared by hardware, and the Timer/Counters start
counting simultaneously.
Bit 1 – PSR0: Prescaler Reset Timer/Counter0
When this bit is written to one, the Timer/Counter0 prescaler will be reset. The bit will be cleared by
hardware after the operation is performed. Writing a zero to this bit will have no effect. This bit will always
be read as zero if Timer/Counter0 is clocked by the internal CPU clock. If this bit is written when Timer/
Counter0 is operating in Asynchronous mode, the bit will remain one until the prescaler has been reset.
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22. 8-bit Timer/Counter2 with PWM
22.1. Features
•Single Channel Counter
•Clear Timer on Compare Match (Auto Reload)
•Glitch-free, phase Correct Pulse Width Modulator (PWM)
•Frequency Generator
•External Event Counter
•10-bit Clock Prescaler
•Overflow and Compare Match Interrupt Sources (TOV2 and OCF2)
22.2. Overview
Timer/Counter2 is a general purpose, single channel, 8-bit Timer/Counter module. A simplified block
diagram of the 8-bit Timer/Counter is shown in the figure below. For the actual placement of I/O pins, refer
to Pin Configurations. CPU accessible I/O Registers, including I/O bits and I/O pins, are shown in bold.
The device-specific I/O Register and bit locations are listed in the Register Description on page 225.
Figure 22-1 8-bit Timer/Counter Block Diagram
Timer/Counte r
DATA BUS
=
TCNTn
Waveform
Gene ration OCn
= 0
Control Logic
= 0xFF
TOPBOTTOM
count
clear
direction
TOVn
(Int. Req.)
OCn
(Int. Req.)
OCRn
TCCRn
clkTn
Edge
Detector
(From Prescaler)
Tn
Clock Select
Related Links
Pin Configurations on page 14
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22.2.1. Registers
The Timer/Counter (TCNT2) and Output Compare Register (OCR2) are 8-bit registers. Interrupt request
(abbreviated to Int.Req. in the figure) signals are all visible in the Timer Interrupt Flag Register (TIFR). All
interrupts are individually masked with the Timer Interrupt Mask Register (TIMSK). TIFR and TIMSK are
not shown in the figure since these registers are shared by other timer units.
The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source on the T2
pin. The Clock Select logic block controls which clock source and edge the Timer/Counter uses to
increment (or decrement) its value. The Timer/Counter is inactive when no clock source is selected. The
output from the clock select logic is referred to as the timer clock (clkT2).
The double buffered Output Compare Register (OCR2) is compared with the Timer/Counter value at all
times. The result of the compare can be used by the waveform generator to generate a PWM or variable
frequency output on the Output Compare Pin (OC2). For details, refer to Output Compare Unit on page
217. The Compare Match event will also set the Compare Flag (OCF2) which can be used to generate an
Output Compare interrupt request.
22.2.2. Definitions
Many register and bit references in this document are written in general form. A lower case “n” replaces
the Timer/Counter number, in this case 2. However, when using the register or bit defines in a program,
the precise form must be used (i.e., TCNT2 for accessing Timer/Counter2 counter value and so on).
The definitions in the following table are also used extensively throughout the document.
Table 22-1 Definitions
BOTTOM The counter reaches the BOTTOM when it becomes zero (0x00).
MAX The counter reaches its MAXimum when it becomes 0xFF (decimal 255).
TOP The counter reaches the TOP when it becomes equal to the highest value in the
count sequence. The TOP value can be assigned to be the fixed value 0xFF (MAX)
or the value stored in the OCR2 Register. The assignment is dependent on the
mode of operation.
22.3. Timer/Counter Clock Sources
The Timer/Counter can be clocked by an internal or an external clock source. The clock source is
selected by the clock select logic which is controlled by the clock select (CS22:0) bits located in the
Timer/Counter Control Register (TCCR2). For details on clock sources and prescaler, see Timer/
Counter3, Timer/Counter2, and Timer/Counter1 Prescalers.
Related Links
Timer/Counter3, Timer/Counter2, and Timer/Counter1 Prescalers on page 134
22.4. Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. The following
figure shows a block diagram of the counter and its surrounding environment.
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Figure 22-2 Counter Unit Block Diagram
DATA BUS
TCNTn Control Logic
count
TOVn
(Int. Req.)
TOPBOTTOM
direction
clear
(From Prescaler)
Tn
Clock Select
Edge
Detector
Signal description (internal signals):
count Increment or decrement TCNT2 by 1.
direction Selects between increment and decrement.
clear Clear TCNT2 (set all bits to zero).
clkT2 Timer/Counter clock.
TOP Signalizes that TCNT2 has reached maximum value.
BOTTOM Signalizes that TCNT2 has reached minimum value (zero).
Depending on the mode of operation used, the counter is cleared, incremented, or decremented at each
timer clock (clkT2). clkT2 can be generated from an external or internal clock source, selected by the clock
select bits (CS22:0). When no clock source is selected (CS22:0 = 0) the timer is stopped. However, the
TCNT2 value can be accessed by the CPU, regardless of whether clkT2 is present or not. A CPU write
overrides (has priority over) all counter clear or count operations.
The counting sequence is determined by the setting of the WGM01 and WGM00 bits located in the Timer/
Counter Control Register (TCCR2). There are close connections between how the counter behaves
(counts) and how waveforms are generated on the Output Compare Output OC2. For more details about
advanced counting sequences and waveform generation, see Modes of Operation on page 220.
The Timer/Counter Overflow (TOV2) Flag is set according to the mode of operation selected by the
WGM21:0 bits. TOV2 can be used for generating a CPU interrupt.
22.5. Output Compare Unit
The 8-bit comparator continuously compares TCNT2 with the Output Compare Register (OCR2).
Whenever TCNT2 equals OCR2, the comparator signals a match. A match will set the Output Compare
Flag (OCF2) at the next timer clock cycle. If enabled (OCIE2 = 1 and global interrupt flag in SREG is set),
the Output Compare Flag generates an Output Compare interrupt. The OCF2 Flag is automatically
cleared when the interrupt is executed. Alternatively, the OCF2 Flag can be cleared by software by writing
a logical one to its I/O bit location. The waveform generator uses the match signal to generate an output
according to operating mode set by the WGM21:0 bits and Compare Output mode (COM21:0) bits. The
max and bottom signals are used by the waveform generator for handling the special cases of the
extreme values in some modes of operation (see Modes of Operation on page 220).
The following figure shows a block diagram of the Output Compare unit.
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Figure 22-3 Output Compare Unit, Block Diagram
OCFn (Int. Req.)
=(8-bit Compa rator )
OCRn
OCn
DATA BUS
TCNTn
WGMn1:0
Waveform Generator
TOP
FOCn
COMn1:0
BOTTOM
The OCR2 Register is double buffered when using any of the Pulse Width Modulation (PWM) modes. For
the normal and Clear Timer on Compare (CTC) modes of operation, the double buffering is disabled. The
double buffering synchronizes the update of the OCR2 Compare Register to either top or bottom of the
counting sequence. The synchronization prevents the occurrence of odd-length, non-symmetrical PWM
pulses, thereby making the output glitch-free.
The OCR2 Register access may seem complex, but this is not case. When the double buffering is
enabled, the CPU has access to the OCR2 Buffer Register, and if double buffering is disabled the CPU
will access the OCR2 directly.
22.5.1. Force Output Compare
In non-PWM Waveform Generation modes, the match output of the comparator can be forced by writing a
one to the Force Output Compare (FOC2) bit. Forcing Compare Match will not set the OCF2 Flag or
reload/clear the timer, but the OC2 pin will be updated as if a real Compare Match had occurred (the
COM21:0 bits settings define whether the OC2 pin is set, cleared or toggled).
22.5.2. Compare Match Blocking by TCNT2 Write
All CPU write operations to the TCNT2 Register will block any Compare Match that occurs in the next
timer clock cycle, even when the timer is stopped. This feature allows OCR2 to be initialized to the same
value as TCNT2 without triggering an interrupt when the Timer/Counter clock is enabled.
22.5.3. Using the Output Compare Unit
Since writing TCNT2 in any mode of operation will block all compare matches for one timer clock cycle,
there are risks involved when changing TCNT2 when using the Output Compare channel, independently
of whether the Timer/Counter is running or not. If the value written to TCNT2 equals the OCR2 value, the
Compare Match will be missed, resulting in incorrect waveform generation. Similarly, do not write the
TCNT2 value equal to BOTTOM when the counter is downcounting.
The setup of the OC2 should be performed before setting the Data Direction Register for the port pin to
output. The easiest way of setting the OC2 value is to use the Force Output Compare (FOC2) strobe bit
in Normal mode. The OC2 Register keeps its value even when changing between waveform generation
modes.
Be aware that the COM21:0 bits are not double buffered together with the compare value. Changing the
COM21:0 bits will take effect immediately.
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22.6. Compare Match Output Unit
The Compare Output mode (COM21:0) bits have two functions. The waveform generator uses the
COM21:0 bits for defining the Output Compare (OC2) state at the next Compare Match. Also, the
COM21:0 bits control the OC2 pin output source. The figure below shows a simplified schematic of the
logic affected by the COM21:0 bit setting. The I/O Registers, I/O bits, and I/O pins in the figure are shown
in bold. Only the parts of the general I/O Port Control Registers (DDR and PORT) that are affected by the
COM21:0 bits are shown. When referring to the OC2 state, the reference is for the internal OC2 Register,
not the OC2 pin. If a System Reset occur, the OC2 Register is reset to "0".
Figure 22-4 Compare Match Output Unit, Schematic
PORT
DDR
D Q
D Q
OCn
Pin
OCn
D Q
Waveform
Generator
COMn1
COMn0
0
1
DATABUS
FOCn
clkI/O
The general I/O port function is overridden by the Output Compare (OC2) from the waveform generator if
either of the COM21:0 bits are set. However, the OC2 pin direction (input or output) is still controlled by
the Data Direction Register (DDR) for the port pin. The Data Direction Register bit for the OC2 pin
(DDR_OC2) must be set as output before the OC2 value is visible on the pin. The port override function is
independent of the Waveform Generation mode.
The design of the Output Compare Pin logic allows initialization of the OC2 state before the output is
enabled. Note that some COM21:0 bit settings are reserved for certain modes of operation. See Register
Description on page 225.
22.6.1. Compare Output Mode and Waveform Generation
The Waveform Generator uses the COM21:0 bits differently in normal, CTC, and PWM modes. For all
modes, setting the COM21:0 = 0 tells the waveform generator that no action on the OC2 Register is to be
performed on the next Compare Match. For compare output actions in the non-PWM modes refer to Table
22-3 Compare Output Mode, Non-PWM Mode on page 227. For fast PWM mode, refer to Table 22-4 
Compare Output Mode, Fast PWM Mode(1) on page 227, and for phase correct PWM refer to Table
22-5 Compare Output Mode, Phase Correct PWM Mode(1) on page 227.
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A change of the COM21:0 bits state will have effect at the first Compare Match after the bits are written.
For non-PWM modes, the action can be forced to have immediate effect by using the FOC2 strobe bits.
22.7. Modes of Operation
The mode of operation (i.e., the behavior of the Timer/Counter and the Output Compare pins) is defined
by the combination of the Waveform Generation mode (WGM21:0) and Compare Output mode
(COM21:0) bits. The Compare Output mode bits do not affect the counting sequence, while the Waveform
Generation mode bits do. The COM21:0 bits control whether the PWM output generated should be
inverted or not (inverted or non-inverted PWM). For non-PWM modes the COM21:0 bits control whether
the output should be set, cleared, or toggled at a Compare Match (see Compare Match Output Unit).
For detailed timing information refer to Timer/Counter Timing Diagrams.
22.7.1. Normal Mode
The simplest mode of operation is the Normal mode (WGM21:0 = 0). In this mode the counting direction
is always up (incrementing), and no counter clear is performed. The counter simply overruns when it
passes its maximum 8-bit value (TOP = 0xFF) and then restarts from the bottom (0x00). In normal
operation the Timer/Counter Overflow Flag (TOV2) will be set in the same timer clock cycle as the TCNT2
becomes zero. The TOV2 Flag in this case behaves like a ninth bit, except that it is only set, not cleared.
However, combined with the timer overflow interrupt that automatically clears the TOV2 Flag, the timer
resolution can be increased by software. There are no special cases to consider in the Normal mode, a
new counter value can be written anytime.
The Output Compare unit can be used to generate interrupts at some given time. Using the Output
Compare to generate waveforms in Normal mode is not recommended, since this will occupy too much of
the CPU time.
22.7.2. Clear Timer on Compare Match (CTC) Mode
In Clear Timer on Compare or CTC mode (WGM21:0 = 2), the OCR2 Register is used to manipulate the
counter resolution. In CTC mode the counter is cleared to zero when the counter value (TCNT2) matches
the OCR2. The OCR2 defines the top value for the counter, hence also its resolution. This mode allows
greater control of the Compare Match output frequency. It also simplifies the operation of counting
external events.
The timing diagram for the CTC mode is shown in the figure below. The counter value (TCNT2) increases
until a Compare Match occurs between TCNT2 and OCR2, and then counter (TCNT2) is cleared.
Figure 22-5 CTC Mode, Timing Diagram
TCNTn
OCn
(Toggle)
OCn Inte rrupt Flag Se t
1 4
Pe riod 2 3
(COMn1:0 = 1)
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An interrupt can be generated each time the counter value reaches the TOP value by using the OCF2
Flag. If the interrupt is enabled, the interrupt handler routine can be used for updating the TOP value.
However, changing the TOP to a value close to BOTTOM when the counter is running with none or a low
prescaler value must be done with care since the CTC mode does not have the double buffering feature.
If the new value written to OCR2 is lower than the current value of TCNT2, the counter will miss the
Compare Match. The counter will then have to count to its maximum value (0xFF) and wrap around
starting at 0x00 before the Compare Match can occur.
For generating a waveform output in CTC mode, the OC2 output can be set to toggle its logical level on
each Compare Match by setting the Compare Output mode bits to toggle mode (COM21:0 = 1). The OC2
value will not be visible on the port pin unless the data direction for the pin is set to output. The waveform
generated will have a maximum frequency of fOC2 = fclk_I/O/2 when OCR2 is set to zero (0x00). The
waveform frequency is defined by the following equation:
OCn =clk_I/O
2   1 + OCRn
The N variable represents the prescaler factor (1, 8, 64, 256, or 1024).
As for the Normal mode of operation, the TOV2 Flag is set in the same timer clock cycle that the counter
counts from MAX to 0x00.
22.7.3. Fast PWM Mode
The fast Pulse Width Modulation or fast PWM mode (WGM21:0 = 3) provides a high frequency PWM
waveform generation option. The fast PWM differs from the other PWM option by its single-slope
operation. The counter counts from BOTTOM to MAX then restarts from BOTTOM. In non-inverting
Compare Output mode, the Output Compare (OC2) is cleared on the Compare Match between TCNT2
and OCR2, and set at BOTTOM. In inverting Compare Output mode, the output is set on Compare Match
and cleared at BOTTOM. Due to the single-slope operation, the operating frequency of the fast PWM
mode can be twice as high as the phase correct PWM mode that uses dual-slope operation. This high
frequency makes the fast PWM mode well suited for power regulation, rectification, and DAC
applications. High frequency allows physically small sized external components (coils, capacitors), and
therefore reduces total system cost.
In fast PWM mode, the counter is incremented until the counter value matches the MAX value. The
counter is then cleared at the following timer clock cycle. The timing diagram for the fast PWM mode is
shown in the figure below. The TCNT2 value is in the timing diagram shown as a histogram for illustrating
the single-slope operation. The diagram includes non-inverted and inverted PWM outputs. The small
horizontal line marks on the TCNT2 slopes represent compare matches between OCR2 and TCNT2.
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Figure 22-6 Fast PWM Mode, Timing Diagram
TCNTn
OCRn Upda te
and
TOVn Interrupt Fla g Se t
1
Pe riod 2 3
OCn
OCn
(COMn1:0 = 2)
(COMn1:0 = 3)
OCRn Inte rrupt Flag Se t
4 5 6 7
The Timer/Counter Overflow Flag (TOV2) is set each time the counter reaches MAX. If the interrupt is
enabled, the interrupt handler routine can be used for updating the compare value.
In fast PWM mode, the compare unit allows generation of PWM waveforms on the OC2 pin. Setting the
COM21:0 bits to 2 will produce a non-inverted PWM and an inverted PWM output can be generated by
setting the COM21:0 to 3 (see Table 22-4 Compare Output Mode, Fast PWM Mode(1) on page 227).
The actual OC2 value will only be visible on the port pin if the data direction for the port pin is set as
output. The PWM waveform is generated by setting (or clearing) the OC2 Register at the Compare Match
between OCR2 and TCNT2, and clearing (or setting) the OC2 Register at the timer clock cycle the
counter is cleared (changes from MAX to BOTTOM).
The PWM frequency for the output can be calculated by the following equation:
OCnPWM =clk_I/O
  256
The N variable represents the prescaler factor (1, 8, 32, 64, 128, 256 or 1024).
The extreme values for the OCR2 Register represent special cases when generating a PWM waveform
output in the fast PWM mode. If the OCR2 is set equal to BOTTOM, the output will be a narrow spike for
each MAX+1 timer clock cycle. Setting the OCR2 equal to MAX will result in a constantly high or low
output (depending on the polarity of the output set by the COM21:0 bits.)
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by setting OC2 to
toggle its logical level on each Compare Match (COM21:0 = 1). The waveform generated will have a
maximum frequency of foc2 = fclk_I/O/2 when OCR2 is set to zero. This feature is similar to the OC2 toggle
in CTC mode, except the double buffer feature of the Output Compare unit is enabled in the fast PWM
mode.
22.7.4. Phase Correct PWM Mode
The phase correct PWM mode (WGM21:0 = 1) provides a high resolution phase correct PWM waveform
generation option. The phase correct PWM mode is based on a dual-slope operation. The counter counts
repeatedly from BOTTOM to MAX and then from MAX to BOTTOM. In non-inverting Compare Output
mode, the Output Compare (OC2) is cleared on the Compare Match between TCNT2 and OCR2 while
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upcounting, and set on the Compare Match while downcounting. In inverting Output Compare mode, the
operation is inverted. The dual-slope operation has lower maximum operation frequency than single slope
operation. However, due to the symmetric feature of the dual-slope PWM modes, these modes are
preferred for motor control applications.
The PWM resolution for the phase correct PWM mode is fixed to eight bits. In phase correct PWM mode
the counter is incremented until the counter value matches MAX. When the counter reaches MAX, it
changes the count direction. The TCNT2 value will be equal to MAX for one timer clock cycle. The timing
diagram for the phase correct PWM mode is shown on the figure below. The TCNT2 value is in the timing
diagram shown as a histogram for illustrating the dual-slope operation. The diagram includes non-
inverted and inverted PWM outputs. The small horizontal line marks on the TCNT2 slopes represent
compare matches between OCR2 and TCNT2.
Figure 22-7 Phase Correct PWM Mode, Timing Diagram
TOVn Interrupt Flag S e t
OCn Interrupt Flag S e t
1 2 3
TCNTn
Pe riod
OCn
OCn
(COMn1:0 = 2)
(COMn1:0 = 3)
OCRn Update
The Timer/Counter Overflow Flag (TOV2) is set each time the counter reaches BOTTOM. The Interrupt
Flag can be used to generate an interrupt each time the counter reaches the BOTTOM value.
In phase correct PWM mode, the compare unit allows generation of PWM waveforms on the OC2 pin.
Setting the COM21:0 bits to 2 will produce a non-inverted PWM. An inverted PWM output can be
generated by setting the COM21:0 to 3 (refer to Table 22-5 Compare Output Mode, Phase Correct PWM
Mode(1) on page 227). The actual OC2 value will only be visible on the port pin if the data direction for
the port pin is set as output. The PWM waveform is generated by clearing (or setting) the OC2 Register at
the Compare Match between OCR2 and TCNT2 when the counter increments, and setting (or clearing)
the OC2 Register at Compare Match between OCR2 and TCNT2 when the counter decrements. The
PWM frequency for the output when using phase correct PWM can be calculated by the following
equation:
OCnPCPWM =clk_I/O
  510
The N variable represents the prescaler factor (1, 8, 32, 64, 128, 256 or 1024).
The extreme values for the OCR2 Register represent special cases when generating a PWM waveform
output in the phase correct PWM mode. If the OCR2 is set equal to BOTTOM, the output will be
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continuously low and if set equal to MAX the output will be continuously high for non-inverted PWM
mode. For inverted PWM the output will have the opposite logic values.
At the very start of period 2 in the timing diagram OCn has a transition from high to low even though there
is no Compare Match. The point of this transition is to guarantee symmetry around BOTTOM. There are
two cases that give a transition without a Compare Match:
• OCR2 changes its value from MAX, like in the timing diagram above. When the OCR2 value is MAX the
OCn pin value is the same as the result of a down-counting Compare Match. To ensure symmetry around
BOTTOM the OCn value at MAX must correspond to the result of an up-counting Compare Match.
• The timer starts counting from a value higher than the one in OCR2, and for that reason misses the
Compare Match and hence the OCn change that would have happened on the way up.
22.8. Timer/Counter Timing Diagrams
The Timer/Counter is a synchronous design and the timer clock (clkT2) is therefore shown as a clock
enable signal in the following figures. The figures include information on when interrupt flags are set. The
first figure below contains timing data for basic Timer/Counter operation. It shows the count sequence
close to the MAX value in all modes other than phase correct PWM mode.
Figure 22-8 Timer/Counter Timing Diagram, no Prescaling
clkTn
(clkI/O/1)
TOVn
clkI/O
TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1
The next figure shows the same timing data, but with the prescaler enabled.
Figure 22-9 Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)
TOVn
TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1
clkI/O
clkTn
(clkI/O/8)
The next figure shows the setting of OCF2 in all modes except CTC mode.
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Figure 22-10 Timer/Counter Timing Diagram, Setting of OCF2, with Prescaler (fclk_I/O/8)
OCFn
OCRn
TCNTn
OCRn Value
OCRn - 1 OCRn OCRn + 1 OCRn + 2
clkI/O
clkTn
(clkI/O/8)
The next figure shows the setting of OCF2 and the clearing of TCNT2 in CTC mode.
Figure 22-11 Timer/Counter Timing Diagram, Clear Timer on Compare Match Mode, with Prescaler (fclk_I/O/8)
OCFn
OCRn
TCNTn
(CTC)
TOP
TOP - 1 TOP BOTTOM BOTTOM + 1
clkI/O
clkTn
(clkI/O/8)
22.9. Register Description
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22.9.1. TCCR2 – Timer/Counter Control Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  TCCR2
Offset:  0x25
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x45
Bit 7 6 5 4 3 2 1 0
FOC2 WGM20 COM21 COM20 WGM21 CS22 CS21 CS20
Access W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bit 7 – FOC2: Force Output Compare
The FOC2 bit is only active when the WGM20 bit specifies a non-PWM mode. However, for ensuring
compatibility with future devices, this bit must be set to zero when TCCR2 is written when operating in
PWM mode. When writing a logical one to the FOC2 bit, an immediate Compare Match is forced on the
waveform generation unit. The OC2 output is changed according to its COM21:0 bits setting. Note that
the FOC2 bit is implemented as a strobe. Therefore it is the value present in the COM21:0 bits that
determines the effect of the forced compare.
A FOC2 strobe will not generate any interrupt, nor will it clear the timer in CTC mode using OCR2 as
TOP.
The FOC2 bit is always read as zero.
Bit 6 – WGM20: Waveform Generation Mode
These bits control the counting sequence of the counter, the source for the maximum (TOP) counter
value, and what type of waveform generation to be used. Modes of operation supported by the Timer/
Counter unit are: Normal mode, Clear Timer on Compare Match (CTC) mode, and two types of Pulse
Width Modulation (PWM) modes. See table below and Modes of Operation on page 220.
Table 22-2 Waveform Generation Mode Bit Description
Mode WGM21
(CTC2)
WGM20
(PWM2)
Timer/Counter Mode of Operation(1) TOP Update of
OCR2
TOV2 Flag
Set
0 0 0 Normal 0xFF Immediate MAX
1 0 1 PWM, Phase Correct 0xFF TOP BOTTOM
2 1 0 CTC OCR2 Immediate MAX
3 1 1 Fast PWM 0xFF BOTTOM MAX
Note:  1. The CTC2 and PWM2 bit definition names are now obsolete. Use the WGM21:0 definitions.
However, the functionality and location of these bits are compatible with previous versions of the timer.
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Bits 5:4 – COM2n: Compare Match Output Mode [n = 1:0]
These bits control the Output Compare Pin (OC2) behavior. If one or both of the COM21:0 bits are set,
the OC2 output overrides the normal port functionality of the I/O pin it is connected to. However, note that
the Data Direction Register (DDR) bit corresponding to the OC2 pin must be set in order to enable the
output driver.
When OC2 is connected to the pin, the function of the COM21:0 bits depends on the WGM21:0 bit
setting. The following table shows the COM21:0 bit functionality when the WGM21:0 bits are set to a
normal or CTC mode (non-PWM).
Table 22-3 Compare Output Mode, Non-PWM Mode
COM21 COM20 Description
0 0 Normal port operation, OC2 disconnected.
0 1 Toggle OC2 on Compare Match
1 0 Clear OC2 on Compare Match
1 1 Set OC2 on Compare Match
The next table shows the COM21:0 bit functionality when the WGM21:0 bits are set to fast PWM mode.
Table 22-4 Compare Output Mode, Fast PWM Mode(1)
COM21 COM20 Description
0 0 Normal port operation, OC2 disconnected.
0 1 Reserved
1 0 Clear OC2 on Compare Match, set OC2 at BOTTOM,
(non-inverting mode)
1 1 Set OC2 on Compare Match, clear OC2 at BOTTOM,
(inverting mode)
Note:  1. A special case occurs when OCR2 equals TOP and COM21 is set. In this case, the Compare
Match is ignored, but the set or clear is done at BOTTOM. See Fast PWM Mode on page 221 for more
details.
The table below shows the COM21:0 bit functionality when the WGM21:0 bits are set to phase correct
PWM mode.
Table 22-5 Compare Output Mode, Phase Correct PWM Mode(1)
COM21 COM20 Description
0 0 Normal port operation, OC2 disconnected.
0 1 Reserved
1 0 Clear OC2 on Compare Match when up-counting. Set OC2 on Compare Match when
downcounting.
1 1 Set OC2 on Compare Match when up-counting. Clear OC2 on Compare Match when
downcounting.
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Note:  1. A special case occurs when OCR2 equals TOP and COM21 is set. In this case, the Compare
Match is ignored, but the set or clear is done at TOP. See Phase Correct PWM Mode on page 222 for
more details.
Bit 3 – WGM21: Waveform Generation Mode [n=0:1]
Refer to WGM20 above.
Bits 2:0 – CS2n: Clock Select [n = 2:0]
The three Clock Select bits select the clock source to be used by the Timer/Counter.
Table 22-6 Clock Select Bit Description
CS22 CS21 CS20 Description
0 0 0 No clock source (Timer/Counter stopped).
0 0 1 clkI/O/1 (No prescaling)
0 1 0 clkI/O/8 (From prescaler)
0 1 1 clkI/O/64 (From prescaler)
1 0 0 clkI/O/256 (From prescaler)
1 0 1 clkI/O/1024 (From prescaler)
1 1 0 External clock source on T2 pin. Clock on falling edge.
1 1 1 External clock source on T2 pin. Clock on falling edge.
If external pin modes are used for the Timer/Counter2, transitions on the T2 pin will clock the counter
even if the pin is configured as an output. This feature allows software control of the counting.
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22.9.2. TCNT0 – Timer/Counter Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
The Timer/Counter Register gives direct access, both for read and write operations, to the Timer/Counter
unit 8-bit counter. Writing to the TCNT0 Register blocks (removes) the Compare Match on the following
timer clock. Modifying the counter (TCNT0) while the counter is running, introduces a risk of missing a
Compare Match between TCNT0 and the OCR0 Register.
Name:  TCNT0
Offset:  0x24
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x44
Bit 7 6 5 4 3 2 1 0
TCNT0[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – TCNT0[7:0]
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22.9.3. OCR0 – Output Compare Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
The Output Compare Register contains an 8-bit value that is continuously compared with the counter
value (TCNT0). A match can be used to generate an Output Compare interrupt, or to generate a
waveform output on the OC0 pin.
Name:  OCR0
Offset:  0x23
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x43
Bit 7 6 5 4 3 2 1 0
OCR0[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – OCR0[7:0]
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22.9.4. TIMSK – Timer/Counter Interrupt Mask Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  TIMSK
Offset:  0x37
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x57
Bit 7 6 5 4 3 2 1 0
OCIE2 TOIE2
Access R/W R/W
Reset 0 0
Bit 7 – OCIE2: Timer/CounterTimer/Counter2 Output Compare Match Interrupt Enable
When the OCIE2 bit is written to one and the I-bit in the Status Register is set (one), the Timer/Counter2
Compare Match interrupt is enabled. The corresponding interrupt is executed if a Compare Match in
Timer/Counter2 occurs (i.e., when the OCF2 bit is set in the Timer/Counter Interrupt Flag Register –
TIFR).
Bit 6 – TOIE2: Timer/CounterTimer/Counter2 Overflow Interrupt Enable
When the TOIE2 bit is written to one and the I-bit in the Status Register is set (one), the Timer/Counter2
Overflow interrupt is enabled. The corresponding interrupt is executed if an overflow in Timer/Counter2
occurs (i.e., when the TOV2 bit is set in the Timer/Counter Interrupt Flag Register – TIFR).
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22.9.5. TIFR – Timer/Counter Interrupt Flag Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  TIFR
Offset:  0x36
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x56
Bit 7 6 5 4 3 2 1 0
OCF2 TOV2
Access R/W R/W
Reset 0 0
Bit 7 – OCF2: Output Compare Flag 2
The OCF2 bit is set (one) when a Compare Match occurs between the Timer/Counter2 and the data in
OCR2 – Output Compare Register2. OCF2 is cleared by hardware when executing the corresponding
interrupt Handling Vector. Alternatively, OCF2 is cleared by writing a logic one to the flag. When the I-bit
in SREG, OCIE2 (Timer/Counter2 Compare Match Interrupt Enable), and OCF2 are set (one), the Timer/
Counter2 Compare Match Interrupt is executed.
Bit 6 – TOV2: Timer/Counter2 Overflow Flag
The TOV2 bit is set (one) when an overflow occurs in Timer/Counter2. TOV2 is cleared by hardware
when executing the corresponding interrupt Handling Vector. Alternatively, TOV2 is cleared by writing a
logic one to the flag. When the SREG I-bit, TOIE2 (Timer/Counter2 Overflow Interrupt Enable), and TOV2
are set (one), the Timer/Counter2 Overflow interrupt is executed. In PWM mode, this bit is set when
Timer/Counter2 changes counting direction at 0x00.
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23. Output Compare Modulator (OCM1C2)
23.1. Overview
The Output Compare Modulator (OCM) allows generation of waveforms modulated with a carrier
frequency. The modulator uses the outputs from the Output Compare Unit C of the 16-bit Timer/Counter1
and the Output Compare Unit of the 8-bit Timer/Counter2. For more details about these Timer/Counters
see 16-bit Timer/Counter (Timer/Counter1 and Timer/Counter3) and 8-bit Timer/Counter2 with PWM.
Note that this feature is not available in ATmega103 compatibility mode.
Figure 23-1 Output Compare Modulator, Block Diagram
OC3B
Pin
OC1C /
OC2 / PB7
Timer/Counter 3
Timer/Counter 4 OC4B
When the modulator is enabled, the two output compare channels are modulated together as shown in
the block diagram above.
Related Links
16-bit Timer/Counter (Timer/Counter1 and Timer/Counter3) on page 137
8-bit Timer/Counter2 with PWM on page 215
23.2. Description
The Output Compare unit 1C and Output Compare unit 2 shares the PB7 port pin for output. The outputs
of the Output Compare units (OC1C and OC2) overrides the normal PORTB7 Register when one of them
is enabled (that is, when COMnx1:0 is not equal to zero). When both OC1C and OC2 are enabled at the
same time, the modulator is automatically enabled.
The functional equivalent schematic of the modulator is shown in the following figure. The schematic
includes part of the Timer/Counter units and the port B pin 7 output driver circuit.
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Figure 23-2 Output Compare Modulator, Schematic
PORTB7 DDRB7
D QD Q
Pin
COM21
COM20
DATABUS
OC1C /
OC2 / PB7
COM1C1
COM1C0
Modulator
1
0
OC1C
D Q
OC2
D Q
( From Waveform Generator )
( From Waveform Generator )
0
1
Vcc
When the modulator is enabled the type of modulation (logical AND or OR) can be selected by the
PORTB7 Register. Note that the DDRB7 controls the direction of the port independent of the COMnx1:0
bit setting.
23.2.1. Timing Example
The figure below illustrates the modulator in action. In this example the Timer/Counter1 is set to operate
in fast PWM mode (non-inverted) and Timer/Counter2 uses CTC waveform mode with toggle Compare
Output mode (COMnx1:0 = 1).
Figure 23-3 Output Compare Modulator, Timing Diagram
1 2
OC2
(CTC Mode)
OC1C
(FPWM Mode)
PB7
(PORTB7 = 0)
PB7
(PORTB7 = 1)
(Period) 3
clk
I/O
In this example, Timer/Counter2 provides the carrier, while the modulating signal is generated by the
Output Compare unit C of the Timer/Counter1.
The resolution of the PWM signal (OC1C) is reduced by the modulation. The reduction factor is equal to
the number of system clock cycles of one period of the carrier (OC2). In this example the resolution is
reduced by a factor of two. The reason for the reduction is illustrated in the figure above at the second
and third period of the PB7 output when PORTB7 equals zero. The period 2 high time is one cycle longer
than the period 3 high time, but the result on the PB7 output is equal in both periods.
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24. SPI – Serial Peripheral Interface
24.1. Features
•Full-duplex, Three-wire Synchronous Data Transfer
•Master or Slave Operation
•LSB First or MSB First Data Transfer
•Seven Programmable Bit Rates
•End of Transmission Interrupt Flag
•Write Collision Flag Protection
•Wake-up from Idle Mode
•Double Speed (CK/2) Master SPI Mode
24.2. Overview
The Serial Peripheral Interface (SPI) allows high-speed synchronous data transfer between the
ATmega64A and peripheral devices or between several AVR devices.
Figure 24-1 SPI Block Diagram(1)
SPI2X
SPI2X
DIVIDER
/2/4/8/16/32/64/128
Note:  1. Refer to Pin Configurations, table Port B Pins Alternate Functions in Alternate Functions of Port
B for SPI pin placement.
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MSB MASTER LSB MSB MISO MISO SLAVE L55 3 BIT SHIFr REGISTER 3 BIT SHIFr REGISTER“ MOSI MOSI SPI SCK SCK CLOCK GENERATOR 7 7 ss 55 W5: r Atmel
The interconnection between Master and Slave CPUs with SPI is shown in the figure below. The system
consists of two shift registers, and a Master Clock generator. The SPI Master initiates the communication
cycle when pulling low the Slave Select SS pin of the desired Slave. Master and Slave prepare the data
to be sent in their respective Shift Registers, and the Master generates the required clock pulses on the
SCK line to interchange data. Data is always shifted from Master to Slave on the Master Out – Slave In,
MOSI, line, and from Slave to Master on the Master In – Slave Out, MISO, line. After each data packet,
the Master will synchronize the Slave by pulling high the Slave Select, SS, line.
When configured as a Master, the SPI interface has no automatic control of the SS line. This must be
handled by user software before communication can start. When this is done, writing a byte to the SPI
Data Register starts the SPI clock generator, and the hardware shifts the eight bits into the Slave. After
shifting one byte, the SPI clock generator stops, setting the end of Transmission Flag (SPIF). If the SPI
interrupt enable bit (SPIE) in the SPCR Register is set, an interrupt is requested. The Master may
continue to shift the next byte by writing it into SPDR, or signal the end of packet by pulling high the Slave
Select, SS line. The last incoming byte will be kept in the Buffer Register for later use.
When configured as a Slave, the SPI interface will remain sleeping with MISO tri-stated as long as the SS
pin is driven high. In this state, software may update the contents of the SPI Data Register, SPDR, but the
data will not be shifted out by incoming clock pulses on the SCK pin until the SS pin is driven low. As one
byte has been completely shifted, the end of Transmission Flag, SPIF is set. If the SPI Interrupt Enable
bit, SPIE, in the SPCR Register is set, an interrupt is requested. The Slave may continue to place new
data to be sent into SPDR before reading the incoming data. The last incoming byte will be kept in the
Buffer Register for later use.
Figure 24-2 SPI Master-slave Interconnection
SHIFT
ENABLE
V
cc
The system is single buffered in the transmit direction and double buffered in the receive direction. This
means that bytes to be transmitted cannot be written to the SPI Data Register before the entire shift cycle
is completed. When receiving data, however, a received character must be read from the SPI Data
Register before the next character has been completely shifted in. Otherwise, the first byte is lost.
In SPI Slave mode, the control logic will sample the incoming signal of the SCK pin. To ensure correct
sampling of the clock signal, the minimum low and high periods should be:
Low period: longer than 2 CPU clock cycles.
High period: longer than 2 CPU clock cycles.
When the SPI is enabled, the data direction of the MOSI, MISO, SCK, and SS pins is overridden
according to the table below. For more details on automatic port overrides, refer to Alternate Port
Functions.
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Table 24-1 SPI Pin Overrides(1)
Pin Direction, Master SPI Direction, Slave SPI
MOSI User Defined Input
MISO Input User Defined
SCK User Defined Input
SS User Defined Input
Note:  1. Refer to table Port B Pins Alternate Functions in Alternate Functions of Port B for a detailed
description of how to define the direction of the user defined SPI pins.
The following code examples show how to initialize the SPI as a Master and how to perform a simple
transmission. DDR_SPI in the examples must be replaced by the actual Data Direction Register
controlling the SPI pins. DD_MOSI, DD_MISO and DD_SCK must be replaced by the actual data direction
bits for these pins. E.g. if MOSI is placed on pin PB5, replace DD_MOSI with DDB5 and DDR_SPI with
DDRB.
Assembly Code Example(1)
SPI_MasterInit:
; Set MOSI and SCK output, all others input
ldi r17,(1<<DD_MOSI)|(1<<DD_SCK)
out DDR_SPI,r17
; Enable SPI, Master, set clock rate fck/16
ldi r17,(1<<SPE)|(1<<MSTR)|(1<<SPR0)
out SPCR,r17
ret
SPI_MasterTransmit:
; Start transmission of data (r16)
out SPDR,r16
Wait_Transmit:
; Wait for transmission complete
sbis SPSR,SPIF
rjmp Wait_Transmit
ret
C Code Example(1)
void SPI_MasterInit(void)
{
/* Set MOSI and SCK output, all others input */
DDR_SPI = (1<<DD_MOSI)|(1<<DD_SCK);
/* Enable SPI, Master, set clock rate fck/16 */
SPCR = (1<<SPE)|(1<<MSTR)|(1<<SPR0);
}
void SPI_MasterTransmit(char cData)
{
/* Start transmission */
SPDR = cData;
/* Wait for transmission complete */
while(!(SPSR & (1<<SPIF)))
;
}
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Note:  1. See About Code Examples.
The following code examples show how to initialize the SPI as a Slave and how to
perform a simple reception.
Assembly Code Example(1)
SPI_SlaveInit:
; Set MISO output, all others input
ldi r17,(1<<DD_MISO)
out DDR_SPI,r17
; Enable SPI
ldi r17,(1<<SPE)
out SPCR,r17
ret
SPI_SlaveReceive:
; Wait for reception complete
sbis SPSR,SPIF
rjmp SPI_SlaveReceive
; Read received data and return
in r16,SPDR
ret
C Code Example(1)
void SPI_SlaveInit(void)
{
/* Set MISO output, all others input */
DDR_SPI = (1<<DD_MISO);
/* Enable SPI */
SPCR = (1<<SPE);
}
char SPI_SlaveReceive(void)
{
/* Wait for reception complete */
while(!(SPSR & (1<<SPIF)))
;
/* Return Data Register */
return SPDR;
}
Note:  1. See About Code Examples.
Related Links
Pin Configurations on page 14
Alternate Functions of Port B on page 100
Alternate Port Functions on page 96
About Code Examples on page 20
24.3. SS Pin Functionality
24.3.1. Slave Mode
When the SPI is configured as a Slave, the Slave Select (SS) pin is always input. When SS is held low,
the SPI is activated, and MISO becomes an output if configured so by the user. All other pins are inputs.
When SS is driven high, all pins are inputs except MISO which can be user configured as an output, and
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the SPI is passive, which means that it will not receive incoming data. The SPI logic will be reset once the
SS pin is driven high.
The SS pin is useful for packet/byte synchronization to keep the slave bit counter synchronous with the
master clock generator. When the SS pin is driven high, the SPI slave will immediately reset the send and
receive logic, and drop any partially received data in the Shift Register.
24.3.2. Master Mode
When the SPI is configured as a Master (MSTR in SPCR is set), the user can determine the direction of
the SS pin.
If SS is configured as an output, the pin is a general output pin which does not affect the SPI system.
Typically, the pin will be driving the SS pin of the SPI Slave.
If SS is configured as an input, it must be held high to ensure Master SPI operation. If the SS pin is driven
low by peripheral circuitry when the SPI is configured as a Master with the SS pin defined as an input, the
SPI system interprets this as another master selecting the SPI as a slave and starting to send data to it.
To avoid bus contention, the SPI system takes the following actions:
1. The MSTR bit in SPCR is cleared and the SPI system becomes a Slave. As a result of the SPI
becoming a Slave, the MOSI and SCK pins become inputs.
2. The SPIF Flag in SPSR is set, and if the SPI interrupt is enabled, and the I-bit in SREG is set, the
interrupt routine will be executed.
Thus, when interrupt-driven SPI transmission is used in Master mode, and there exists a possibility that
SS is driven low, the interrupt should always check that the MSTR bit is still set. If the MSTR bit has been
cleared by a slave select, it must be set by the user to re-enable SPI Master mode.
24.4. Data Modes
There are four combinations of SCK phase and polarity with respect to serial data, which are determined
by control bits CPHA and CPOL. The SPI data transfer formats are shown in the figures in this section.
Data bits are shifted out and latched in on opposite edges of the SCK signal, ensuring sufficient time for
data signals to stabilize. This is clearly seen by summarizing Table 24-3 CPOL Functionality on page
241 and Table 24-4 CPHA Functionality on page 242, as done below:
Table 24-2 CPOL and CPHA Functionality
SPI Mode Conditions Leading Edge Trailing Edge
0 CPOL=0, CPHA=0 Sample (Rising) Setup (Falling)
1 CPOL=0, CPHA=1 Setup (Rising) Sample (Falling)
2 CPOL=1, CPHA=0 Sample (Falling) Setup (Rising)
3 CPOL=1, CPHA=1 Setup (Falling) Sample (Rising)
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Figure 24-3 SPI Transfer Format with CPHA = 0
Bit 1
Bit 6
LSB
MSB
SCK (CPOL = 0)
mode 0
SAMPLE I
MOSI/MISO
CHANGE 0
MOSI PIN
CHANGE 0
MISO PIN
SCK (CPOL = 1)
mode 2
SS
MSB
LSB
Bit 6
Bit 1
Bit 5
Bit 2
Bit 4
Bit 3
Bit 3
Bit 4
Bit 2
Bit 5
MSB first (DORD = 0)
LSB first (DORD = 1)
Figure 24-4 SPI Transfer Format with CPHA = 1
SCK (CPOL = 0)
mode 1
SAMPLE I
MOSI/MISO
CHANGE 0
MOSI PIN
CHANGE 0
MISO PIN
SCK (CPOL = 1)
mode 3
SS
MSB
LSB
Bit 6
Bit 1
Bit 5
Bit 2
Bit 4
Bit 3
Bit 3
Bit 4
Bit 2
Bit 5
Bit 1
Bit 6
LSB
MSB
MSB first (DORD = 0)
LSB first (DORD = 1)
24.5. Register Description
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24.5.1. SPCR – SPI Control Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  SPCR
Offset:  0x0D
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x2D
Bit 7 6 5 4 3 2 1 0
SPIE SPE DORD MSTR CPOL CPHA SPR1 SPR0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bit 7 – SPIE: SPI Interrupt Enable
This bit causes the SPI interrupt to be executed if SPIF bit in the SPSR Register is set and if the Global
Interrupt Enable bit in SREG is set.
Bit 6 – SPE: SPI Enable
When the SPE bit is written to one, the SPI is enabled. This bit must be set to enable any SPI operations.
Bit 5 – DORD: Data Order
When the DORD bit is written to one, the LSB of the data word is transmitted first.
When the DORD bit is written to zero, the MSB of the data word is transmitted first.
Bit 4 – MSTR: Master/Slave Select
This bit selects Master SPI mode when written to one, and Slave SPI mode when written logic zero. If SS
is configured as an input and is driven low while MSTR is set, MSTR will be cleared, and SPIF in SPSR
will become set. The user will then have to set MSTR to re-enable SPI Master mode.
Bit 3 – CPOL: Clock Polarity
When this bit is written to one, SCK is high when idle. When CPOL is written to zero, SCK is low when
idle. Refer to the figures in Data Modes on page 239 for an example. The CPOL functionality is
summarized below:
Table 24-3 CPOL Functionality
CPOL Leading Edge Trailing Edge
0 Rising Falling
1 Falling Rising
Bit 2 – CPHA: Clock Phase
The settings of the Clock Phase bit (CPHA) determine if data is sampled on the leading (first) or trailing
(last) edge of SCK. Refer to the figures in Data Modes on page 239 for an example. The CPHA
functionality is summarized below:
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Table 24-4 CPHA Functionality
CPHA Leading Edge Trailing Edge
0 Sample Setup
1 Setup Sample
Bits 1:0 – SPRn: SPI Clock Rate Select [n = 1:0]
These two bits control the SCK rate of the device configured as a Master. SPR1 and SPR0 have no effect
on the Slave. The relationship between SCK and the Oscillator Clock frequency fosc is shown in the table
below.
Table 24-5 Relationship between SCK and Oscillator Frequency
SPI2X SPR1 SPR0 SCK Frequency
0 0 0 fosc/4
0 0 1 fosc/16
0 1 0 fosc/64
0 1 1 fosc/128
1 0 0 fosc/2
1 0 1 fosc/8
1 1 0 fosc/32
1 1 1 fosc/64
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24.5.2. SPSR – SPI Status Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  SPSR
Offset:  0x0E
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x2E
Bit 7 6 5 4 3 2 1 0
SPIF WCOL SPI2X
Access R R R/W
Reset 0 0 0
Bit 7 – SPIF: SPI Interrupt Flag
When a serial transfer is complete, the SPIF Flag is set. An interrupt is generated if SPIE in SPCR is set
and global interrupts are enabled. If SS is an input and is driven low when the SPI is in Master mode, this
will also set the SPIF Flag. SPIF is cleared by hardware when executing the corresponding interrupt
handling vector. Alternatively, the SPIF bit is cleared by first reading the SPI Status Register with SPIF
set, then accessing the SPI Data Register (SPDR).
Bit 6 – WCOL: Write Collision Flag
The WCOL bit is set if the SPI Data Register (SPDR) is written during a data transfer. The WCOL bit (and
the SPIF bit) are cleared by first reading the SPI Status Register with WCOL set, and then accessing the
SPI Data Register.
Bit 0 – SPI2X: Double SPI Speed Bit
When this bit is written logic one the SPI speed (SCK Frequency) will be doubled when the SPI is in
Master mode (refer to Table 24-5 Relationship between SCK and Oscillator Frequency on page 242).
This means that the minimum SCK period will be two CPU clock periods. When the SPI is configured as
Slave, the SPI is only guaranteed to work at fosc/4 or lower.
The SPI interface on the ATmega64A is also used for program memory and EEPROM downloading or
uploading. Refer to section Serial Downloading in Memory Programming for serial programming and
verification.
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24.5.3. SPDR – SPI Data Register is a read/write register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  SPDR
Offset:  0x0F
Reset:  0xXX
Property:
 
When addressing I/O Registers as data space the offset address is 0x2F
Bit 7 6 5 4 3 2 1 0
SPID7 SPID6 SPID5 SPID4 SPID3 SPID2 SPID1 SPID0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset x x x x x x x x
Bits 7:0 – SPIDn: SPI Data
The SPI Data Register is a read/write register used for data transfer between the Register File and the
SPI Shift Register. Writing to the register initiates data transmission. Reading the register causes the Shift
Register Receive buffer to be read.
•SPID7 is MSB
•SPID0 is LSB
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25. USART
25.1. Features
•Full Duplex Operation (Independent Serial Receive and Transmit Registers)
•Asynchronous or Synchronous Operation
•Master or Slave Clocked Synchronous Operation
•High Resolution Baud Rate Generator
•Supports Serial Frames with 5, 6, 7, 8, or 9 data bits and 1 or 2 stop bits
•Odd or Even Parity Generation and Parity Check Supported by Hardware
•Data OverRun Detection
•Framing Error Detection
•Noise Filtering Includes False Start Bit Detection and Digital Low Pass Filter
•Three Separate Interrupts on TX Complete, TX Data Register Empty and RX Complete
•Multi-processor Communication Mode
•Double Speed Asynchronous Communication Mode
25.1.1. Dual USART
The Universal Synchronous and Asynchronous serial Receiver and Transmitter (USART) is a highly
flexible serial communication device. The ATmega64A has two USARTs, USART0 and USART1. The
functionality for both USARTs is described below. USART0 and USART1 have different I/O registers as
shown in Register Summary. Note that in ATmega103 compatibility mode, USART1 is not available,
neither is the UBRR0H or UCRS0C Registers. This means that in ATmega103 compatibility mode, the
ATmega64A supports asynchronous operation of USART0 only.
Related Links
Register Summary on page 492
25.2. Overview
The Universal Synchronous and Asynchronous serial Receiver and Transmitter (USART) is a highly-
flexible serial communication device. A simplified block diagram of the USART Transmitter is shown in the
figure below. CPU accessible I/O Registers and I/O pins are shown in bold.
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Figure 25-1 USART Block Diagram(1)
PARITY
GENERATOR
UBRRn [H:L]
UDRn(Transmit)
UCSRnA UCSRnB UCSRnC
BAUD RATE GENERATOR
TRANSMIT SHIFT REGISTER
RECEIVE SHIFT REGISTER RxDn
TxDn
PIN
CONTROL
UDRn (Receive)
PIN
CONTROL
XCKn
DATA
RECOVERY
CLOCK
RECOVERY
PIN
CONTROL
TX
CONTROL
RX
CONTROL
PARITY
CHECKER
DATA BUS
OSC
SYNC LOGIC
Clock Generator
Transmitter
Receiver
Note:  1. Refer to Pin Configurations, table Port D Pins Alternate Functions in Alternate Functions of Port
D and table Port E Pins Alternate Functions in Alternate Functions of Port E for USART pin placement.
The dashed boxes in the block diagram separate the three main parts of the USART (listed from the top):
Clock Generator, Transmitter, and Receiver. Control registers are shared by all units. The clock
generation logic consists of synchronization logic for external clock input used by synchronous slave
operation, and the baud rate generator. The XCK (Transfer Clock) pin is only used by Synchronous
Transfer mode. The Transmitter consists of a single write buffer, a serial Shift Register, parity generator
and control logic for handling different serial frame formats. The write buffer allows a continuous transfer
of data without any delay between frames. The Receiver is the most complex part of the USART module
due to its clock and data recovery units. The recovery units are used for asynchronous data reception. In
addition to the recovery units, the receiver includes a parity checker, control logic, a Shift Register and a
two level receive buffer (UDR). The receiver supports the same frame formats as the Transmitter, and can
detect frame error, data overrun and parity errors.
Related Links
Pin Configurations on page 14
Alternate Functions of Port D on page 103
Alternate Functions of Port E on page 105
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25.2.1. AVR USART vs. AVR UART – Compatibility
The USART is fully compatible with the AVR UART regarding:
•Bit locations inside all USART Registers.
•Baud Rate Generation.
•Transmitter Operation.
•Transmit Buffer Functionality.
•Receiver Operation.
However, the receive buffering has two improvements that will affect the compatibility in some special
cases:
•A second Buffer Register has been added. The two Buffer Registers operate as a circular FIFO
buffer. Therefore the UDR must only be read once for each incoming data! More important is the
fact that the Error Flags (FE and DOR) and the ninth data bit (RXB8) are buffered with the data in
the receive buffer. Therefore the status bits must always be read before the UDR Register is read.
Otherwise the error status will be lost since the buffer state is lost.
•The Receiver Shift Register can now act as a third buffer level. This is done by allowing the
received data to remain in the serial Shift Register (see Block Diagram in previous section) if the
Buffer Registers are full, until a new start bit is detected. The USART is therefore more resistant to
Data OverRun (DOR) error conditions.
The following control bits have changed name, but have same functionality and register location:
•CHR9 is changed to UCSZ2.
•OR is changed to DOR.
25.3. Clock Generation
The clock generation logic generates the base clock for the Transmitter and Receiver. The USART
supports four modes of clock operation: normal asynchronous, double speed asynchronous, Master
synchronous and Slave Synchronous mode. The UMSEL bit in USART Control and Status Register C
(UCSRC) selects between asynchronous and synchronous operation. Double speed (Asynchronous
mode only) is controlled by the U2X found in the UCSRA Register. When using Synchronous mode
(UMSEL = 1), the Data Direction Register for the XCK pin (DDR_XCK) controls whether the clock source
is internal (Master mode) or external (Slave mode). The XCK pin is only active when using Synchronous
mode.
Below is a block diagram of the clock generation logic.
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Figure 25-2 Clock Generation Logic, Block Diagram
Prescaling
Down-Counter /2
UBRRn
/4 /2
foscn
UBRRn+1
Sync
Register
OSC
XCKn
Pin
txclk
U2Xn
UMSELn
DDR_XCKn
0
1
0
1
xcki
xcko
DDR_XCKn rxclk
0
1
1
0
Edge
Detector
UCPOLn
Signal description:
txclk Transmitter clock (internal signal).
rxclk Receiver base clock (internal signal).
xcki Input from XCK pin (internal Signal). Used for synchronous slave operation.
xcko Clock output to XCK pin (internal signal). Used for synchronous master operation.
fosc XTAL pin frequency (System Clock).
25.3.1. Internal Clock Generation – The Baud Rate Generator
Internal clock generation is used for the asynchronous and the synchronous master modes of operation.
The description in this section refers to the block diagram above.
The USART Baud Rate Register (UBRR) and the down-counter connected to it function as a
programmable prescaler or baud rate generator. The down-counter, running at system clock (fosc), is
loaded with the UBRR value each time the counter has counted down to zero or when the UBRRL
Register is written. A clock is generated each time the counter reaches zero. This clock is the baud rate
generator clock output (= fosc/(UBRR+1)). The Transmitter divides the baud rate generator clock output
by 2, 8, or 16 depending on mode. The baud rate generator output is used directly by the Receiver’s clock
and data recovery units. However, the recovery units use a state machine that uses 2, 8, or 16 states
depending on mode set by the state of the UMSEL, U2X and DDR_XCK bits.
The table below contains equations for calculating the baud rate (in bits per second) and for calculating
the UBRR value for each mode of operation using an internally generated clock source.
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fosc fosc EAUD = m ”3““ = 7151mm) ‘ 1 fosc fosc EAUD ‘ 8(UBRR + 1) "ERR ‘ BEAUD _ 1 fOSC fOSC MUD—m ”BM—m‘l Atmel
Table 25-1 Equations for Calculating Baud Rate Register Setting
Operating Mode Equation for Calculating Baud
Rate(1)
Equation for Calculating UBRR
Value
Asynchronous Normal
mode (U2X = 0) BAUD = OSC
16  + 1  =OSC
16BAUD 1
Asynchronous Double
Speed mode (U2X = 1) BAUD = OSC
8 + 1  =OSC
8BAUD 1
Synchronous Master mode BAUD = OSC
2+1  =OSC
2BAUD 1
Note:  1. The baud rate is defined to be the transfer rate in bit per second (bps).
BAUD Baud rate (in bits per second, bps).
fOSC System oscillator clock frequency.
UBRR Contents of the UBRRH and UBRRL Registers, (0-4095).
Some examples of UBRR values for some system clock frequencies are found in Table 25-4 Examples of
UBRR Settings for Commonly Used Oscillator Frequencies on page 262.
25.3.2. Double Speed Operation (U2X)
The transfer rate can be doubled by setting the U2X bit in UCSRA. Setting this bit only has effect for the
asynchronous operation. Set this bit to zero when using synchronous operation.
Setting this bit will reduce the divisor of the baud rate divider from 16 to 8, effectively doubling the transfer
rate for asynchronous communication. Note however that the Receiver will in this case only use half the
number of samples (reduced from 16 to 8) for data sampling and clock recovery, and therefore a more
accurate baud rate setting and system clock are required when this mode is used.
For the Transmitter, there are no downsides.
25.3.3. External Clock
External clocking is used by the synchronous slave modes of operation. The description in this section
refers to Figure 25-2 Clock Generation Logic, Block Diagram on page 248.
External clock input from the XCK pin is sampled by a synchronization register to minimize the chance of
meta-stability. The output from the synchronization register must then pass through an edge detector
before it can be used by the Transmitter and Receiver. This process introduces a two CPU clock period
delay and therefore the maximum external XCK clock frequency is limited by the following equation:
XCK <OSC
4
The value of fosc depends on the stability of the system clock source. It is therefore recommended to add
some margin to avoid possible loss of data due to frequency variations.
25.3.4. Synchronous Clock Operation
When Synchronous mode is used (UMSEL = 1), the XCK pin will be used as either clock input (Slave) or
clock output (Master). The dependency between the clock edges and data sampling or data change is the
same. The basic principle is that data input (on RxD) is sampled at the opposite XCK clock edge of the
edge the data output (TxD) is changed.
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+——+ \/XXXXXXXXX/1\ Atmel
Figure 25-3 Synchronous Mode XCK Timing
RxD / TxD
XCK
RxD / TxD
XCK UCPOL = 0
UCPOL = 1
Sample
Sample
The UCPOL bit UCRSC selects which XCK clock edge is used for data sampling and which is used for
data change. As the figure above shows, when UCPOL is zero the data will be changed at rising XCK
edge and sampled at falling XCK edge. If UCPOL is set, the data will be changed at falling XCK edge and
sampled at rising XCK edge.
25.4. Frame Formats
A serial frame is defined to be one character of data bits with synchronization bits (start and stop bits),
and optionally a parity bit for error checking. The USART accepts all 30 combinations of the following as
valid frame formats:
•1 start bit
•5, 6, 7, 8, or 9 data bits
•no, even or odd parity bit
•1 or 2 stop bits
A frame starts with the start bit followed by the least significant data bit. Then the next data bits, up to a
total of nine, are succeeding, ending with the most significant bit. If enabled, the parity bit is inserted after
the data bits, before the stop bits. When a complete frame is transmitted, it can be directly followed by a
new frame, or the communication line can be set to an idle (high) state. The figure below illustrates the
possible combinations of the frame formats. Bits inside brackets are optional.
Figure 25-4 Frame Formats
10 2 3 4 [5] [6] [7] [8] [P]St Sp1 [Sp2] (St / IDLE)(IDLE)
FRAME
St Start bit, always low.
(n) Data bits (0 to 8).
PParity bit. Can be odd or even.
Sp Stop bit, always high.
IDLE No transfers on the communication line (RxD or TxD). An IDLE line must be high.
The frame format used by the USART is set by the UCSZ2:0, UPM1:0 and USBS bits in UCSRB and
UCSRC. The Receiver and Transmitter use the same setting. Note that changing the setting of any of
these bits will corrupt all ongoing communication for both the Receiver and Transmitter.
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The USART Character Size (UCSZ2:0) bits select the number of data bits in the frame. The USART
Parity mode (UPM1:0) bits enable and set the type of parity bit. The selection between one or two stop
bits is done by the USART Stop Bit Select (USBS) bit. The Receiver ignores the second stop bit. An FE
(Frame Error) will therefore only be detected in the cases where the first stop bit is zero
25.4.1. Parity Bit Calculation
The parity bit is calculated by doing an exclusive-or of all the data bits. If odd parity is used, the result of
the exclusive or is inverted. The relation between the parity bit and data bits is as follows:
even =1… 3 2 1 01
odd =1… 3 2 1 01
Peven Parity bit using even parity
Podd Parity bit using odd parity
dnData bit n of the character
If used, the parity bit is located between the last data bit and first stop bit of a serial frame.
25.5. USART Initialization
The USART has to be initialized before any communication can take place. The initialization process
normally consists of setting the baud rate, setting frame format and enabling the Transmitter or the
Receiver depending on the usage. For interrupt driven USART operation, the Global Interrupt Flag should
be cleared (and interrupts globally disabled) when doing the initialization.
Before doing a re-initialization with changed baud rate or frame format, be sure that there are no ongoing
transmissions during the period the registers are changed. The TXC Flag can be used to check that the
Transmitter has completed all transfers, and the RXC Flag can be used to check that there are no unread
data in the receive buffer. Note that the TXC Flag must be cleared before each transmission (before UDR
is written) if it is used for this purpose.
The following simple USART initialization code examples show one assembly and one C function that are
equal in functionality. The examples assume asynchronous operation using polling (no interrupts enabled)
and a fixed frame format. The baud rate is given as a function parameter. For the assembly code, the
baud rate parameter is assumed to be stored in the r17:r16 Registers. When the function writes to the
UCSRC Register, the URSEL bit (MSB) must be set due to the sharing of I/O location by UBRRH and
UCSRC.
Assembly Code Example(1)
USART_Init:
; Set baud rate
out UBRRH, r17
out UBRRL, r16
; Enable receiver and transmitter
ldi r16, (1<<RXEN)|(1<<TXEN)
out UCSRB,r16
; Set frame format: 8data, 2stop bit
ldi r16, (1<<USBS)|(3<<UCSZ0)
out UCSRC,r16
ret
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C Code Example(1)
#define FOSC 1843200 // Clock Speed
#define BAUD 9600
#define MYUBRR FOSC/16/BAUD-1
void main( void )
{
:.
USART_Init(MYUBRR);
:.
}
void USART_Init( unsigned int ubrr)
{
/*Set baud rate */
UBRRH = (unsigned char)(ubrr>>8);
UBRRL = (unsigned char)ubrr;
Enable receiver and transmitter */
UCSRB = (1<<RXEN)|(1<<TXEN);
/* Set frame format: 8data, 2stop bit */
UCSRC = (1<<USBS)|(3<<UCSZ0);
}
Note:  1. See About Code Examples.
More advanced initialization routines can be written to include frame format as
parameters, disable interrupts, and so on. However, many applications use a fixed setting
of the baud and control registers, and for these types of applications the initialization
code can be placed directly in the main routine, or be combined with initialization code for
other I/O modules.
25.6. Data Transmission – The USART Transmitter
The USART Transmitter is enabled by setting the Transmit Enable (TXEN) bit in the UCSRB Register.
When the Transmitter is enabled, the normal port operation of the TxD pin is overridden by the USART
and given the function as the Transmitter’s serial output. The baud rate, mode of operation and frame
format must be set up once before doing any transmissions. If synchronous operation is used, the clock
on the XCK pin will be overridden and used as transmission clock.
25.6.1. Sending Frames with 5 to 8 Data Bits
A data transmission is initiated by loading the transmit buffer with the data to be transmitted. The CPU
can load the transmit buffer by writing to the UDR I/O location. The buffered data in the transmit buffer will
be moved to the Shift Register when the Shift Register is ready to send a new frame. The Shift Register is
loaded with new data if it is in idle state (no ongoing transmission) or immediately after the last stop bit of
the previous frame is transmitted. When the Shift Register is loaded with new data, it will transfer one
complete frame at the rate given by the Baud Register, U2X bit or by XCK depending on mode of
operation.
The following code examples show a simple USART transmit function based on polling of the Data
Register Empty (UDRE) Flag. When using frames with less than eight bits, the most significant bits
written to the UDR are ignored. The USART has to be initialized before the function can be used. For the
assembly code, the data to be sent is assumed to be stored in Register R16.
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Assembly Code Example(1)
USART_Transmit:
; Wait for empty transmit buffer
sbis UCSRA,UDRE
rjmp USART_Transmit
; Put data (r16) into buffer, sends the data
out UDR,r16
ret
C Code Example(1)
void USART_Transmit( unsigned char data )
{
/* Wait for empty transmit buffer */
while ( !( UCSRA & (1<<UDRE)) )
;
/* Put data into buffer, sends the data */
UDR = data;
}
Note:  1. See About Code Examples.
The function simply waits for the transmit buffer to be empty by checking the UDRE Flag,
before loading it with new data to be transmitted. If the Data Register Empty Interrupt is
utilized, the interrupt routine writes the data into the buffer.
Related Links
About Code Examples on page 20
25.6.2. Sending Frames with 9 Data Bits
If 9-bit characters are used (UCSZ = 7), the ninth bit must be written to the TXB8 bit in UCSRB before the
Low byte of the character is written to UDR. The following code examples show a transmit function that
handles 9-bit characters. For the assembly code, the data to be sent is assumed to be stored in registers
R17:R16.
Assembly Code Example(1)
USART_Transmit:
; Wait for empty transmit buffer
sbis UCSRA,UDRE
rjmp USART_Transmit
; Copy 9th bit from r17 to TXB8
cbi UCSRB,TXB8
sbrc r17,0
sbi UCSRB,TXB8
; Put LSB data (r16) into buffer, sends the data
out UDR,r16
ret
C Code Example(1)
void USART_Transmit( unsigned int data )
{
/* Wait for empty transmit buffer */
while ( !( UCSRA & (1<<UDRE))) )
;
/* Copy 9th bit to TXB8 */
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UCSRB &= ~(1<<TXB8);
if ( data & 0x0100 )
UCSRB |= (1<<TXB8);
/* Put data into buffer, sends the data */
UDR = data;
}
Note:  1. These transmit functions are written to be general functions. They can be
optimized if the contents of the UCSRB is static. For example, only the TXB8 bit of the
UCSRB Register is used after initialization. For I/O registers located in extended I/O map,
“IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI” instructions must be replaced with
instructions that allow access to extended I/O. Typically “LDS” and “STS” combined with
“SBRS”, “SBRC”, “SBR”, and “CBR”.
The ninth bit can be used for indicating an address frame when using multi processor
communication mode or for other protocol handling as for example synchronization.
25.6.3. Transmitter Flags and Interrupts
The USART Transmitter has two flags that indicate its state: USART Data Register Empty (UDRE) and
Transmit Complete (TXC). Both flags can be used for generating interrupts.
The Data Register Empty (UDRE) Flag indicates whether the transmit buffer is ready to receive new data.
This bit is set when the transmit buffer is empty, and cleared when the transmit buffer contains data to be
transmitted that has not yet been moved into the Shift Register. For compatibility with future devices,
always write this bit to zero when writing the UCSRA Register.
When the Data Register empty Interrupt Enable (UDRIE) bit in UCSRB is written to one, the USART Data
Register Empty Interrupt will be executed as long as UDRE is set (provided that global interrupts are
enabled). UDRE is cleared by writing UDR. When interrupt-driven data transmission is used, the Data
Register empty Interrupt routine must either write new data to UDR in order to clear UDRE or disable the
Data Register empty Interrupt, otherwise a new interrupt will occur once the interrupt routine terminates.
The Transmit Complete (TXC) Flag bit is set one when the entire frame in the transmit Shift Register has
been shifted out and there are no new data currently present in the transmit buffer. The TXC Flag bit is
automatically cleared when a transmit complete interrupt is executed, or it can be cleared by writing a one
to its bit location. The TXC Flag is useful in half-duplex communication interfaces (like the RS485
standard), where a transmitting application must enter Receive mode and free the communication bus
immediately after completing the transmission.
When the Transmit Compete Interrupt Enable (TXCIE) bit in UCSRB is set, the USART Transmit
Complete Interrupt will be executed when the TXC Flag becomes set (provided that global interrupts are
enabled). When the transmit complete interrupt is used, the interrupt handling routine does not have to
clear the TXC Flag, this is done automatically when the interrupt is executed.
25.6.4. Parity Generator
The Parity Generator calculates the parity bit for the serial frame data. When parity bit is enabled (UPM1
= 1), the Transmitter control logic inserts the parity bit between the last data bit and the first stop bit of the
frame that is sent.
25.6.5. Disabling the Transmitter
The disabling of the Transmitter (setting the TXEN to zero) will not become effective until ongoing and
pending transmissions are completed (i.e., when the Transmit Shift Register and Transmit Buffer Register
do not contain data to be transmitted). When disabled, the Transmitter will no longer override the TxD pin.
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25.7. Data Reception – The USART Receiver
The USART Receiver is enabled by writing the Receive Enable (RXEN) bit in the UCSRB Register to
one. When the Receiver is enabled, the normal pin operation of the RxD pin is overridden by the USART
and given the function as the Receiver’s serial input. The baud rate, mode of operation and frame format
must be set up once before any serial reception can be done. If synchronous operation is used, the clock
on the XCK pin will be used as transfer clock.
25.7.1. Receiving Frames with 5 to 8 Data Bits
The Receiver starts data reception when it detects a valid start bit. Each bit that follows the start bit will be
sampled at the baud rate or XCK clock, and shifted into the Receive Shift Register until the first stop bit of
a frame is received. A second stop bit will be ignored by the Receiver. When the first stop bit is received
(i.e., a complete serial frame is present in the Receive Shift Register), the contents of the Shift Register
will be moved into the receive buffer. The receive buffer can then be read by reading the UDR I/O
location.
The following code example shows a simple USART receive function based on polling of the Receive
Complete (RXC) Flag. When using frames with less than eight bits the most significant bits of the data
read from the UDR will be masked to zero. The USART has to be initialized before the function can be
used.
Assembly Code Example(1)
USART_Receive:
; Wait for data to be received
sbis UCSRA, RXC
rjmp USART_Receive
; Get and return received data from buffer
in r16, UDR
ret
C Code Example(1)
unsigned char USART_Receive( void )
{
/* Wait for data to be received */
while ( !(UCSRA & (1<<RXC)) )
;
/* Get and return received data from buffer */
return UDR;
}
Note:  1. See About Code Examples.
The function simply waits for data to be present in the receive buffer by checking the
RXC Flag, before reading the buffer and returning the value.
Related Links
About Code Examples on page 20
25.7.2. Receiving Frames with 9 Data Bits
If 9-bit characters are used (UCSZ=7) the ninth bit must be read from the RXB8 bit in UCSRB before
reading the low bits from the UDR. This rule applies to the FE, DOR and UPE Status Flags as well. Read
status from UCSRA, then data from UDR. Reading the UDR I/O location will change the state of the
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receive buffer FIFO and consequently the TXB8, FE, DOR, and UPE bits, which all are stored in the
FIFO, will change.
The following code example shows a simple USART receive function that handles both 9-bit characters
and the status bits.
Assembly Code Example(1)
USART_Receive:
; Wait for data to be received
sbis UCSRA, RXC
rjmp USART_Receive
; Get status and 9th bit, then data from buffer
in r18, UCSRA
in r17, UCSRB
in r16, UDR
; If error, return -1
andi r18,(1<<FE)|(1<<DOR)|(1<<UPE)
breq USART_ReceiveNoError
ldi r17, HIGH(-1)
ldi r16, LOW(-1)
USART_ReceiveNoError:
; Filter the 9th bit, then return
lsr r17
andi r17, 0x01
ret
C Code Example(1)
unsigned int USART_Receive( void )
{
unsigned char status, resh, resl;
/* Wait for data to be received */
while ( !(UCSRA & (1<<RXC)) )
;
/* Get status and 9th bit, then data */
/* from buffer */
status = UCSRA;
resh = UCSRB;
resl = UDR;
/* If error, return -1 */
if (status & ((1<<FE)|(1<<DOR)|(1<<UPE)))
return -1;
/* Filter the 9th bit, then return */
resh = (resh >> 1) & 0x01;
return ((resh << 8) | resl);
}
Note:  1. See About Code Examples.
The receive function example reads all the I/O Registers into the Register File before any
computation is done. This gives an optimal receive buffer utilization since the buffer
location read will be free to accept new data as early as possible.
25.7.3. Receive Compete Flag and Interrupt
The USART Receiver has one flag that indicates the Receiver state.
The Receive Complete (RXC) Flag indicates if there are unread data present in the receive buffer. This
flag is one when unread data exist in the receive buffer, and zero when the receive buffer is empty (i.e.,
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does not contain any unread data). If the Receiver is disabled (RXEN = 0), the receive buffer will be
flushed and consequently the RXC bit will become zero.
When the Receive Complete Interrupt Enable (RXCIE) in UCSRB is set, the USART Receive Complete
Interrupt will be executed as long as the RXC Flag is set (provided that global interrupts are enabled).
When interrupt-driven data reception is used, the receive complete routine must read the received data
from UDR in order to clear the RXC Flag, otherwise a new interrupt will occur once the interrupt routine
terminates.
25.7.4. Receiver Error Flags
The USART Receiver has three error flags: Frame Error (FE), Data OverRun (DOR) and Parity Error
(UPE). All can be accessed by reading UCSRA. Common for the error flags is that they are located in the
receive buffer together with the frame for which they indicate the error status. Due to the buffering of the
error flags, the UCSRA must be read before the receive buffer (UDR), since reading the UDR I/O location
changes the buffer read location. Another equality for the error flags is that they can not be altered by
software doing a write to the flag location. However, all flags must be set to zero when the UCSRA is
written for upward compatibility of future USART implementations. None of the error flags can generate
interrupts.
The Frame Error (FE) Flag indicates the state of the first stop bit of the next readable frame stored in the
receive buffer. The FE Flag is zero when the stop bit was correctly read (as one), and the FE Flag will be
one when the stop bit was incorrect (zero). This flag can be used for detecting out-of-sync conditions,
detecting break conditions and protocol handling. The FE Flag is not affected by the setting of the USBS
bit in UCSRC since the Receiver ignores all, except for the first, stop bits. For compatibility with future
devices, always set this bit to zero when writing to UCSRA.
The Data OverRun (DOR) Flag indicates data loss due to a Receiver buffer full condition. A Data
OverRun occurs when the receive buffer is full (two characters), it is a new character waiting in the
Receive Shift Register, and a new start bit is detected. If the DOR Flag is set there was one or more serial
frame lost between the frame last read from UDR, and the next frame read from UDR. For compatibility
with future devices, always write this bit to zero when writing to UCSRA. The DOR Flag is cleared when
the frame received was successfully moved from the Shift Register to the receive buffer.
The Parity Error (UPE) Flag indicates that the next frame in the receive buffer had a parity error when
received. If parity check is not enabled the UPE bit will always be read zero. For compatibility with future
devices, always set this bit to zero when writing to UCSRA. For more details, refer to Parity Bit
Calculation on page 251 and Parity Checker on page 257.
25.7.5. Parity Checker
The Parity Checker is active when the high USART Parity mode (UPM1) bit is set. Type of parity check to
be performed (odd or even) is selected by the UPM0 bit. When enabled, the Parity Checker calculates the
parity of the data bits in incoming frames and compares the result with the parity bit from the serial frame.
The result of the check is stored in the receive buffer together with the received data and stop bits. The
Parity Error (UPE) Flag can then be read by software to check if the frame had a parity error.
The UPE bit is set if the next character that can be read from the receive buffer had a parity error when
received and the parity checking was enabled at that point (UPM1 = 1). This bit is valid until the receive
buffer (UDR) is read.
25.7.6. Disabling the Receiver
In contrast to the Transmitter, disabling of the Receiver will be immediate. Data from ongoing receptions
will therefore be lost. When disabled (i.e., the RXEN is set to zero) the Receiver will no longer override
the normal function of the RxD port pin. The Receiver buffer FIFO will be flushed when the Receiver is
disabled. Remaining data in the buffer will be lost.
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25.7.7. Flushing the Receive Buffer
The Receiver buffer FIFO will be flushed when the Receiver is disabled (i.e., the buffer will be emptied of
its contents). Unread data will be lost. If the buffer has to be flushed during normal operation, due to for
instance an error condition, read the UDR I/O location until the RXC Flag is cleared. The following code
example shows how to flush the receive buffer.
Assembly Code Example(1)
USART_Flush:
sbis UCSRA, RXC
ret
in r16, UDR
rjmp USART_Flush
C Code Example(1)
void USART_Flush( void )
{
unsigned char dummy;
while ( UCSRA & (1<<RXC) ) dummy = UDR;
}
Note:  1. See About Code Examples.
The USART includes a clock recovery and a data recovery unit for handling
asynchronous data reception. The clock recovery logic is used for synchronizing the
internally generated baud rate clock to the incoming asynchronous serial frames at the
RxD pin. The data recovery logic samples and low pass filters each incoming bit, thereby
improving the noise immunity of the receiver. The asynchronous reception operational
range depends on the accuracy of the internal baud rate clock, the rate of the incoming
frames, and the frame size in number of bits.
Related Links
About Code Examples on page 20
25.8. Asynchronous Data Reception
The USART includes a clock recovery and a data recovery unit for handling asynchronous data reception.
The clock recovery logic is used for synchronizing the internally generated baud rate clock to the
incoming asynchronous serial frames at the RxD pin. The data recovery logic samples and low pass
filters each incoming bit, thereby improving the noise immunity of the Receiver. The asynchronous
reception operational range depends on the accuracy of the internal baud rate clock, the rate of the
incoming frames, and the frame size in number of bits.
25.8.1. Asynchronous Clock Recovery
The clock recovery logic synchronizes internal clock to the incoming serial frames. The figure below
illustrates the sampling process of the start bit of an incoming frame. The sample rate is 16 times the
baud rate for Normal mode, and eight times the baud rate for Double Speed mode. The horizontal arrows
illustrate the synchronization variation due to the sampling process. Note the larger time variation when
using the Double Speed mode (U2X = 1) of operation. Samples denoted zero are samples done when the
RxD line is idle (i.e., no communication activity).
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Figure 25-5 Start Bit Sampling
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1 2
STARTIDLE
00
BIT 0
3
1 2 3 4 5 6 7 8 1 20
RxD
Sample
(U2X = 0)
Sample
(U2X = 1)
When the clock recovery logic detects a high (idle) to low (start) transition on the RxD line, the start bit
detection sequence is initiated. Let sample 1 denote the first zero-sample as shown in the figure. The
clock recovery logic then uses samples 8, 9 and 10 for Normal mode, and samples 4, 5 and 6 for Double
Speed mode (indicated with sample numbers inside boxes on the figure), to decide if a valid start bit is
received. If two or more of these three samples have logical high levels (the majority wins), the start bit is
rejected as a noise spike and the Receiver starts looking for the next high to low-transition. If however, a
valid start bit is detected, the clock recovery logic is synchronized and the data recovery can begin. The
synchronization process is repeated for each start bit.
25.8.2. Asynchronous Data Recovery
When the Receiver clock is synchronized to the start bit, the data recovery can begin. The data recovery
unit uses a state machine that has 16 states for each bit in Normal mode and eight states for each bit in
Double Speed mode. The following figure shows the sampling of the data bits and the parity bit. Each of
the samples is given a number that is equal to the state of the recovery unit.
Figure 25-6 Sampling of Data and Parity Bit
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1
BIT n
1 2 3 4 5 6 7 8 1
RxD
Sample
(U2X = 0)
Sample
(U2X = 1)
The decision of the logic level of the received bit is taken by doing a majority voting of the logic value to
the three samples in the center of the received bit. The center samples are emphasized on the figure by
having the sample number inside boxes. The majority voting process is done as follows: If two or all three
samples have high levels, the received bit is registered to be a logic 1. If two or all three samples have
low levels, the received bit is registered to be a logic 0. This majority voting process acts as a low pass
filter for the incoming signal on the RxD pin. The recovery process is then repeated until a complete
frame is received. Including the first stop bit. Note that the Receiver only uses the first stop bit of a frame.
The following figure shows the sampling of the stop bit and the earliest possible beginning of the start bit
of the next frame.
Figure 25-7 Stop Bit Sampling and Next Start Bit Sampling
1 2 3 4 5 6 7 8 9 10 0/1 0/1 0/1
STOP 1
1 2 3 4 5 6 0/1
RxD
Sample
(U2X = 0)
Sample
(U2X = 1)
(A) (B) (C)
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R slow = (D + 1)5 S—1+DvS+SF R _ (D+2)S mr‘ (D+1)S+SM Atmel
The same majority voting is done to the stop bit as done for the other bits in the frame. If the stop bit is
registered to have a logic 0 value, the Frame Error (FE) Flag will be set.
A new high to low transition indicating the start bit of a new frame can come right after the last of the bits
used for majority voting. For Normal Speed mode, the first low level sample can be at point marked (A) in
the figure above. For Double Speed mode the first low level must be delayed to (B). (C) marks a stop bit
of full length. The early start bit detection influences the operational range of the Receiver.
25.8.3. Asynchronous Operational Range
The operational range of the Receiver is dependent on the mismatch between the received bit rate and
the internally generated baud rate. If the Transmitter is sending frames at too fast or too slow bit rates, or
the internally generated baud rate of the Receiver does not have a similar (refer to next table) base
frequency, the Receiver will not be able to synchronize the frames to the start bit.
The following equations can be used to calculate the ratio of the incoming data rate and internal receiver
baud rate.
slow =+ 1
1+ +
fast =+ 2
+ 1 +
DSum of character size and parity size (D = 5- to 10-bit).
SSamples per bit. S = 16 for Normal Speed mode and S = 8 for Double Speed mode.
SFFirst sample number used for majority voting. SF = 8 for Normal Speed and SF = 4 for Double
Speed mode.
SMMiddle sample number used for majority voting. SM = 9 for Normal Speed and SM = 5 for Double
Speed mode.
Rslow is the ratio of the slowest incoming data rate that can be accepted in relation to the Receiver
baud rate.
Rfast is the ratio of the fastest incoming data rate that can be accepted in relation to the Receiver baud
rate.
The following tables list the maximum receiver baud rate error that can be tolerated. Note that Normal
Speed mode has higher toleration of baud rate variations.
Table 25-2 Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode (U2X = 0)
D
# (Data+Parity Bit)
Rslow [%] Rfast [%] Max. Total Error [%] Recommended Max Receiver Error
[%]
5 93.20 106.67 +6.67/-6.8 ±3.0
6 94.12 105.79 +5.79/-5.88 ±2.5
7 94.81 105.11 +5.11/-5.19 ±2.0
8 95.36 104.58 +4.58/-4.54 ±2.0
9 95.81 104.14 +4.14/-4.19 ±1.5
10 96.17 103.78 +3.78/-3.83 ±1.5
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Table 25-3 Recommended Maximum Receiver Baud Rate Error for Double Speed Mode (U2X = 1)
D
# (Data+Parity Bit)
Rslow [%] Rfast [%] Max Total Error [%] Recommended Max
Receiver Error [%]
5 94.12 105.66 +5.66/-5.88 ±2.5
6 94.92 104.92 +4.92/-5.08 ±2.0
7 95.52 104.35 +4.35/-4.48 ±1.5
8 96.00 103.90 +3.90/-4.00 ±1.5
9 96.39 103.53 +3.53/-3.61 ±1.5
10 96.70 103.23 +3.23/-3.30 ±1.0
The recommendations of the maximum Receiver baud rate error was made under the assumption that
the Receiver and Transmitter equally divides the maximum total error.
There are two possible sources for the Receivers Baud Rate error. The Receiver’s system clock (XTAL)
will always have some minor instability over the supply voltage range and the temperature range. When
using a crystal to generate the system clock, this is rarely a problem, but for a resonator the system clock
may differ more than 2% depending of the resonators tolerance. The second source for the error is more
controllable. The baud rate generator can not always do an exact division of the system frequency to get
the baud rate wanted. In this case an UBRR value that gives an acceptable low error can be used if
possible.
25.9. Multi-Processor Communication Mode
Setting the Multi-processor Communication mode (MPCM) bit in UCSRA enables a filtering function of
incoming frames received by the USART Receiver. Frames that do not contain address information will
be ignored and not put into the receive buffer. This effectively reduces the number of incoming frames
that has to be handled by the CPU, in a system with multiple MCUs that communicate via the same serial
bus. The Transmitter is unaffected by the MPCM setting, but has to be used differently when it is a part of
a system utilizing the Multi-processor Communication mode.
If the Receiver is set up to receive frames that contain 5 to 8 data bits, then the first stop bit indicates if
the frame contains data or address information. If the Receiver is set up for frames with nine data bits,
then the ninth bit (RXB8) is used for identifying address and data frames. When the frame type bit (the
first stop or the ninth bit) is one, the frame contains an address. When the frame type bit is zero the frame
is a data frame.
The Multi-processor Communication mode enables several Slave MCUs to receive data from a Master
MCU. This is done by first decoding an address frame to find out which MCU has been addressed. If a
particular Slave MCU has been addressed, it will receive the following data frames as normal, while the
other Slave MCUs will ignore the received frames until another address frame is received.
25.9.1. Using MPCM
For an MCU to act as a Master MCU, it can use a 9-bit character frame format (UCSZ = 7). The ninth bit
(TXB8) must be set when an address frame (TXB8 = 1) or cleared when a data frame (TXB = 0) is being
transmitted. The Slave MCUs must in this case be set to use a 9-bit character frame format.
The following procedure should be used to exchange data in Multi-Processor Communication Mode:
1. All Slave MCUs are in Multi-processor Communication mode (MPCM in UCSRA is set).
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2. The Master MCU sends an address frame, and all slaves receive and read this frame. In the Slave
MCUs, the RXC Flag in UCSRA will be set as normal.
3. Each Slave MCU reads the UDR Register and determines if it has been selected. If so, it clears the
MPCM bit in UCSRA, otherwise it waits for the next address byte and keeps the MPCM setting.
4. The addressed MCU will receive all data frames until a new address frame is received. The other
Slave MCUs, which still have the MPCM bit set, will ignore the data frames.
5. When the last data frame is received by the addressed MCU, the addressed MCU sets the MPCM
bit and waits for a new address frame from Master. The process then repeats from 2.
Using any of the 5- to 8-bit character frame formats is possible, but impractical since the Receiver must
change between using n and n+1 character frame formats. This makes full-duplex operation difficult since
the Transmitter and Receiver uses the same character size setting. If 5- to 8-bit character frames are
used, the Transmitter must be set to use two stop bit (USBS = 1) since the first stop bit is used for
indicating the frame type.
Do not use Read-Modify-Write instructions (SBI and CBI) to set or clear the MPCM bit. The MPCM bit
shares the same I/O location as the TXC Flag and this might accidentally be cleared when using SBI or
CBI instructions.
25.10. Examples of Baud Rate Setting
For standard crystal and resonator frequencies, the most commonly used baud rates for asynchronous
operation can be generated by using the UBRR settings as listed in the table below.
UBRR values which yield an actual baud rate differing less than 0.5% from the target baud rate, are bold
in the table. Higher error ratings are acceptable, but the Receiver will have less noise resistance when the
error ratings are high, especially for large serial frames (see Asynchronous Operational Range). The error
values are calculated using the following equation:
 % = BaudRateClosest Match
BaudRate 1× 100 %
Table 25-4 Examples of UBRR Settings for Commonly Used Oscillator Frequencies
Baud
Rate
[bps]
fosc = 1.0000MHz fosc = 1.8432MHz fosc = 2.0000MHz
U2X = 0 U2X = 1 U2X= 0 U2X = 1 U2X = 0 U2X = 1
UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error
2400 25 0.2% 51 0.2% 47 0.0% 95 0.0% 51 0.2% 103 0.2%
4800 12 0.2% 25 0.2% 23 0.0% 47 0.0% 25 0.2% 51 0.2%
9600 6 -7.0% 12 0.2% 11 0.0% 23 0.0% 12 0.2% 25 0.2%
14.4k 3 8.5% 8 -3.5% 7 0.0% 15 0.0% 8 -3.5% 16 2.1%
19.2k 2 8.5% 6 -7.0% 5 0.0% 11 0.0% 6 -7.0% 12 0.2%
28.8k 1 8.5% 3 8.5% 3 0.0% 7 0.0% 3 8.5% 8 -3.5%
38.4k 1 -18.6% 2 8.5% 2 0.0% 5 0.0% 2 8.5% 6 -7.0%
57.6k 0 8.5% 1 8.5% 1 0.0% 3 0.0% 1 8.5% 3 8.5%
76.8k – – 1 -18.6% 1 -25.0% 2 0.0% 1 -18.6% 2 8.5%
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Baud
Rate
[bps]
fosc = 1.0000MHz fosc = 1.8432MHz fosc = 2.0000MHz
U2X = 0 U2X = 1 U2X= 0 U2X = 1 U2X = 0 U2X = 1
UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error
115.2k – – 0 8.5% 0 0.0% 1 0.0% 0 8.5% 1 8.5%
230.4k – – – – – – 0 0.0% – – – –
250k – – – – – – – – – – 0 0.0%
Max(1) 62.5kbps 125kbps 115.2kbps 230.4kbps 125kbps 250kbps
Note:  1. UBRR = 0, Error = 0.0%
Table 25-5 Examples of UBRR Settings for Commonly Used Oscillator Frequencies (Continued)
Baud
Rate
[bps]
fosc = 3.6864MHz fosc = 4.0000MHz fosc = 7.3728MHz
U2X = 0 U2X = 1 U2X = 0 U2X = 1 U2X = 0 U2X = 1
UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error
2400 95 0.0% 191 0.0% 103 0.2% 207 0.2% 191 0.0% 383 0.0%
4800 47 0.0% 95 0.0% 51 0.2% 103 0.2% 95 0.0% 191 0.0%
9600 23 0.0% 47 0.0% 25 0.2% 51 0.2% 47 0.0% 95 0.0%
14.4k 15 0.0% 31 0.0% 16 2.1% 34 -0.8% 31 0.0% 63 0.0%
19.2k 11 0.0% 23 0.0% 12 0.2% 25 0.2% 23 0.0% 47 0.0%
28.8k 7 0.0% 15 0.0% 8 -3.5% 16 2.1% 15 0.0% 31 0.0%
38.4k 5 0.0% 11 0.0% 6 -7.0% 12 0.2% 11 0.0% 23 0.0%
57.6k 3 0.0% 7 0.0% 3 8.5% 8 -3.5% 7 0.0% 15 0.0%
76.8k 2 0.0% 5 0.0% 2 8.5% 6 -7.0% 5 0.0% 11 0.0%
115.2k 1 0.0% 3 0.0% 1 8.5% 3 8.5% 3 0.0% 7 0.0%
230.4k 0 0.0% 1 0.0% 0 8.5% 1 8.5% 1 0.0% 3 0.0%
250k 0 -7.8% 1 -7.8% 0 0.0% 1 0.0% 1 -7.8% 3 -7.8%
0.5M – – 0 -7.8% – – 0 0.0% 0 -7.8% 1 -7.8%
1M – – – – – – – – – – 0 -7.8%
Max.(1) 230.4kbps 460.8kbps 250kbps 0.5Mbps 460.8kbps 921.6kbps
Note:  1. UBRR = 0, Error = 0.0%
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Table 25-6 Examples of UBRR Settings for Commonly Used Oscillator Frequencies (Continued)
Baud
Rate
[bps]
fosc = 8.0000MHz fosc = 11.0592MHz fosc = 14.7456MHz
U2X = 0 U2X = 1 U2X = 0 U2X = 1 U2X = 0 U2X = 1
UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error
2400 207 0.2% 416 -0.1% 287 0.0% 575 0.0% 383 0.0% 767 0.0%
4800 103 0.2% 207 0.2% 143 0.0% 287 0.0% 191 0.0% 383 0.0%
9600 51 0.2% 103 0.2% 71 0.0% 143 0.0% 95 0.0% 191 0.0%
14.4k 34 -0.8% 68 0.6% 47 0.0% 95 0.0% 63 0.0% 127 0.0%
19.2k 25 0.2% 51 0.2% 35 0.0% 71 0.0% 47 0.0% 95 0.0%
28.8k 16 2.1% 34 -0.8% 23 0.0% 47 0.0% 31 0.0% 63 0.0%
38.4k 12 0.2% 25 0.2% 17 0.0% 35 0.0% 23 0.0% 47 0.0%
57.6k 8 -3.5% 16 2.1% 11 0.0% 23 0.0% 15 0.0% 31 0.0%
76.8k 6 -7.0% 12 0.2% 8 0.0% 17 0.0% 11 0.0% 23 0.0%
115.2k 3 8.5% 8 -3.5% 5 0.0% 11 0.0% 7 0.0% 15 0.0%
230.4k 1 8.5% 3 8.5% 2 0.0% 5 0.0% 3 0.0% 7 0.0%
250k 1 0.0% 3 0.0% 2 -7.8% 5 -7.8% 3 -7.8% 6 5.3%
0.5M 0 0.0% 1 0.0% – – 2 -7.8% 1 -7.8% 3 -7.8%
1M – – 0 0.0% – – – – 0 -7.8% 1 -7.8%
Max.(1) 0.5Mbps 1Mbps 691.2kbps 1.3824Mbps 921.6kbps 1.8432Mbps
Note:  1. UBRR = 0, Error = 0.0%
Table 25-7 Examples of UBRR Settings for Commonly Used Oscillator Frequencies (Continued)
Baud
Rate
[bps]
fosc = 16.0000MHz fosc = 18.4320MHz fosc = 20.0000MHz
U2X = 0 U2X = 1 U2X = 0 U2X = 1 U2X = 0 U2X = 1
UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error
2400 416 -0.1% 832 0.0% 479 0.0% 959 0.0% 520 0.0% 1041 0.0%
4800 207 0.2% 416 -0.1% 239 0.0% 479 0.0% 259 0.2% 520 0.0%
9600 103 0.2% 207 0.2% 119 0.0% 239 0.0% 129 0.2% 259 0.2%
14.4k 68 0.6% 138 -0.1% 79 0.0% 159 0.0% 86 -0.2% 173 -0.2%
19.2k 51 0.2% 103 0.2% 59 0.0% 119 0.0% 64 0.2% 129 0.2%
28.8k 34 -0.8% 68 0.6% 39 0.0% 79 0.0% 42 0.9% 86 -0.2%
38.4k 25 0.2% 51 0.2% 29 0.0% 59 0.0% 32 -1.4% 64 0.2%
57.6k 16 2.1% 34 -0.8% 19 0.0% 39 0.0% 21 -1.4% 42 0.9%
76.8k 12 0.2% 25 0.2% 14 0.0% 29 0.0% 15 1.7% 32 -1.4%
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Baud
Rate
[bps]
fosc = 16.0000MHz fosc = 18.4320MHz fosc = 20.0000MHz
U2X = 0 U2X = 1 U2X = 0 U2X = 1 U2X = 0 U2X = 1
UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error
115.2k 8 -3.5% 16 2.1% 9 0.0% 19 0.0% 10 -1.4% 21 -1.4%
230.4k 3 8.5% 8 -3.5% 4 0.0% 9 0.0% 4 8.5% 10 -1.4%
250k 3 0.0% 7 0.0% 4 -7.8% 8 2.4% 4 0.0% 9 0.0%
0.5M 1 0.0% 3 0.0% – – 4 -7.8% – – 4 0.0%
1M 0 0.0% 1 0.0% – – – – – – – –
Max.(1) 1Mbps 2Mbps 1.152Mbps 2.304Mbps 1.25Mbps 2.5Mbps
Note:  1. UBRR = 0, Error = 0.0%
25.11. Register Description
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25.11.1. UDRn – USART I/O Data Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  UDRn
Offset:  0x0C
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x2C
Bit 7 6 5 4 3 2 1 0
TXB / RXB[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – TXB / RXB[7:0]: USART Transmit / Receive Data Buffer
The USARTn Transmit Data Buffer Register and USARTn Receive Data Buffer Registers share the same
I/O address referred to as USARTn Data Register or UDRn. The Transmit Data Buffer Register (TXBn)
will be the destination for data written to the UDRn Register location. Reading the UDRn Register location
will return the contents of the Receive Data Buffer Register (RXBn).
For 5-, 6-, or 7-bit characters the upper unused bits will be ignored by the Transmitter and set to zero by
the Receiver.
The transmit buffer can only be written when the UDREn Flag in the UCSRAn Register is set. Data
written to UDRn when the UDREn Flag is not set, will be ignored by the USARTn Transmitter. When data
is written to the transmit buffer, and the Transmitter is enabled, the Transmitter will load the data into the
Transmit Shift Register when the Shift Register is empty. Then the data will be serially transmitted on the
TxDn pin.
The receive buffer consists of a two level FIFO. The FIFO will change its state whenever the receive
buffer is accessed. Due to this behavior of the receive buffer, do not use Read-Modify-Write instructions
(SBI and CBI) on this location. Be careful when using bit test instructions (SBIC and SBIS), since these
also will change the state of the FIFO.
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25.11.2. UCSRmA – USART Control and Status Register A
Name:  UCSRmA
Offset:  0x9B
Reset:  0x20
Property:
 
–
Bit 7 6 5 4 3 2 1 0
RXCm TXCm UDREm FEm DORm UPEm U2Xm MPCMm
Access R R/W R R R R R/W R/W
Reset 0 0 1 0 0 0 0 0
Bit 7 – RXCm: USART Receive Complete
This flag bit is set when there are unread data in the receive buffer and cleared when the receive buffer is
empty (that is, does not contain any unread data). If the receiver is disabled, the receive buffer will be
flushed and consequently the RXCm bit will become zero. The RXCm flag can be used to generate a
Receive Complete interrupt (see description of the RXCIEm bit).
Bit 6 – TXCm: USART Transmit Complete
This flag bit is set when the entire frame in the Transmit Shift Register has been shifted out and there are
no new data currently present in the transmit buffer (UDRm). The TXCm flag bit is automatically cleared
when a transmit complete interrupt is executed, or it can be cleared by writing a one to its bit location. The
TXCm flag can generate a Transmit Complete interrupt (see description of the TXCIEm bit).
Bit 5 – UDREm: USART Data Register Empty
The UDREm flag indicates if the transmit buffer (UDRm) is ready to receive new data. If UDREm is one,
the buffer is empty, and therefore ready to be written. The UDREm flag can generate a Data Register
Empty interrupt (see description of the UDRIEm bit).
UDREm is set after a reset to indicate that the Transmitter is ready.
Bit 4 – FEm: Frame Error
This bit is set if the next character in the receive buffer had a Frame Error when received, that is, when
the first stop bit of the next character in the receive buffer is zero. This bit is valid until the receive buffer
(UDRm) is read. The FEm bit is zero when the stop bit of received data is one. Always set this bit to zero
when writing to UCSRmA.
Bit 3 – DORm: Data OverRun
This bit is set if a Data OverRun condition is detected. A data overrun occurs when the receive buffer is
full (two characters), it is a new character waiting in the Receive Shift Register, and a new start bit is
detected. This bit is valid until the receive buffer (UDRm) is read. Always set this bit to zero when writing
to UCSRmA.
Bit 2 – UPEm: Parity Error
This bit is set if the next character in the receive buffer had a Parity Error when received and the parity
checking was enabled at that point (UPMm1 = 1). This bit is valid until the receive buffer (UDRm) is read.
Always set this bit to zero when writing to UCSRmA.
Bit 1 – U2Xm: Double the USART Transmission Speed
This bit only has effect for the asynchronous operation. Write this bit to zero when using synchronous
operation.
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Writing this bit to one will reduce the divisor of the baud rate divider from 16 to 8 effectively doubling the
transfer rate for asynchronous communication.
Bit 0 – MPCMm: Multi-processor Communication Mode
This bit enables the Multi-processor Communication mode. When the MPCMm bit is written to one, all the
incoming frames received by the USART Receiver that do not contain address information will be
ignored. The transmitter is unaffected by the MPCMm setting. For more detailed information, refer to
Multi-Processor Communication Mode on page 261.
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25.11.3. UCSRmB – USART Control and Status Register B
Name:  UCSRmB
Offset:  0x9A
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
RXCIEm TXCIEm UDRIEm RXENm TXENm UCSZm2 RXB8m TXB8m
Access R/W R/W R/W R/W R/W R/W R R/W
Reset 0 0 0 0 0 0 0 0
Bit 7 – RXCIEm: RX Complete Interrupt Enable
Writing this bit to one enables interrupt on the RXC flag. A USART Receive Complete interrupt will be
generated only if the RXCIE bit is written to one, the global interrupt flag in SREG is written to one and
the RXC bit in UCSRmA is set.
Bit 6 – TXCIEm: TX Complete Interrupt Enable
Writing this bit to one enables interrupt on the TXCm flag. A USARTm Transmit Complete interrupt will be
generated only if the TXCIEm bit is written to one, the global interrupt flag in SREG is written to one and
the TXCm bit in UCSRmA is set.
Bit 5 – UDRIEm: USART Data Register Empty Interrupt Enable
Writing this bit to one enables interrupt on the UDREm flag. A Data Register Empty interrupt will be
generated only if the UDRIEm bit is written to one, the global interrupt flag in SREG is written to one and
the UDREm bit in UCSRmA is set.
Bit 4 – RXENm: Receiver Enable
Writing this bit to one enables the USARTm Receiver. The Receiver will override normal port operation for
the RxDm pin when enabled. Disabling the Receiver will flush the receive buffer invalidating the FEm,
DORm and UPEm flags..
Bit 3 – TXENm: Transmitter Enable
Writing this bit to one enables the USARTm Transmitter. The Transmitter will override normal port
operation for the TxDm pin when enabled. The disabling of the Transmitter (writing TXENm to zero) will
not become effective until ongoing and pending transmissions are completed, that is, when the Transmit
Shift Register and transmit buffer register do not contain data to be transmitted. When disabled, the
transmitter will no longer override the TxDm port.
Bit 2 – UCSZm2: Character Size
The UCSZm2 bits combined with the UCSZm1:0 bit in UCSRmC sets the number of data bits (character
size) in a frame the Receiver and Transmitter use.
Bit 1 – RXB8m: Receive Data Bit 8
RXB8m is the ninth data bit of the received character when operating with serial frames with 9-data bits.
Must be read before reading the low bits from UDRm.
Bit 0 – TXB8m: Transmit Data Bit 8
TXB8m is the 9th data bit in the character to be transmitted when operating with serial frames with 9 data
bits. Must be written before writing the low bits to UDRm.
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25.11.4. UCSRmC – USART Control and Status Register C
Note:  This register is not available in ATmega103 compatibility mode.
Name:  UCSRmC
Offset:  0x20
Reset:  0x06
Property:
 
When addressing I/O Registers as data space the offset address is 0x40
Bit 7 6 5 4 3 2 1 0
UMSELm UPMm1 UPMm0 USBSm UCSZm1 UCSZm0 UCPOLm
Access R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 1 0
Bit 6 – UMSELm: Mode Select
This bit selects between Asynchronous and Synchronous mode of operation.
Table 25-8 UMSEL Bit Settings
UMSEL Bit Settings Mode
0 Asynchronous Operation
1 Synchronous Operation
Bits 5:4 – UPMmn: Parity Mode [n = 1:0]
UPMm1 and UPMm0 bits enable and set type of parity generation and check. If enabled, the Transmitter
will automatically generate and send the parity of the transmitted data bits within each frame. The
Receiver will generate a parity value for the incoming data and compare it to the UPMm0 setting. If a
mismatch is detected, the UPEm flag in UCSRmA will be set.
Table 25-9 UPM Bits Settings
UPMm1 UPMm0 ParityMode
0 0 Disabled
0 1 Reserved
1 0 Enabled, Even Parity
1 1 Enabled, Odd Parity
Bit 3 – USBSm: Stop Bit Select
This bit selects the number of stop bits to be inserted by the Transmitter. The Receiver ignores this
setting.
Table 25-10 USBS Bit Settings
USBSm Stop Bit(s)
0 1-bit
1 2-bit
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Bits 2:1 – UCSZmn: Character Size [n = 1:0]
The UCSZm1:0 bits combined with the UCSZm2 bit in UCSRmB sets the number of data bits (Character
Size) in a frame the Receiver and Transmitter use.
Table 25-11 UCSZ Bits Settings
UCSZm2 UCSZm1 UCSZm0 Character Size
0 0 0 5-bit
0 0 1 6-bit
0 1 0 7-bit
0 1 1 8-bit
1 0 0 Reserved
1 0 1 Reserved
1 1 0 Reserved
1 1 1 9-bit
Bit 0 – UCPOLm: Clock Polarity
This bit is used for synchronous mode only. Write this bit to zero when Asynchronous mode is used. The
UCPOLm bit sets the relationship between data output change and data input sample, and the
synchronous clock (XCKm).
Table 25-12 UCPOLm Bit Settings
UCPOLm Transmitted Data Changed
(Output of TxDm Pin)
Received Data Sampled
(Input on RxDm Pin)
0 Rising XCKm Edge Falling XCKm Edge
1 Falling XCKm Edge Rising XCKm Edge
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25.11.5. UBRRmL – USART Baud Rate Register Low
Name:  UBRRmL
Offset:  0x99
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
UBBRm[7:0]
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – UBBRm[7:0]: USARTm Baud Rate Register
This is a 12-bit register which contains the USARTm baud rate. The UBRRmH contains the four most
significant bits, and the UBRRmL contains the eight least significant bits of the USARTm baud rate.
Ongoing transmissions by the transmitter and receiver will be corrupted if the baud rate is changed.
Writing UBRRmL will trigger an immediate update of the baud rate prescaler.
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25.11.6. UBBRmH – USART Baud Rate Register High
Note:  UBRRmH is not available in mega103 compatibility mode.
Name:  UBBRmH
Offset:  0x20
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
UBRRm[3:0]
Access R/W R/W R/W R/W
Reset 0 0 0 0
Bits 3:0 – UBRRm[3:0]: USART Baud Rate Register
The bits in this register ranges from UBRRm[11:8]. Refer to UBBRmL.
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26. TWI - Two-wire Serial Interface
26.1. Features
•Simple, yet Powerful and Flexible Communication Interface, only two Bus Lines Needed
•Both Master and Slave Operation Supported
•Device can Operate as Transmitter or Receiver
•7-bit Address Space Allows up to 128 Different Slave Addresses
•Multi-master Arbitration Support
•Up to 400kHz Data Transfer Speed
•Slew-rate Limited Output Drivers
•Noise Suppression Circuitry Rejects Spikes on Bus Lines
•Fully Programmable Slave Address with General Call Support
•Address Recognition Causes Wake-up When AVR is in Sleep Mode
26.2. Overview
The TWI module is comprised of several submodules, as shown in the following figure. All registers
drawn in a thick line are accessible through the AVR data bus.
Figure 26-1 Overview of the TWI Module
TWI Unit
Address Register
(TW AR)
Address Match Unit
Address Compar ator
Control Unit
Control Register
(TWCR)
Status Register
(TWSR)
State Machine and
Status control
SCL
Sle w-r ate
Control
Spik e
Filter
SD A
Sle w-r ate
Control
Spik e
Filter
Bit Rate Gener ator
Bit Rate Register
(TWBR)
Prescaler
Bus Interf ace Unit
ST AR T / ST OP
Control
Arbitration detection Ack
Spik e Suppression
Address/Data Shift
Register (TWDR)
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26.2.1. SCL and SDA Pins
These pins interface the AVR TWI with the rest of the MCU system. The output drivers contain a slew-
rate limiter in order to conform to the TWI specification. The input stages contain a spike suppression unit
removing spikes shorter than 50 ns. Note that the internal pull-ups in the AVR pads can be enabled by
setting the PORT bits corresponding to the SCL and SDA pins, as explained in the I/O Port section. The
internal pull-ups can in some systems eliminate the need for external ones.
26.2.2. Bit Rate Generator Unit
This unit controls the period of SCL when operating in a Master mode. The SCL period is controlled by
settings in the TWI Bit Rate Register (TWBR) and the Prescaler bits in the TWI Status Register (TWSR).
Slave operation does not depend on Bit Rate or Prescaler settings, but the CPU clock frequency in the
Slave must be at least 16 times higher than the SCL frequency. Note that slaves may prolong the SCL
low period, thereby reducing the average TWI bus clock period.
The SCL frequency is generated according to the following equation:
SCL frequency = CPU Clock frequency
16 + 2(TWBR) PrescalerValue
•TWBR = Value of the TWI Bit Rate Register
•PrescalerValue = Value of the prescaler, see description of the TWI Prescaler bit in the TWSR
Status Register description (TWSR.TWPS)
Note:  Pull-up resistor values should be selected according to the SCL frequency and the capacitive bus
line load. See the Two-Wire Serial Interface Characteristics for a suitable value of the pull-up resistor.
Related Links
Two-wire Serial Interface Characteristics on page 419
26.2.3. Bus Interface Unit
This unit contains the Data and Address Shift Register (TWDR), a START/STOP Controller and
Arbitration detection hardware. The TWDR contains the address or data bytes to be transmitted, or the
address or data bytes received. In addition to the 8-bit TWDR, the Bus Interface Unit also contains a
register containing the (N)ACK bit to be transmitted or received. This (N)ACK Register is not directly
accessible by the application software. However, when receiving, it can be set or cleared by manipulating
the TWI Control Register (TWCR). When in Transmitter mode, the value of the received (N)ACK bit can
be determined by the value in the TWSR.
The START/STOP Controller is responsible for generation and detection of START, REPEATED START,
and STOP conditions. The START/STOP controller is able to detect START and STOP conditions even
when the AVR MCU is in one of the sleep modes, enabling the MCU to wake up if addressed by a Master.
If the TWI has initiated a transmission as Master, the Arbitration Detection hardware continuously
monitors the transmission trying to determine if arbitration is in process. If the TWI has lost an arbitration,
the Control Unit is informed. Correct action can then be taken and appropriate status codes generated.
26.2.4. Address Match Unit
The Address Match unit checks if received address bytes match the seven-bit address in the TWI
Address Register (TWAR). If the TWI General Call Recognition Enable (TWGCE) bit in the TWAR is
written to one, all incoming address bits will also be compared against the General Call address. Upon an
address match, the Control Unit is informed, allowing correct action to be taken. The TWI may or may not
acknowledge its address, depending on settings in the TWCR. The Address Match unit is able to
compare addresses even when the AVR MCU is in sleep mode, enabling the MCU to wake up if
addressed by a Master. If another interrupt (e.g., INT0) occurs during TWI Power-down address match
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and wakes up the CPU, the TWI aborts operation and return to it’s idle state. If this cause any problems,
ensure that TWI Address Match is the only enabled interrupt when entering Power-down.
26.2.5. Control Unit
The Control unit monitors the TWI bus and generates responses corresponding to settings in the TWI
Control Register (TWCR). When an event requiring the attention of the application occurs on the TWI
bus, the TWI Interrupt Flag (TWINT) is asserted. In the next clock cycle, the TWI Status Register (TWSR)
is updated with a status code identifying the event. The TWSR only contains relevant status information
when the TWI Interrupt Flag is asserted. At all other times, the TWSR contains a special status code
indicating that no relevant status information is available. As long as the TWINT Flag is set, the SCL line
is held low. This allows the application software to complete its tasks before allowing the TWI
transmission to continue.
The TWINT Flag is set in the following situations:
•After the TWI has transmitted a START/REPEATED START condition.
•After the TWI has transmitted SLA+R/W.
•After the TWI has transmitted an address byte.
•After the TWI has lost arbitration.
•After the TWI has been addressed by own slave address or general call.
•After the TWI has received a data byte.
•After a STOP or REPEATED START has been received while still addressed as a Slave.
•When a bus error has occurred due to an illegal START or STOP condition.
26.3. Two-Wire Serial Interface Bus Definition
The Two-wire Serial Interface (TWI) is ideally suited for typical microcontroller applications. The TWI
protocol allows the systems designer to interconnect up to 128 different devices using only two bi-
directional bus lines, one for clock (SCL) and one for data (SDA). The only external hardware needed to
implement the bus is a single pullup resistor for each of the TWI bus lines. All devices connected to the
bus have individual addresses, and mechanisms for resolving bus contention are inherent in the TWI
protocol.
Figure 26-2 TWI Bus Interconnection
SD A
SCL
........ R1 R2
VCC
Device 1 Device 2 Device 3 Device n
26.3.1. TWI Terminology
The following definitions are frequently encountered in this section.
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Table 26-1 TWI Terminology
Term Description
Master The device that initiates and terminates a transmission. The Master also generates the SCL clock.
Slave The device addressed by a Master.
Transmitter The device placing data on the bus.
Receiver The device reading data from the bus.
26.3.2. Electrical Interconnection
As depicted in Figure 26-2 TWI Bus Interconnection on page 276, both bus lines are connected to the
positive supply voltage through pull-up resistors. The bus drivers of all TWI-compliant devices are open-
drain or open-collector. This implements a wired-AND function which is essential to the operation of the
interface. A low level on a TWI bus line is generated when one or more TWI devices output a zero. A high
level is output when all TWI devices tri-state their outputs, allowing the pull-up resistors to pull the line
high. Note that all AVR devices connected to the TWI bus must be powered in order to allow any bus
operation.
The number of devices that can be connected to the bus is only limited by the bus capacitance limit of
400pF and the 7-bit slave address space. A detailed specification of the electrical characteristics of the
TWI is given in Two-wire Serial Interface Characteristics. Two different sets of specifications are
presented there, one relevant for bus speeds below 100kHz, and one valid for bus speeds up to 400kHz.
Related Links
Two-wire Serial Interface Characteristics on page 419
26.4. Data Transfer and Frame Format
26.4.1. Transferring Bits
Each data bit transferred on the TWI bus is accompanied by a pulse on the clock line. The level of the
data line must be stable when the clock line is high. The only exception to this rule is for generating start
and stop conditions.
Figure 26-3 Data Validity
SD
A
SCL
Data Stab le Data Stab le
Data Change
26.4.2. START and STOP Conditions
The Master initiates and terminates a data transmission. The transmission is initiated when the Master
issues a START condition on the bus, and it is terminated when the Master issues a STOP condition.
Between a START and a STOP condition, the bus is considered busy, and no other master should try to
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STOP REPEATED START START
seize control of the bus. A special case occurs when a new START condition is issued between a START
and STOP condition. This is referred to as a REPEATED START condition, and is used when the Master
wishes to initiate a new transfer without relinquishing control of the bus. After a REPEATED START, the
bus is considered busy until the next STOP. This is identical to the START behavior, and therefore START
is used to describe both START and REPEATED START for the remainder of this datasheet, unless
otherwise noted. As depicted below, START and STOP conditions are signalled by changing the level of
the SDA line when the SCL line is high.
Figure 26-4 START, REPEATED START and STOP conditions
SDA
SCL
START STOPREPEATED STARTSTOP START
26.4.3. Address Packet Format
All address packets transmitted on the TWI bus are 9 bits long, consisting of 7 address bits, one READ/
WRITE control bit and an acknowledge bit. If the READ/WRITE bit is set, a read operation is to be
performed, otherwise a write operation should be performed. When a Slave recognizes that it is being
addressed, it should acknowledge by pulling SDA low in the ninth SCL (ACK) cycle. If the addressed
Slave is busy, or for some other reason can not service the Master’s request, the SDA line should be left
high in the ACK clock cycle. The Master can then transmit a STOP condition, or a REPEATED START
condition to initiate a new transmission. An address packet consisting of a slave address and a READ or
a WRITE bit is called SLA+R or SLA+W, respectively.
The MSB of the address byte is transmitted first. Slave addresses can freely be allocated by the designer,
but the address 0000 000 is reserved for a general call.
When a general call is issued, all slaves should respond by pulling the SDA line low in the ACK cycle. A
general call is used when a Master wishes to transmit the same message to several slaves in the system.
When the general call address followed by a Write bit is transmitted on the bus, all slaves set up to
acknowledge the general call will pull the SDA line low in the ack cycle. The following data packets will
then be received by all the slaves that acknowledged the general call. Note that transmitting the general
call address followed by a Read bit is meaningless, as this would cause contention if several slaves
started transmitting different data.
All addresses of the format 1111 xxx should be reserved for future purposes.
Figure 26-5 Address Packet Format
SD A
SCL
ST AR T
1 2 7 8 9
Addr MSB Addr LSB R/W ACK
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26.4.4. Data Packet Format
All data packets transmitted on the TWI bus are nine bits long, consisting of one data byte and an
acknowledge bit. During a data transfer, the Master generates the clock and the START and STOP
conditions, while the Receiver is responsible for acknowledging the reception. An Acknowledge (ACK) is
signalled by the Receiver pulling the SDA line low during the ninth SCL cycle. If the Receiver leaves the
SDA line high, a NACK is signalled. When the Receiver has received the last byte, or for some reason
cannot receive any more bytes, it should inform the Transmitter by sending a NACK after the final byte.
The MSB of the data byte is transmitted first.
Figure 26-6 Data Packet Format
1 2 7 8 9
Data MSB Data LSB ACK
Aggregate
SD A
SDA from
Transmitter
SDA from
Receiv er
SCL from
Master
SLA+R/W Data Byte
ST OP, REPEA TED
ST AR T or Ne xt
Data Byte
26.4.5. Combining Address and Data Packets Into a Transmission
A transmission basically consists of a START condition, a SLA+R/W, one or more data packets and a
STOP condition. An empty message, consisting of a START followed by a STOP condition, is illegal. Note
that the Wired-ANDing of the SCL line can be used to implement handshaking between the Master and
the Slave. The Slave can extend the SCL low period by pulling the SCL line low. This is useful if the clock
speed set up by the Master is too fast for the Slave, or the Slave needs extra time for processing between
the data transmissions. The Slave extending the SCL low period will not affect the SCL high period, which
is determined by the Master. As a consequence, the Slave can reduce the TWI data transfer speed by
prolonging the SCL duty cycle.
The following figure depicts a typical data transmission. Note that several data bytes can be transmitted
between the SLA+R/W and the STOP condition, depending on the software protocol implemented by the
application software.
Figure 26-7 Typical Data Transmission
1 2 7 8 9
Data Byte
Data MSB Data LSB ACK
SD A
SCL
ST AR T
1 2 7 8 9
Addr MSB Addr LSB R/W ACK
SLA+R/W ST OP
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TA TA \Jn wwwwwww ‘ww , , , , , S Counting High P eriod Counting Lo w Period
26.5. Multi-master Bus Systems, Arbitration and Synchronization
The TWI protocol allows bus systems with several masters. Special concerns have been taken in order to
ensure that transmissions will proceed as normal, even if two or more masters initiate a transmission at
the same time. Two problems arise in multi-master systems:
•An algorithm must be implemented allowing only one of the masters to complete the transmission.
All other masters should cease transmission when they discover that they have lost the selection
process. This selection process is called arbitration. When a contending master discovers that it
has lost the arbitration process, it should immediately switch to Slave mode to check whether it is
being addressed by the winning master. The fact that multiple masters have started transmission at
the same time should not be detectable to the slaves, i.e. the data being transferred on the bus
must not be corrupted.
•Different masters may use different SCL frequencies. A scheme must be devised to synchronize
the serial clocks from all masters, in order to let the transmission proceed in a lockstep fashion.
This will facilitate the arbitration process.
The wired-ANDing of the bus lines is used to solve both these problems. The serial clocks from all
masters will be wired-ANDed, yielding a combined clock with a high period equal to the one from the
Master with the shortest high period. The low period of the combined clock is equal to the low period of
the Master with the longest low period. Note that all masters listen to the SCL line, effectively starting to
count their SCL high and low time-out periods when the combined SCL line goes high or low,
respectively.
Figure 26-8 SCL Synchronization Between Multiple Masters
T Alow T Ahigh
SCL from
Master A
SCL from
Master B
SCL Bus
Line
TBlow TBhigh
Masters Star t
Counting Lo w P er iod
Masters Star t
Counting High P er iod
Arbitration is carried out by all masters continuously monitoring the SDA line after outputting data. If the
value read from the SDA line does not match the value the Master had output, it has lost the arbitration.
Note that a Master can only lose arbitration when it outputs a high SDA value while another Master
outputs a low value. The losing Master should immediately go to Slave mode, checking if it is being
addressed by the winning Master. The SDA line should be left high, but losing masters are allowed to
generate a clock signal until the end of the current data or address packet. Arbitration will continue until
only one Master remains, and this may take many bits. If several masters are trying to address the same
Slave, arbitration will continue into the data packet.
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““““““““
Figure 26-9 Arbitration Between Two Masters
SD A from
Master A
SD A from
Master B
SD A Line
Synchroniz ed
SCL Line
START Master A Loses
Arbitration, SD AA SD A
Note that arbitration is not allowed between:
•A REPEATED START condition and a data bit.
•A STOP condition and a data bit.
•A REPEATED START and a STOP condition.
It is the user software’s responsibility to ensure that these illegal arbitration conditions never occur. This
implies that in multi-master systems, all data transfers must use the same composition of SLA+R/W and
data packets. In other words: All transmissions must contain the same number of data packets, otherwise
the result of the arbitration is undefined.
26.6. Using the TWI
The AVR TWI is byte-oriented and interrupt based. Interrupts are issued after all bus events, like
reception of a byte or transmission of a START condition. Because the TWI is interrupt-based, the
application software is free to carry on other operations during a TWI byte transfer. Note that the TWI
Interrupt Enable (TWIE) bit in TWCR together with the Global Interrupt Enable bit in SREG allow the
application to decide whether or not assertion of the TWINT Flag should generate an interrupt request. If
the TWIE bit is cleared, the application must poll the TWINT Flag in order to detect actions on the TWI
bus.
When the TWINT Flag is asserted, the TWI has finished an operation and awaits application response. In
this case, the TWI Status Register (TWSR) contains a value indicating the current state of the TWI bus.
The application software can then decide how the TWI should behave in the next TWI bus cycle by
manipulating the TWCR and TWDR Registers.
The following figure is a simple example of how the application can interface to the TWI hardware. In this
example, a Master wishes to transmit a single data byte to a Slave. This description is quite abstract, a
more detailed explanation follows later in this section. A simple code example implementing the desired
behavior is also presented.
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Figure 26-10 Interfacing the Application to the TWI in a Typical Transmission
START SLA+W A Data A STOP
1. Application
writes to TWCR to
initiate
transmission of
START
2.TWINT set.
Status code indicates
START condition sent
4.TWINT set.
Status code indicates
SLA+W sent, ACK
received
6.TWINT set.
Status code indicates
data sent, ACK received
3. Check TWSR to see if START was
sent. Application loads SLA+W into
TWDR, and loads appropriate control
signals into TWCR, making sure that
TWINT is written to one,
and TWSTA is written to zero.
5. CheckTWSR to see if SLA+W was
sent and ACK received.
Application loads data into TWDR, and
loads appropriate control signals into
TWCR, making sure that TWINT is
written to one
7. CheckTWSR to see if data was sent
and ACK received.
Application loads appropriate control
signals to send STOP into TWCR,
making sure that TWINT is written to one
TWI bus
Indicates
TWINT set
Application
Action
TWI
Hardware
Action
1. The first step in a TWI transmission is to transmit a START condition. This is done by writing a
specific value into TWCR, instructing the TWI hardware to transmit a START condition. Which value
to write is described later on. However, it is important that the TWINT bit is set in the value written.
Writing a one to TWINT clears the flag. The TWI will not start any operation as long as the TWINT
bit in TWCR is set. Immediately after the application has cleared TWINT, the TWI will initiate
transmission of the START condition.
2. When the START condition has been transmitted, the TWINT Flag in TWCR is set, and TWSR is
updated with a status code indicating that the START condition has successfully been sent.
3. The application software should now examine the value of TWSR, to make sure that the START
condition was successfully transmitted. If TWSR indicates otherwise, the application software might
take some special action, like calling an error routine. Assuming that the status code is as
expected, the application must load SLA+W into TWDR. Remember that TWDR is used both for
address and data. After TWDR has been loaded with the desired SLA+W, a specific value must be
written to TWCR, instructing the TWI hardware to transmit the SLA+W present in TWDR. Which
value to write is described later on. However, it is important that the TWINT bit is set in the value
written. Writing a one to TWINT clears the flag. The TWI will not start any operation as long as the
TWINT bit in TWCR is set. Immediately after the application has cleared TWINT, the TWI will initiate
transmission of the address packet.
4. When the address packet has been transmitted, the TWINT Flag in TWCR is set, and TWSR is
updated with a status code indicating that the address packet has successfully been sent. The
status code will also reflect whether a Slave acknowledged the packet or not.
5. The application software should now examine the value of TWSR, to make sure that the address
packet was successfully transmitted, and that the value of the ACK bit was as expected. If TWSR
indicates otherwise, the application software might take some special action, like calling an error
routine. Assuming that the status code is as expected, the application must load a data packet into
TWDR. Subsequently, a specific value must be written to TWCR, instructing the TWI hardware to
transmit the data packet present in TWDR. Which value to write is described later on. However, it is
important that the TWINT bit is set in the value written. Writing a one to TWINT clears the flag. The
TWI will not start any operation as long as the TWINT bit in TWCR is set. Immediately after the
application has cleared TWINT, the TWI will initiate transmission of the data packet.
6. When the data packet has been transmitted, the TWINT Flag in TWCR is set, and TWSR is
updated with a status code indicating that the data packet has successfully been sent. The status
code will also reflect whether a Slave acknowledged the packet or not.
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7. The application software should now examine the value of TWSR, to make sure that the data
packet was successfully transmitted, and that the value of the ACK bit was as expected. If TWSR
indicates otherwise, the application software might take some special action, like calling an error
routine. Assuming that the status code is as expected, the application must write a specific value to
TWCR, instructing the TWI hardware to transmit a STOP condition. Which value to write is
described later on. However, it is important that the TWINT bit is set in the value written. Writing a
one to TWINT clears the flag. The TWI will not start any operation as long as the TWINT bit in
TWCR is set. Immediately after the application has cleared TWINT, the TWI will initiate
transmission of the STOP condition. Note that TWINT is NOT set after a STOP condition has been
sent.
Even though this example is simple, it shows the principles involved in all TWI transmissions. These can
be summarized as follows:
•When the TWI has finished an operation and expects application response, the TWINT Flag is set.
The SCL line is pulled low until TWINT is cleared.
•When the TWINT Flag is set, the user must update all TWI Registers with the value relevant for the
next TWI bus cycle. As an example, TWDR must be loaded with the value to be transmitted in the
next bus cycle.
•After all TWI Register updates and other pending application software tasks have been completed,
TWCR is written. When writing TWCR, the TWINT bit should be set. Writing a one to TWINT clears
the flag. The TWI will then commence executing whatever operation was specified by the TWCR
setting.
The following table lists assembly and C implementation examples. Note that the code below assumes
that several definitions have been made, e.g. by using include-files.
Table 26-2 Assembly and C Code Example
Assembly Code Example C Example Comments
1ldi r16, (1<<TWINT)|(1<<TWSTA)|
(1<<TWEN)
out TWCR, r16
TWCR = (1<<TWINT)|
(1<<TWSTA)|(1<<TWEN) Send START condition
2
wait1:
in r16,TWCR
sbrs r16,TWINT
rjmp wait1
while (!(TWCR &
(1<<TWINT)));
Wait for TWINT Flag set. This indicates
that the START condition has been
transmitted.
3
in r16,TWSR
andi r16, 0xF8
cpi r16, START
brne ERROR
if ((TWSR & 0xF8) !=
START)
ERROR();
Check value of TWI Status Register.
Mask prescaler bits. If status different
from START go to ERROR.
ldi r16, SLA_W
out TWDR, r16
ldi r16, (1<<TWINT) | (1<<TWEN)
out TWCR, r16
TWDR = SLA_W;
TWCR = (1<<TWINT) |
(1<<TWEN);
Load SLA_W into TWDR Register. Clear
TWINT bit in TWCR to start transmission
of address.
4
wait2:
in r16,TWCR
sbrs r16,TWINT
rjmp wait2
while (!(TWCR &
(1<<TWINT)));
Wait for TWINT Flag set. This indicates
that the SLA+W has been transmitted,
and ACK/NACK has been received.
5
in r16,TWSR
andi r16, 0xF8
cpi r16, MT_SLA_ACK
brne ERROR
if ((TWSR & 0xF8) !=
MT_SLA_ACK)
ERROR();
Check value of TWI Status Register.
Mask prescaler bits. If status different
from MT_SLA_ACK go to ERROR.
ldi r16, DATA
out TWDR, r16
ldi r16, (1<<TWINT) | (1<<TWEN)
out TWCR, r16
TWDR = DATA;
TWCR = (1<<TWINT) |
(1<<TWEN);
Load DATA into TWDR Register. Clear
TWINT bit in TWCR to start transmission
of data.
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Assembly Code Example C Example Comments
6
wait3:
in r16,TWCR
sbrs r16,TWINT
rjmp wait3
while (!(TWCR &
(1<<TWINT)));
Wait for TWINT Flag set. This indicates
that the DATA has been transmitted, and
ACK/NACK has been received.
7
in r16,TWSR
andi r16, 0xF8
cpi r16, MT_DATA_ACK
brne ERROR
if ((TWSR & 0xF8) !=
MT_DATA_ACK)
ERROR();
Check value of TWI Status Register.
Mask prescaler bits. If status different
from MT_DATA_ACK go to ERROR.
ldi r16, (1<<TWINT)|(1<<TWEN)|
(1<<TWSTO)
out TWCR, r16
TWCR = (1<<TWINT)|
(1<<TWEN)|(1<<TWSTO); Transmit STOP condition.
26.6.1. Transmission Modes
The TWI can operate in one of four major modes:
•Master Transmitter (MT)
•Master Receiver (MR)
•Slave Transmitter (ST)
•Slave Receiver (SR)
Several of these modes can be used in the same application. As an example, the TWI can use MT mode
to write data into a TWI EEPROM, MR mode to read the data back from the EEPROM. If other masters
are present in the system, some of these might transmit data to the TWI, and then SR mode would be
used. It is the application software that decides which modes are legal.
The following sections describe each of these modes. Possible status codes are described along with
figures detailing data transmission in each of the modes. These figures use the following abbreviations:
S START condition
Rs REPEATED START condition
R Read bit (high level at SDA)
W Write bit (low level at SDA)
A Acknowledge bit (low level at SDA)
A Not acknowledge bit (high level at SDA)
Data 8-bit data byte
P STOP condition
SLA Slave Address
Circles are used to indicate that the TWINT Flag is set. The numbers in the circles show the status code
held in TWSR, with the prescaler bits masked to zero. At these points, actions must be taken by the
application to continue or complete the TWI transfer. The TWI transfer is suspended until the TWINT Flag
is cleared by software.
When the TWINT Flag is set, the status code in TWSR is used to determine the appropriate software
action. For each status code, the required software action and details of the following serial transfer are
given below in the Status Code table for each mode. Note that the prescaler bits are masked to zero in
these tables.
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26.6.2. Master Transmitter Mode
In the Master Transmitter (MT) mode, a number of data bytes are transmitted to a Slave Receiver, see
figure below. In order to enter a Master mode, a START condition must be transmitted. The format of the
following address packet determines whether MT or Master Receiver (MR) mode is to be entered: If SLA
+W is transmitted, MT mode is entered, if SLA+R is transmitted, MR mode is entered. All the status codes
mentioned in this section assume that the prescaler bits are zero or are masked to zero.
Figure 26-11 Data Transfer in Master Transmitter Mode
Device 1
MASTER
TRANSMITTER
Device 2
SLA VE
RECEIVER
Device 3 Device n
SD A
SCL
........ R1 R2
VCC
A START condition is sent by writing a value to the TWI Control Register (TWCR) of the type
TWCR=1x10x10x:
•The TWI Enable bit (TWCR.TWEN) must be written to '1' to enable the 2-wire Serial Interface
•The TWI Start Condition bit (TWCR.TWSTA) must be written to '1' to transmit a START condition
•The TWI Interrupt Flag (TWCR.TWINT) must be written to '1' to clear the flag.
The TWI will then test the 2-wire Serial Bus and generate a START condition as soon as the bus
becomes free. After a START condition has been transmitted, the TWINT Flag is set by hardware, and
the status code in TWSR will be 0x08 (see Status Code table below). In order to enter MT mode, SLA+W
must be transmitted. This is done by writing SLA+W to the TWI Data Register (TWDR). Thereafter, the
TWCR.TWINT Flag should be cleared (by writing a '1' to it) to continue the transfer. This is accomplished
by writing a value to TWRC of the type TWCR=1x00x10x.
When SLA+W have been transmitted and an acknowledgment bit has been received, TWINT is set again
and a number of status codes in TWSR are possible. Possible status codes in Master mode are 0x18,
0x20, or 0x38. The appropriate action to be taken for each of these status codes is detailed in the Status
Code table below.
When SLA+W has been successfully transmitted, a data packet should be transmitted. This is done by
writing the data byte to TWDR. TWDR must only be written when TWINT is high. If not, the access will be
discarded, and the Write Collision bit (TWWC) will be set in the TWCR Register. After updating TWDR,
the TWINT bit should be cleared (by writing '1' to it) to continue the transfer. This is accomplished by
writing again a value to TWCR of the type TWCR=1x00x10x.
This scheme is repeated until the last byte has been sent and the transfer is ended, either by generating
a STOP condition or a by a repeated START condition. A repeated START condition is accomplished by
writing a regular START value TWCR=1x10x10x. A STOP condition is generated by writing a value of the
type TWCR=1x01x10x.
After a repeated START condition (status code 0x10), the 2-wire Serial Interface can access the same
Slave again, or a new Slave without transmitting a STOP condition. Repeated START enables the Master
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to switch between Slaves, Master Transmitter mode and Master Receiver mode without losing control of
the bus.
Table 26-3 Status Codes for Master Transmitter Mode
Status Code
(TWSR)
Prescaler Bits
are 0
Status of the 2-wire Serial Bus
and 2-wire Serial Interface
Hardware
Application Software Response Next Action Taken by TWI Hardware
To/from TWDR To TWCR
STA STO TWIN
T
TWE
A
0x08 A START condition has been
transmitted
Load SLA+W 0 0 1 X SLA+W will be transmitted;
ACK or NOT ACK will be received
0x10 A repeated START condition has
been transmitted
Load SLA+W or
Load SLA+R
0
0
0
0
1
1
X
X
SLA+W will be transmitted;
ACK or NOT ACK will be received
SLA+R will be transmitted;
Logic will switch to Master Receiver mode
0x18 SLA+W has been transmitted;
ACK has been received
Load data byte or
No TWDR action or
No TWDR action or
No TWDR action
0
1
0
1
0
0
1
1
1
1
1
1
X
X
X
X
Data byte will be transmitted and ACK or NOT ACK will
be received
Repeated START will be transmitted
STOP condition will be transmitted and
TWSTO Flag will be reset
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
0x20 SLA+W has been transmitted;
NOT ACK has been received
Load data byte or
No TWDR action or
No TWDR action or
No TWDR action
0
1
0
1
0
0
1
1
1
1
1
1
X
X
X
X
Data byte will be transmitted and ACK or NOT ACK will
be received
Repeated START will be transmitted
STOP condition will be transmitted and
TWSTO Flag will be reset
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
0x28 Data byte has been transmitted;
ACK has been received
Load data byte or
No TWDR action or
No TWDR action or
No TWDR action
0
1
0
1
0
0
1
1
1
1
1
1
X
X
X
X
Data byte will be transmitted and ACK or NOT ACK will
be received
Repeated START will be transmitted
STOP condition will be transmitted and
TWSTO Flag will be reset
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
0x30 Data byte has been transmitted;
NOT ACK has been received
Load data byte or
No TWDR action or
No TWDR action or
No TWDR action
0
1
0
1
0
0
1
1
1
1
1
1
X
X
X
X
Data byte will be transmitted and ACK or NOT ACK will
be received
Repeated START will be transmitted
STOP condition will be transmitted and
TWSTO Flag will be reset
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
0x38 Arbitration lost in SLA+W or data
bytes
No TWDR action or
No TWDR action
0
1
0
0
1
1
X
X
2-wire Serial Bus will be released and not addressed
Slave mode entered
A START condition will be transmitted when the bus
becomes free
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>x m9» Atmel prescnlm bus I.» I m or mask ad m 1 cm
Figure 26-12 Formats and States in the Master Transmitter Mode
S SLA W A DAT A A P
$08 $18 $28
R SLA W
$10
A P
$20
P
$30
A or A
$38
A
Other master
contin ues
A or A
$38
Other master
contin ues
R
A
$68
Other master
contin ues
$78 $B0
To corresponding
states in sla v e mode
MT
MR
Successfull
transmission
to a sla v e
receiv er
Next transfer
star ted with a
repeated star t
condition
Not acknowledge
received after the
slave address
Not acknowledge
receiv ed after a data
byte
Arbitration lost in sla v e
address or data b yte
Arbitration lost and
addressed as sla v e
DAT A A
n
From master to sla v e
From sla v e to master
Any number of data b ytes
and their associated ac kno wledge bits
This number (contained in TWSR) corresponds
to a defined state of the 2-Wire Ser ial Bus. The
prescaler bits are z ero or mask ed to z ero
S
26.6.3. Master Receiver Mode
In the Master Receiver (MR) mode, a number of data bytes are received from a Slave Transmitter (see
next figure). In order to enter a Master mode, a START condition must be transmitted. The format of the
following address packet determines whether Master Transmitter (MT) or MR mode is to be entered. If
SLA+W is transmitted, MT mode is entered, if SLA+R is transmitted, MR mode is entered. All the status
codes mentioned in this section assume that the prescaler bits are zero or are masked to zero.
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Figure 26-13 Data Transfer in Master Receiver Mode
Device 1
MASTER
RECEIVER
Device 2
SLA VE
TRANSMITTER
Device 3 Device n
SD A
SCL
........ R1 R2
VCC
A START condition is sent by writing to the TWI Control register (TWCR) a value of the type
TWCR=1x10x10x:
•TWCR.TWEN must be written to '1' to enable the 2-wire Serial Interface
•TWCR.TWSTA must be written to '1' to transmit a START condition
•TWCR.TWINT must be cleared by writing a '1' to it.
The TWI will then test the 2-wire Serial Bus and generate a START condition as soon as the bus
becomes free. After a START condition has been transmitted, the TWINT Flag is set by hardware, and
the status code in TWSR will be 0x08 (see Status Code table below). In order to enter MR mode, SLA+R
must be transmitted. This is done by writing SLA+R to TWDR. Thereafter, the TWINT flag should be
cleared (by writing '1' to it) to continue the transfer. This is accomplished by writing the a value to TWCR
of the type TWCE=1x00x10x.
When SLA+R have been transmitted and an acknowledgment bit has been received, TWINT is set again
and a number of status codes in TWSR are possible. Possible status codes in Master mode are 0x38,
0x40, or 0x48. The appropriate action to be taken for each of these status codes is detailed in the table
below. Received data can be read from the TWDR Register when the TWINT Flag is set high by
hardware. This scheme is repeated until the last byte has been received. After the last byte has been
received, the MR should inform the ST by sending a NACK after the last received data byte. The transfer
is ended by generating a STOP condition or a repeated START condition. A repeated START condition is
sent by writing to the TWI Control register (TWCR) a value of the type TWCR=1x10x10x again. A STOP
condition is generated by writing TWCR=1xx01x10x:
After a repeated START condition (status code 0x10) the 2-wire Serial Interface can access the same
Slave again, or a new Slave without transmitting a STOP condition. Repeated START enables the Master
to switch between Slaves, Master Transmitter mode and Master Receiver mode without losing control
over the bus.
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Table 26-4 Status codes for Master Receiver Mode
Status Code
(TWSR)
Prescaler Bits
are 0
Status of the 2-wire Serial Bus
and 2-wire Serial Interface
Hardware
Application Software Response Next Action Taken by TWI Hardware
To/from TWD To TWCR
STA STO TWIN
T
TWE
A
0x08 A START condition has been
transmitted
Load SLA+R 0 0 1 X SLA+R will be transmitted
ACK or NOT ACK will be received
0x10 A repeated START condition has
been transmitted
Load SLA+R or
Load SLA+W
0
0
0
0
1
1
X
X
SLA+R will be transmitted
ACK or NOT ACK will be received
SLA+W will be transmitted
Logic will switch to Master Transmitter mode
0x38 Arbitration lost in SLA+R or NOT
ACK bit
No TWDR action or
No TWDR action
0
1
0
0
1
1
X
X
2-wire Serial Bus will be released and not addressed
Slave mode will be entered
A START condition will be transmitted when the bus
becomes free
0x40 SLA+R has been transmitted;
ACK has been received
No TWDR action or
No TWDR action
0
0
0
0
1
1
0
1
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
0x48 SLA+R has been transmitted;
NOT ACK has been received
No TWDR action or
No TWDR action or
No TWDR action
1
0
1
0
1
1
1
1
1
X
X
X
Repeated START will be transmitted
STOP condition will be transmitted and TWSTO Flag
will be reset
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
0x50 Data byte has been received;
ACK has been returned
Read data byte or
Read data byte
0
0
0
0
1
1
0
1
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
0x58 Data byte has been received;
NOT ACK has been returned
Read data byte or
Read data byte or
Read data byte
1
0
1
0
1
1
1
1
1
X
X
X
Repeated START will be transmitted
STOP condition will be transmitted and TWSTO Flag
will be reset
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
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as
Figure 26-14 Formats and States in the Master Receiver Mode
S SLA R A DAT A A
$08 $40 $50
SLA R
$10
A P
$48
A or A
$38
Other master
contin ues
$38
Other master
contin ues
W
A
$68
Other master
contin ues
$78 $B0
To corresponding
states in sla v e mode
MR
MT
Successfull
reception
from a sla v e
receiv er
Next transf er
star ted with a
repeated star t
condition
Not ac kno wledge
received after the
slave address
Arbitration lost in sla v e
address or data b yte
Arbitration lost and
addressed as sla v e
DAT A A
n
From master to sla v e
From slave to master
Any number of data b ytes
and their associated ac kno wledge bits
This number (contained in TWSR) corresponds
to a defined state of the 2-Wire Ser ial Bus. The
prescaler bits are z ero or mask ed to z ero
PDATA A
$58
A
RS
26.6.4. Slave Receiver Mode
In the Slave Receiver (SR) mode, a number of data bytes are received from a Master Transmitter (see
figure below). All the status codes mentioned in this section assume that the prescaler bits are zero or are
masked to zero.
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Figure 26-15 Data transfer in Slave Receiver mode
Device 3 Device n
SD A
SCL
........ R1 R2
VCC
Device 2
MASTER
TRANSMITTER
Device 1
SLA VE
RECEIVER
To initiate the SR mode, the TWI (Slave) Address Register (TWAR) and the TWI Control Register
(TWCR) must be initialized as follows:
The upper seven bits of TWAR are the address to which the 2-wire Serial Interface will respond when
addressed by a Master (TWAR.TWA[6:0]). If the LSB of TWAR is written to TWAR.TWGCI=1, the TWI will
respond to the general call address (0x00), otherwise it will ignore the general call address.
TWCR must hold a value of the type TWCR=0100010x - TWCR.TWEN must be written to '1' to enable
the TWI. TWCR.TWEA bit must be written to '1' to enable the acknowledgement of the device’s own slave
address or the general call address. TWCR.TWSTA and TWSTO must be written to zero.
When TWAR and TWCR have been initialized, the TWI waits until it is addressed by its own slave
address (or the general call address, if enabled) followed by the data direction bit. If the direction bit is '0'
(write), the TWI will operate in SR mode, otherwise ST mode is entered. After its own slave address and
the write bit have been received, the TWINT Flag is set and a valid status code can be read from TWSR.
The status code is used to determine the appropriate software action, as detailed in the table below. The
SR mode may also be entered if arbitration is lost while the TWI is in the Master mode (see states 0x68
and 0x78).
If the TWCR.TWEA bit is reset during a transfer, the TWI will return a "Not Acknowledge" ('1') to SDA
after the next received data byte. This can be used to indicate that the Slave is not able to receive any
more bytes. While TWEA is zero, the TWI does not acknowledge its own slave address. However, the 2-
wire Serial Bus is still monitored and address recognition may resume at any time by setting TWEA. This
implies that the TWEA bit may be used to temporarily isolate the TWI from the 2-wire Serial Bus.
In all sleep modes other than Idle mode, the clock system to the TWI is turned off. If the TWEA bit is set,
the interface can still acknowledge its own slave address or the general call address by using the 2-wire
Serial Bus clock as a clock source. The part will then wake up from sleep and the TWI will hold the SCL
clock low during the wake up and until the TWINT Flag is cleared (by writing '1' to it). Further data
reception will be carried out as normal, with the AVR clocks running as normal. Observe that if the AVR is
set up with a long start-up time, the SCL line may be held low for a long time, blocking other data
transmissions.
Note:  The 2-wire Serial Interface Data Register (TWDR) does not reflect the last byte present on the bus
when waking up from these Sleep modes.
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Table 26-5 Status Codes for Slave Receiver Mode
Status
Code
(TWSR)
Prescaler
Bits are 0
Status of the 2-wire Serial
Bus and 2-wire Serial
Interface Hardware
Application Software Response Next Action Taken by TWI Hardware
To/from TWDR To TWCR
STA STO TWI
NT
TWE
A
0x60 Own SLA+W has been
received;
ACK has been returned
No TWDR action
or
No TWDR action
X
X
0
0
1
1
0
1
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
0x68 Arbitration lost in SLA+R/W
as Master; own SLA+W has
been
received; ACK has been
returned
No TWDR action
or
No TWDR action
X
X
0
0
1
1
0
1
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
0x70 General call address has
been
received; ACK has been
returned
No TWDR action
or
No TWDR action
X
X
0
0
1
1
0
1
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
0x78 Arbitration lost in SLA+R/W
as Master; General call
address has been received;
ACK has been returned
No TWDR action
or
No TWDR action
X
X
0
0
1
1
0
1
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
0x80 Previously addressed with
own SLA+W; data has been
received; ACK has been
returned
Read data byte or
Read data byte
X
X
0
0
1
1
0
1
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
0x88 Previously addressed with
own SLA+W; data has been
received; NOT ACK has been
returned
Read data byte or
Read data byte or
Read data byte or
Read data byte
0
0
1
1
0
0
0
0
1
1
1
1
0
1
0
1
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA;
a START condition will be transmitted when the bus
becomes free
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”; a START
condition will be transmitted when the bus becomes
free
0x90 Previously addressed with
general call; data has been
received; ACK has been
returned
Read data byte or
Read data byte
X
X
0
0
1
1
0
1
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
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Status
Code
(TWSR)
Prescaler
Bits are 0
Status of the 2-wire Serial
Bus and 2-wire Serial
Interface Hardware
Application Software Response Next Action Taken by TWI Hardware
To/from TWDR To TWCR
STA STO TWI
NT
TWE
A
0x98 Previously addressed with
general call; data has been
received; NOT ACK has been
returned
Read data byte or
Read data byte or
Read data byte or
Read data byte
0
0
1
1
0
0
0
0
1
1
1
1
0
1
0
1
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA;
a START condition will be transmitted when the bus
becomes free
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”; a START
condition will be transmitted when the bus becomes
free
0xA0 A STOP condition or repeated
START condition has been
received while still addressed
as Slave
No action 0
0
1
1
0
0
0
0
1
1
1
1
0
1
0
1
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA;
a START condition will be transmitted when the bus
becomes free
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”; a START
condition will be transmitted when the bus becomes
free
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Figure 26-16 Formats and States in the Slave Receiver Mode
S SLA W A DATA A
$60 $80
$88
A
$68
Reception of the o wn
sla v e address and one or
more data b ytes. All are
acknowledged
Last data b yte receiv ed
is not ac kno wledged
Arbitration lost as master
and addressed as sla v e
Reception of the gener al call
address and one or more data
bytes
Last data b yte receiv ed is
not ac knowledged
n
From master to sla v e
From sla v e to master
Any number of data b ytes
and their associated ac kno wledge bits
This n umber (contained in TWSR) corresponds
to a defined state of the 2-Wire Ser ial Bus. The
prescaler bits are z ero or mask ed to z ero
P or SDATA A
$80 $A0
P or SA
A DATA A
$70 $90
$98
A
$78
P or SDATA A
$90 $A0
P or SA
General Call
Arbitration lost as master and
addressed as sla v e b y gener al call
DATA A
26.6.5. Slave Transmitter Mode
In the Slave Transmitter (ST) mode, a number of data bytes are transmitted to a Master Receiver, as in
the figure below. All the status codes mentioned in this section assume that the prescaler bits are zero or
are masked to zero.
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Figure 26-17 Data Transfer in Slave Transmitter Mode
Device 3 Device n
SD A
SCL
........ R1 R2
VCC
Device 2
MASTER
RECEIVER
Device 1
SLA VE
TRANSMITTER
To initiate the SR mode, the TWI (Slave) Address Register (TWAR) and the TWI Control Register
(TWCR) must be initialized as follows:
The upper seven bits of TWAR are the address to which the 2-wire Serial Interface will respond when
addressed by a Master (TWAR.TWA[6:0]). If the LSB of TWAR is written to TWAR.TWGCI=1, the TWI will
respond to the general call address (0x00), otherwise it will ignore the general call address.
TWCR must hold a value of the type TWCR=0100010x - TWEN must be written to one to enable the TWI.
The TWEA bit must be written to one to enable the acknowledgement of the device’s own slave address
or the general call address. TWSTA and TWSTO must be written to zero.
When TWAR and TWCR have been initialized, the TWI waits until it is addressed by its own slave
address (or the general call address if enabled) followed by the data direction bit. If the direction bit is “1”
(read), the TWI will operate in ST mode, otherwise SR mode is entered. After its own slave address and
the write bit have been received, the TWINT Flag is set and a valid status code can be read from TWSR.
The status code is used to determine the appropriate software action. The appropriate action to be taken
for each status code is detailed in the table below. The ST mode may also be entered if arbitration is lost
while the TWI is in the Master mode (see state 0xB0).
If the TWCR.TWEA bit is written to zero during a transfer, the TWI will transmit the last byte of the
transfer. State 0xC0 or state 0xC8 will be entered, depending on whether the Master Receiver transmits a
NACK or ACK after the final byte. The TWI is switched to the not addressed Slave mode, and will ignore
the Master if it continues the transfer. Thus the Master Receiver receives all '1' as serial data. State 0xC8
is entered if the Master demands additional data bytes (by transmitting ACK), even though the Slave has
transmitted the last byte (TWEA zero and expecting NACK from the Master).
While TWCR.TWEA is zero, the TWI does not respond to its own slave address. However, the 2-wire
Serial Bus is still monitored and address recognition may resume at any time by setting TWEA. This
implies that the TWEA bit may be used to temporarily isolate the TWI from the 2-wire Serial Bus.
In all sleep modes other than Idle mode, the clock system to the TWI is turned off. If the TWEA bit is set,
the interface can still acknowledge its own slave address or the general call address by using the 2-wire
Serial Bus clock as a clock source. The part will then wake up from sleep and the TWI will hold the SCL
clock will low during the wake up and until the TWINT Flag is cleared (by writing '1' to it). Further data
transmission will be carried out as normal, with the AVR clocks running as normal. Observe that if the
AVR is set up with a long start-up time, the SCL line may be held low for a long time, blocking other data
transmissions.
Note:  The 2-wire Serial Interface Data Register (TWDR) does not reflect the last byte present on the bus
when waking up from these Sleep modes.
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Table 26-6 Status Codes for Slave Transmitter Mode
Status
Code
(TWSR)
Prescaler
Bits are 0
Status of the 2-wire Serial
Bus and 2-wire Serial
Interface Hardware
Application Software Response Next Action Taken by TWI Hardware
To/from TWDR To TWCR
STA STO TWI
NT
TWE
A
0xA8 Own SLA+R has been
received;
ACK has been returned
Load data byte or
Load data byte
X
X
0
0
1
1
0
1
Last data byte will be transmitted and NOT ACK
should be received
Data byte will be transmitted and ACK should be
received
0xB0 Arbitration lost in SLA+R/W
as Master; own SLA+R has
been
received; ACK has been
returned
Load data byte or
Load data byte
X
X
0
0
1
1
0
1
Last data byte will be transmitted and NOT ACK
should be received
Data byte will be transmitted and ACK should be
received
0xB8 Data byte in TWDR has
been
transmitted; ACK has been
received
Load data byte or
Load data byte
X
X
0
0
1
1
0
1
Last data byte will be transmitted and NOT ACK
should be received
Data byte will be transmitted and ACK should be
received
0xC0 Data byte in TWDR has
been
transmitted; NOT ACK has
been
received
No TWDR action
or
No TWDR action
or
No TWDR action
or
No TWDR action
0
0
1
1
0
0
0
0
1
1
1
1
0
1
0
1
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA;
a START condition will be transmitted when the bus
becomes free
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”; a START
condition will be transmitted when the bus becomes
free
0xC8 Last data byte in TWDR has
been transmitted (TWEA =
“0”); ACK has been received
No TWDR action
or
No TWDR action
or
No TWDR action
or
No TWDR action
0
0
1
1
0
0
0
0
1
1
1
1
0
1
0
1
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA;
a START condition will be transmitted when the bus
becomes free
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”; a START
condition will be transmitted when the bus becomes
free
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Figure 26-18 Formats and States in the Slave Transmitter Mode
S SLA R A DATA A
$A8 $B8
A
$B0
Reception of the o wn
sla v e address and one or
more data b ytes
Last data b yte tr ansmitted.
Switched to not addressed
slave (TWEA = '0')
Arbitration lost as master
and addressed as sla v e
n
From master to sla v e
From slave to master
Any number of data b ytes
and their associated ac kno wledge bits
This number (contained in TWSR) corresponds
to a defined state of the 2-Wire Ser ial Bus. The
prescaler bits are z ero or mask ed to z ero
P or SDATA
$C0
DATA A
A
$C8
P or SAll 1's
A
26.6.6. Miscellaneous States
There are two status codes that do not correspond to a defined TWI state, see the table below.
Status 0xF8 indicates that no relevant information is available because the TWINT Flag is not set. This
occurs between other states, and when the TWI is not involved in a serial transfer.
Status 0x00 indicates that a bus error has occurred during a Two-wire Serial Bus transfer. A bus error
occurs when a START or STOP condition occurs at an illegal position in the format frame. Examples of
such illegal positions are during the serial transfer of an address byte, a data byte, or an acknowledge bit.
When a bus error occurs, TWINT is set. To recover from a bus error, the TWSTO Flag must set and
TWINT must be cleared by writing a logic one to it. This causes the TWI to enter the not addressed Slave
mode and to clear the TWSTO Flag (no other bits in TWCR are affected). The SDA and SCL lines are
released, and no STOP condition is transmitted.
Table 26-7 Miscellaneous States
Status
Code
(TWSR)
Prescaler
Bits are 0
Status of the 2-wire Serial
Bus and 2-wire Serial
Interface Hardware
Application Software Response Next Action Taken by TWI Hardware
To/from TWDR To TWCR
STA STO TWI
NT
TWE
A
0xF8 No relevant state
information available;
TWINT = “0”
No TWDR action No TWCR action Wait or proceed current transfer
0x00 Bus error due to an illegal
START or STOP condition
No TWDR action 0 1 1 X Only the internal hardware is affected, no STOP
condition is sent on the bus. In all cases, the bus
is released and TWSTO is cleared.
26.6.7. Combining Several TWI Modes
In some cases, several TWI modes must be combined in order to complete the desired action. Consider
for example reading data from a serial EEPROM. Typically, such a transfer involves the following steps:
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Muster Transmitter Mastcr Racaiv cr AA Atmel
1. The transfer must be initiated.
2. The EEPROM must be instructed what location should be read.
3. The reading must be performed.
4. The transfer must be finished.
Note that data is transmitted both from Master to Slave and vice versa. The Master must instruct the
Slave what location it wants to read, requiring the use of the MT mode. Subsequently, data must be read
from the Slave, implying the use of the MR mode. Thus, the transfer direction must be changed. The
Master must keep control of the bus during all these steps, and the steps should be carried out as an
atomical operation. If this principle is violated in a multimaster system, another Master can alter the data
pointer in the EEPROM between steps 2 and 3, and the Master will read the wrong data location. Such a
change in transfer direction is accomplished by transmitting a REPEATED START between the
transmission of the address byte and reception of the data. After a REPEATED START, the Master keeps
ownership of the bus. The following figure shows the flow in this transfer.
Figure 26-19 Combining Several TWI Modes to Access a Serial EEPROM
Master Transmitter Master Receiv er
S = ST AR T Rs = REPEA TED ST AR T P = ST OP
Transmitted from master to sla v e Transmitted from sla v e to master
S SLA+W A ADDRESS A Rs SLA+R A DATA A P
26.7. Multi-master Systems and Arbitration
If multiple masters are connected to the same bus, transmissions may be initiated simultaneously by one
or more of them. The TWI standard ensures that such situations are handled in such a way that one of
the masters will be allowed to proceed with the transfer, and that no data will be lost in the process. An
example of an arbitration situation is depicted below, where two masters are trying to transmit data to a
Slave Receiver.
Figure 26-20 An Arbitration Example
Device 1
MASTER
TRANSMITTER
Device 2
MASTER
TRANSMITTER
Device 3
SLA VE
RECEIVER
Device n
SD A
SCL
........ R1 R2
VCC
Several different scenarios may arise during arbitration, as described below:
•Two or more masters are performing identical communication with the same Slave. In this case,
neither the Slave nor any of the masters will know about the bus contention.
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•Two or more masters are accessing the same Slave with different data or direction bit. In this case,
arbitration will occur, either in the READ/WRITE bit or in the data bits. The masters trying to output
a '1' on SDA while another Master outputs a zero will lose the arbitration. Losing masters will switch
to not addressed Slave mode or wait until the bus is free and transmit a new START condition,
depending on application software action.
•Two or more masters are accessing different slaves. In this case, arbitration will occur in the SLA
bits. Masters trying to output a '1' on SDA while another Master outputs a zero will lose the
arbitration. Masters losing arbitration in SLA will switch to Slave mode to check if they are being
addressed by the winning Master. If addressed, they will switch to SR or ST mode, depending on
the value of the READ/WRITE bit. If they are not being addressed, they will switch to not addressed
Slave mode or wait until the bus is free and transmit a new START condition, depending on
application software action.
This is summarized in the next figure. Possible status values are given in circles.
Figure 26-21 Possible Status Codes Caused by Arbitration
Own
Address / General Call
received
Arbitration lost in SLA
TWI bus will be released and not addressed slave mode will be entered
A START condition will be transmitted when the bus becomes free
No
Arbitration lost in Data
Direction
Yes
Write Data byte will be received and NOT ACK will be returned
Data byte will be received and ACK will be returned
Last data byte will be transmitted and NOT ACK should be received
Data byte will be transmitted and ACK should be received
Read
B0
68/78
38
SLASTART Data STOP
26.8. Register Description
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26.8.1. TWBR – TWI Bit Rate Register
Name:  TWBR
Offset:  0x70
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
TWBR7 TWBR6 TWBR5 TWBR4 TWBR3 TWBR2 TWBR1 TWBR0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – TWBRn: TWI Bit Rate Register [n = 7:0]
TWBR selects the division factor for the bit rate generator. The bit rate generator is a frequency divider
which generates the SCL clock frequency in the Master modes. Refer to Bit Rate Generator Unit on page
275 for calculating bit rates.
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26.8.2. TWCR – TWI Control Register
The TWCR is used to control the operation of the TWI. It is used to enable the TWI, to initiate a Master
access by applying a START condition to the bus, to generate a Receiver acknowledge, to generate a
stop condition, and to control halting of the bus while the data to be written to the bus are written to the
TWDR. It also indicates a write collision if data is attempted written to TWDR while the register is
inaccessible.
Name:  TWCR
Offset:  0x74
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
TWINT TWEA TWSTA TWSTO TWWC TWEN TWIE
Access R/W R/W R/W R/W R R/W R/W
Reset 0 0 0 0 0 0 0
Bit 7 – TWINT: TWI Interrupt Flag
This bit is set by hardware when the TWI has finished its current job and expects application software
response. If the I-bit in SREG and TWIE in TWCR are set, the MCU will jump to the TWI Interrupt Vector.
While the TWINT Flag is set, the SCL low period is stretched. The TWINT Flag must be cleared by
software by writing a logic one to it.
Note that this flag is not automatically cleared by hardware when executing the interrupt routine. Also
note that clearing this flag starts the operation of the TWI, so all accesses to the TWI Address Register
(TWAR), TWI Status Register (TWSR), and TWI Data Register (TWDR) must be complete before clearing
this flag.
Bit 6 – TWEA: TWI Enable Acknowledge
The TWEA bit controls the generation of the acknowledge pulse. If the TWEA bit is written to one, the
ACK pulse is generated on the TWI bus if the following conditions are met:
1. The device’s own slave address has been received.
2. A general call has been received, while the TWGCE bit in the TWAR is set.
3. A data byte has been received in Master Receiver or Slave Receiver mode.
By writing the TWEA bit to zero, the device can be virtually disconnected from the 2-wire Serial Bus
temporarily. Address recognition can then be resumed by writing the TWEA bit to one again.
Bit 5 – TWSTA: TWI START Condition
The application writes the TWSTA bit to one when it desires to become a Master on the 2-wire Serial Bus.
The TWI hardware checks if the bus is available, and generates a START condition on the bus if it is free.
However, if the bus is not free, the TWI waits until a STOP condition is detected, and then generates a
new START condition to claim the bus Master status. TWSTA must be cleared by software when the
START condition has been transmitted.
Bit 4 – TWSTO: TWI STOP Condition
Writing the TWSTO bit to one in Master mode will generate a STOP condition on the 2-wire Serial Bus.
When the STOP condition is executed on the bus, the TWSTO bit is cleared automatically. In Slave
mode, setting the TWSTO bit can be used to recover from an error condition. This will not generate a
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STOP condition, but the TWI returns to a well-defined unaddressed Slave mode and releases the SCL
and SDA lines to a high impedance state.
Bit 3 – TWWC: TWI Write Collision Flag
The TWWC bit is set when attempting to write to the TWI Data Register – TWDR when TWINT is low.
This flag is cleared by writing the TWDR Register when TWINT is high.
Bit 2 – TWEN: TWI Enable
The TWEN bit enables TWI operation and activates the TWI interface. When TWEN is written to one, the
TWI takes control over the I/O pins connected to the SCL and SDA pins, enabling the slew-rate limiters
and spike filters. If this bit is written to zero, the TWI is switched off and all TWI transmissions are
terminated, regardless of any ongoing operation.
Bit 0 – TWIE: TWI Interrupt Enable
When this bit is written to one, and the I-bit in SREG is set, the TWI interrupt request will be activated for
as long as the TWINT Flag is high.
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26.8.3. TWSR – TWI Status Register
Name:  TWSR
Offset:  0x71
Reset:  0xF8
Property:
 
–
Bit 7 6 5 4 3 2 1 0
TWS7 TWS6 TWS5 TWS4 TWS3 TWPS1 TWPS0
Access R R R R R R/W R/W
Reset 1 1 1 1 1 0 0
Bit 7 – TWS7: TWI Status Bit 7
The TWS[7:3] reflect the status of the TWI logic and the 2-wire Serial Bus. The different status codes are
described later in this section. Note that the value read from TWSR contains both the 5-bit status value
and the 2-bit prescaler value. The application designer should mask the prescaler bits to zero when
checking the Status bits. This makes status checking independent of prescaler setting. This approach is
used in this datasheet, unless otherwise noted.
Bit 6 – TWS6: TWI Status Bit 6
Bit 5 – TWS5: TWI Status Bit 5
Bit 4 – TWS4: TWI Status Bit 4
Bit 3 – TWS3: TWI Status Bit 3
Bits 1:0 – TWPSn: TWI Prescaler [n = 1:0]
These bits can be read and written, and control the bit rate prescaler.
Table 26-8 TWI Bit Rate Prescaler
TWPS1 TWPS0 Prescaler Value
0 0 1
0 1 4
1 0 16
1 1 64
To calculate bit rates, refer to Bit Rate Generator Unit on page 275. The value of TWPS1:0 is used in the
equation.
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26.8.4. TWDR – TWI Data Register
In Transmit mode, TWDR contains the next byte to be transmitted. In Receive mode, the TWDR contains
the last byte received. It is writable while the TWI is not in the process of shifting a byte. This occurs when
the TWI Interrupt Flag (TWINT) is set by hardware. Note that the Data Register cannot be initialized by
the user before the first interrupt occurs. The data in TWDR remains stable as long as TWINT is set.
While data is shifted out, data on the bus is simultaneously shifted in. TWDR always contains the last
byte present on the bus, except after a wake up from a sleep mode by the TWI interrupt. In this case, the
contents of TWDR is undefined. In the case of a lost bus arbitration, no data is lost in the transition from
Master to Slave. Handling of the ACK bit is controlled automatically by the TWI logic, the CPU cannot
access the ACK bit directly.
Name:  TWDR
Offset:  0x73
Reset:  0xFF
Property:
 
–
Bit 7 6 5 4 3 2 1 0
TWD7 TWD6 TWD5 TWD4 TWD3 TWD2 TWD1 TWD0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 1 1 1 1 1 1 1 1
Bits 7:0 – TWDn: TWI Data [n = 7:0]
These eight bits constitute the next data byte to be transmitted, or the latest data byte received on the 2-
wire Serial Bus.
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26.8.5. TWAR – TWI (Slave) Address Register
The TWAR should be loaded with the 7-bit Slave address (in the seven most significant bits of TWAR) to
which the TWI will respond when programmed as a Slave Transmitter or Receiver, and not needed in the
Master modes. In multimaster systems, TWAR must be set in masters which can be addressed as Slaves
by other Masters.
The LSB of TWAR is used to enable recognition of the general call address (0x00). There is an
associated address comparator that looks for the slave address (or general call address if enabled) in the
received serial address. If a match is found, an interrupt request is generated.
Name:  TWAR
Offset:  0x72
Reset:  0x7F
Property:
 
–
Bit 7 6 5 4 3 2 1 0
TWA6 TWA5 TWA4 TWA3 TWA2 TWA1 TWA0 TWGCE
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 1 1 1 1 1 1 1 0
Bits 7:1 – TWAn: TWI (Slave) Address [n = 6:0]
These seven bits constitute the slave address of the TWI unit.
Bit 0 – TWGCE: TWI General Call Recognition Enable Bit
If set, this bit enables the recognition of a General Call given over the Two-wire Serial Bus.
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27. Analog Comparator
27.1. Overview
The Analog Comparator compares the input values on the positive pin AIN0 and negative pin AIN1. When
the voltage on the positive pin AIN0 is higher than the voltage on the negative pin AIN1, the Analog
Comparator Output, ACO, is set. The comparator’s output can be set to trigger the Timer/Counter1 Input
Capture function. In addition, the comparator can trigger a separate interrupt, exclusive to the Analog
Comparator. The user can select Interrupt triggering on comparator output rise, fall or toggle. A block
diagram of the comparator and its surrounding logic is shown in the figure below.
Figure 27-1 Analog Comparator Block Diagram
ACBG
BANDGAP
REFERENCE
ADC MULTIPLEXER
OUTPUT
ACME
ADEN
(1)
Note: 
1. See table Analog Comparator Multiplexed Input in the section below.
2. Refer to figure Pinout ATmega64A in Pin Configurations and table Port E Pins Alternate Functions
in Alternate Functions of Port E for Analog Comparator pin placement.
Related Links
Pin Configurations on page 14
Alternate Functions of Port E on page 105
27.2. Analog Comparator Multiplexed Input
It is possible to select any of the ADC7:0 pins to replace the negative input to the Analog Comparator.
The ADC multiplexer is used to select this input, and consequently, the ADC must be switched off to
utilize this feature. If the Analog Comparator Multiplexer Enable bit (ACME in SFIOR) is set and the ADC
is switched off (ADEN in ADCSRA is zero), MUX2:0 in ADMUX select the input pin to replace the
negative input to the Analog Comparator, as shown in the following table. If ACME is cleared or ADEN is
set, AIN1 is applied to the negative input to the Analog Comparator.
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Table 27-1 Analog Comparator Multiplexed Input
ACME ADEN MUX2:0 Analog Comparator Negative Input
0 x xxx AIN1
1 1 xxx AIN1
1 0 000 ADC0
1 0 001 ADC1
1 0 010 ADC2
1 0 011 ADC3
1 0 100 ADC4
1 0 101 ADC5
1 0 110 ADC6
1 0 111 ADC7
27.3. Register Description
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27.3.1. SFIOR – Analog Comparator Control and Status Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  SFIOR
Offset:  0x20
Reset:  N/A
Property:
 
When addressing I/O Registers as data space the offset address is 0x40
Bit 7 6 5 4 3 2 1 0
ACME
Access R/W
Reset 0
Bit 3 – ACME: Analog Comparator Multiplexer Enable
When this bit is written logic one and the ADC is switched off (ADEN in ADCSRA is zero), the ADC
multiplexer selects the negative input to the Analog Comparator. When this bit is written logic zero, AIN1
is applied to the negative input of the Analog Comparator. For a detailed description of this bit, see
Analog Comparator Multiplexed Input on page 306.
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27.3.2. ACSR – Analog Comparator Control and Status Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  ACSR
Offset:  0x08
Reset:  N/A
Property:
 
When addressing I/O Registers as data space the offset address is 0x28
Bit 7 6 5 4 3 2 1 0
ACD ACBG ACO ACI ACIE ACIC ACIS1 ACIS0
Access R/W R/W R R/W R/W R/W R/W R/W
Reset 0 0 x 0 0 0 0 0
Bit 7 – ACD: Analog Comparator Disable
When this bit is written logic one, the power to the Analog Comparator is switched off. This bit can be set
at any time to turn off the Analog Comparator. This will reduce power consumption in Active and Idle
mode. When changing the ACD bit, the Analog Comparator Interrupt must be disabled by clearing the
ACIE bit in ACSR. Otherwise an interrupt can occur when the bit is changed.
Bit 6 – ACBG: Analog Comparator Bandgap Select
When this bit is set, a fixed bandgap reference voltage replaces the positive input to the Analog
Comparator. When this bit is cleared, AIN0 is applied to the positive input of the Analog Comparator.
Refer to Internal Voltage Reference on page 71.
Bit 5 – ACO: Analog Comparator Output
The output of the Analog Comparator is synchronized and then directly connected to ACO. The
synchronization introduces a delay of 1 - 2 clock cycles.
Bit 4 – ACI: Analog Comparator Interrupt Flag
This bit is set by hardware when a comparator output event triggers the interrupt mode defined by ACIS1
and ACIS0. The Analog Comparator interrupt routine is executed if the ACIE bit is set and the I-bit in
SREG is set. ACI is cleared by hardware when executing the corresponding interrupt handling vector.
Alternatively, ACI is cleared by writing a logic one to the flag.
Bit 3 – ACIE: Analog Comparator Interrupt Enable
When the ACIE bit is written logic one and the I-bit in the Status Register is set, the Analog Comparator
interrupt is activated. When written logic zero, the interrupt is disabled.
Bit 2 – ACIC: Analog Comparator Input Capture Enable
When written logic one, this bit enables the input capture function in Timer/Counter1 to be triggered by
the Analog Comparator. The comparator output is in this case directly connected to the input capture
front-end logic, making the comparator utilize the noise canceler and edge select features of the Timer/
Counter1 Input Capture interrupt. When written logic zero, no connection between the Analog
Comparator and the input capture function exists. To make the comparator trigger the Timer/Counter1
Input Capture interrupt, the TICIE1 bit in the Timer Interrupt Mask Register (TIMSK) must be set.
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Bits 1:0 – ACISn: Analog Comparator Interrupt Mode Select [n = 1:0]
These bits determine which comparator events that trigger the Analog Comparator interrupt.
Table 27-2 ACIS[1:0] Settings
ACIS1 ACIS0 Interrupt Mode
0 0 Comparator Interrupt on Output Toggle.
0 1 Reserved
1 0 Comparator Interrupt on Falling Output Edge.
1 1 Comparator Interrupt on Rising Output Edge.
When changing the ACIS1/ACIS0 bits, the Analog Comparator Interrupt must be disabled by clearing its
Interrupt Enable bit in the ACSR Register. Otherwise an interrupt can occur when the bits are changed.
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28. ADC - Analog to Digital Converter
28.1. Features
•10-bit Resolution
•0.75 LSB Integral Non-Linearity
•±1.5 LSB Absolute Accuracy
•13 - 260μs Conversion Time
•Up to 15ksps at Maximum Resolution
•8 Multiplexed Single Ended Input Channels
•7 Differential Input Channels
•2 Differential Input Channels with Optional Gain of 10x and 200x
•Optional Left Adjustment for ADC Result Readout
•0 - VCC ADC Input Voltage Range
•2.7 - VCC Differential ADC Voltage Range
•Selectable 2.56V ADC Reference Voltage
•Free Running or Single Conversion Mode
•ADC Start Conversion by Auto Triggering on Interrupt Sources
•Interrupt on ADC Conversion Complete
•Sleep Mode Noise Canceler
28.2. Overview
The ATmega64A features a 10-bit successive approximation ADC. The ADC is connected to an 8-
channel Analog Multiplexer which allows 8 single-ended voltage inputs constructed from the pins of Port
F. The single-ended voltage inputs refer to 0V (GND).
The device also supports 16 differential voltage input combinations. Two of the differential inputs (ADC1,
ADC0 and ADC3, ADC2) are equipped with a programmable gain stage, providing amplification steps of
0dB (1x), 20dB (10x), or 46dB (200x) on the differential input voltage before the A/D conversion. Seven
differential analog input channels share a common negative terminal (ADC1), while any other ADC input
can be selected as the positive input terminal. If 1x or 10x gain is used, 8-bit resolution can be expected.
If 200x gain is used, 7-bit resolution can be expected.
The ADC contains a Sample and Hold circuit which ensures that the input voltage to the ADC is held at a
constant level during conversion. A block diagram of the ADC is shown below.
The ADC has a separate analog supply voltage pin, AVCC. AVCC must not differ more than ±0.3V from
VCC. See section ADC Noise Canceler on page 318 on how to connect this pin.
Internal reference voltages of nominally 2.56V or AVCC are provided On-chip. The voltage reference may
be externally decoupled at the AREF pin by a capacitor for better noise performance.
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Figure 28-1 Analog to Digital Converter Block Schematic Operation
ADC CONVERSION
COMPLETE IRQ
8-BIT DATA BUS
15 0
ADC MULTIPLEXER
SELECT (ADMUX) ADC CTRL. & STATUS
REGISTER (ADCSRA) ADC DATA REGISTER
(ADCH/ADCL)
MUX2
ADIE
ADFR
ADSC
ADEN
ADIF ADIF
MUX1
MUX0
ADPS0
ADPS1
ADPS2
MUX3
CONVERSION LOGIC
10-BIT DAC
+
-
SAMPLE & HOLD
COMPARATOR
INTERNAL 2.56V
REFERENCE
MUX DECODER
AVCC
ADC7
ADC6
ADC5
ADC4
ADC3
ADC2
ADC1
ADC0
REFS0
REFS1
ADLAR
CHANNEL SELECTION
ADC[9:0]
ADC MULTIPLEXER
OUTPUT
AREF
BANDGAP
REFERENCE
PRESCALER
AGND
MUX4
+
-
SINGLE ENDED / DIFFERENTIAL SELECTION
POS.
INPUT
MUX
NEG.
INPUT
MUX
The ADC converts an analog input voltage to a 10-bit digital value through successive approximation. The
minimum value represents GND and the maximum value represents the voltage on the AREF pin minus 1
LSB. Optionally, AVCC or an internal 2.56V reference voltage may be connected to the AREF pin by
writing to the REFSn bits in the ADMUX Register. The internal voltage reference may thus be decoupled
by an external capacitor at the AREF pin to improve noise immunity.
The analog input channel and differential gain are selected by writing to the MUX bits in ADMUX. Any of
the ADC input pins, as well as GND and a fixed bandgap voltage reference, can be selected as single
ended inputs to the ADC. A selection of ADC input pins can be selected as positive and negative inputs to
the differential gain amplifier.
If differential channels are selected, the differential gain stage amplifies the voltage difference between
the selected input channel pair by the selected gain factor. This amplified value then becomes the analog
input to the ADC. If single ended channels are used, the gain amplifier is bypassed altogether.
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The ADC is enabled by setting the ADC Enable bit, ADEN in ADCSRA. Voltage reference and input
channel selections will not go into effect until ADEN is set. The ADC does not consume power when
ADEN is cleared, so it is recommended to switch off the ADC before entering power saving sleep modes.
The ADC generates a 10-bit result which is presented in the ADC Data Registers, ADCH and ADCL. By
default, the result is presented right adjusted, but can optionally be presented left adjusted by setting the
ADLAR bit in ADMUX.
If the result is left adjusted and no more than 8-bit precision is required, it is sufficient to read ADCH.
Otherwise, ADCL must be read first, then ADCH, to ensure that the content of the data registers belongs
to the same conversion. Once ADCL is read, ADC access to data registers is blocked. This means that if
ADCL has been read, and a conversion completes before ADCH is read, neither register is updated and
the result from the conversion is lost. When ADCH is read, ADC access to the ADCH and ADCL
Registers is re-enabled.
The ADC has its own interrupt which can be triggered when a conversion completes. When ADC access
to the data registers is prohibited between reading of ADCH and ADCL, the interrupt will trigger even if
the result is lost.
28.3. Starting a Conversion
A single conversion is started by writing a logical one to the ADC Start Conversion bit, ADSC. This bit
stays high as long as the conversion is in progress and will be cleared by hardware when the conversion
is completed. If a different data channel is selected while a conversion is in progress, the ADC will finish
the current conversion before performing the channel change.
Alternatively, a conversion can be triggered automatically by various sources. Auto Triggering is enabled
by setting the ADC Auto Trigger Enable bit, ADATE in ADCSRA. The trigger source is selected by setting
the ADC Trigger Select bits, ADTS in ADCSRB (see description of the ADTS bits for a list of the trigger
sources). When a positive edge occurs on the selected trigger signal, the ADC prescaler is reset and a
conversion is started. This provides a method of starting conversions at fixed intervals. If the trigger signal
still is set when the conversion completes, a new conversion will not be started. If another positive edge
occurs on the trigger signal during conversion, the edge will be ignored. Note that an interrupt flag will be
set even if the specific interrupt is disabled or the Global Interrupt Enable bit in SREG is cleared. A
conversion can thus be triggered without causing an interrupt. However, the interrupt flag must be cleared
in order to trigger a new conversion at the next interrupt event.
Figure 28-2 ADC Auto Trigger Logic
ADSC
ADIF
SOURCE 1
SOURCE n
ADTS[2:0]
CONVERSION
LOGIC
PRESCALER
START CLKADC
.
.
.
.EDGE
DETECTOR
ADATE
Using the ADC Interrupt Flag as a trigger source makes the ADC start a new conversion as soon as the
ongoing conversion has finished. The ADC then operates in Free Running mode, constantly sampling
and updating the ADC Data Register. The first conversion must be started by writing a logical one to the
ADSC bit in ADCSRA. In this mode the ADC will perform successive conversions independently of
whether the ADC Interrupt Flag, ADIF is cleared or not.
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If Auto Triggering is enabled, single conversions can be started by writing ADSC in ADCSRA to one.
ADSC can also be used to determine if a conversion is in progress. The ADSC bit will be read as one
during a conversion, independently of how the conversion was started.
28.4. Prescaling and Conversion Timing
Figure 28-3 ADC Prescaler
7-BIT ADC PRESCALER
ADC CLOCK SOURCE
CK
ADPS0
ADPS1
ADPS2
CK/128
CK/2
CK/4
CK/8
CK/16
CK/32
CK/64
Reset
ADEN
START
By default, the successive approximation circuitry requires an input clock frequency between 50kHz and
200kHz to get maximum resolution. If a lower resolution than 10 bits is needed, the input clock frequency
to the ADC can be higher than 200kHz to get a higher sample rate.
The ADC module contains a prescaler, which generates an acceptable ADC clock frequency from any
CPU frequency above 100kHz. The prescaling is set by the ADPS bits in ADCSRA. The prescaler starts
counting from the moment the ADC is switched on by setting the ADEN bit in ADCSRA. The prescaler
keeps running for as long as the ADEN bit is set, and is continuously reset when ADEN is low.
When initiating a single ended conversion by setting the ADSC bit in ADCSRA, the conversion starts at
the following rising edge of the ADC clock cycle. See Differential Gain Channels on page 316 for details
on differential conversion timing.
A normal conversion takes 13 ADC clock cycles. The first conversion after the ADC is switched on (ADEN
in ADCSRA is set) takes 25 ADC clock cycles in order to initialize the analog circuitry.
The actual sample-and-hold takes place 1.5 ADC clock cycles after the start of a normal conversion and
13.5 ADC clock cycles after the start of a first conversion. When a conversion is complete, the result is
written to the ADC Data Registers, and ADIF is set. In single conversion mode, ADSC is cleared
simultaneously. The software may then set ADSC again, and a new conversion will be initiated on the first
rising ADC clock edge.
When Auto Triggering is used, the prescaler is reset when the trigger event occurs. This assures a fixed
delay from the trigger event to the start of conversion. In this mode, the sample-and-hold takes place two
ADC clock cycles after the rising edge on the trigger source signal. Three additional CPU clock cycles are
used for synchronization logic.
When using Differential mode, along with auto trigging from a source other that the ADC Conversion
Complete, each conversion will require 25 ADC clocks. This is because the ADC must be disabled and
re-enabled after every conversion.
In Free Running mode, a new conversion will be started immediately after the conversion completes,
while ADSC remains high. For a summary of conversion times, see table ADC Conversion Time at the
end of this section.
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Figure 28-4 ADC Timing Diagram, First Conversion (Single Conversion Mode)
Sign and MSB of Result
LSB of Result
ADC Clock
ADSC
Sample and Hold
ADIF
ADCH
ADCL
Cycle Number
ADEN
1 2 12 13 14 15 16 17 18 19 20 21 22 23 24 25 1 2
First Conversion Next
Conversion
3
MUX and REFS
Update
MUX and REFS
Update
Conversion
Complete
Figure 28-5 ADC Timing Diagram, Single Conversion
12 3 4 5 6 7 8 9 10 11 12 13
Sign and MSB of Result
LSB of Result
ADC Clock
ADSC
ADIF
ADCH
ADCL
Cycle Number 1 2
One Conversion Next Conversion
3
Sample and Hold
MUX and REFS
Update
Conversion
Complete
MUX and REFS
Update
Figure 28-6 ADC Timing Diagram, Auto Triggered Conversion
1 2 3 4 5 6 7 8 910 11 12 13
Sign and MSB of Result
LSB of Result
ADC Clock
Trigger
Source
ADIF
ADCH
ADCL
Cycle Number 1 2
One Conversion Next Conversion
Conversion
Complete
Prescaler
Reset
ADATE
Prescaler
Reset
Sample &
Hold
MUX and REFS
Update
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One Canvusmn Nexl Canvusmn Atmel
Figure 28-7 ADC Timing Diagram, Free Running Conversion
11 12 13
Sign and MSB of Result
LSB of Result
ADC Clock
ADSC
ADIF
ADCH
ADCL
Cycle Number 12
One Conversion Next Conversion
3 4
Conversion
Complete
Sample and Hold
MUX and REFS
Update
Table 28-1 ADC Conversion Time
Condition Sample & Hold
(Cycles from Start of Conversion)
Conversion Time
(Cycles)
First conversion 13.5 25
Normal conversions, single ended 1.5 13
Auto Triggered conversions 2 13.5
Normal conversions, differential 1.5/2.5 13/14
28.4.1. Differential Gain Channels
When using differential gain channels, certain aspects of the conversion need to be taken into
consideration.
Differential conversions are synchronized to the internal clock CKADC2 equal to half the ADC clock. This
synchronization is done automatically by the ADC interface in such a way that the sample-and-hold
occurs at a specific edge of CKADC2. A conversion initiated by the user (that is, all single conversions, and
the first free running conversion) when CKADC2 is low will take the same amount of time as a single ended
conversion (13 ADC clock cycles from the next prescaled clock cycle). A conversion initiated by the user
when CKADC2 is high will take 14 ADC clock cycles due to the synchronization mechanism. In free
running mode, a new conversion is initiated immediately after the previous conversion completes, and
since CKADC2 is high at this time, all automatically started (that is, all but the first) free running
conversions will take 14 ADC clock cycles.
The gain stage is optimized for a bandwidth of 4kHz at all gain settings. Higher frequencies may be
subjected to non-linear amplification. An external low-pass filter should be used if the input signal
contains higher frequency components than the gain stage bandwidth. Note that the ADC clock frequency
is independent of the gain stage bandwidth limitation. For example the ADC clock period may be 6μs,
allowing a channel to be sampled at 12kSPS, regardless of the bandwidth of this channel.
If differential gain channels are used and conversions are started by Auto Triggering, the ADC must be
switched off between conversions. When Auto Triggering is used, the ADC prescaler is reset before the
conversion is started. Since the gain stage is dependent of a stable ADC clock prior to the conversion,
this conversion will not be valid. By disabling and then re-enabling the ADC between each conversion
(writing ADEN in ADCSRA to “0” then to “1”), only extended conversions are performed. The result from
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the extended conversions will be valid. Refer to Prescaling and Conversion Timing on page 314 for timing
details.
28.5. Changing Channel or Reference Selection
The MUXn and REFS1:0 bits in the ADMUX Register are single buffered through a temporary register to
which the CPU has random access. This ensures that the channels and reference selection only takes
place at a safe point during the conversion. The channel and reference selection is continuously updated
until a conversion is started. Once the conversion starts, the channel and reference selection is locked to
ensure a sufficient sampling time for the ADC. Continuous updating resumes in the last ADC clock cycle
before the conversion completes (ADIF in ADCSRA is set). Note that the conversion starts on the
following rising ADC clock edge after ADSC is written. The user is thus advised not to write new channel
or reference selection values to ADMUX until one ADC clock cycle after ADSC is written.
If Auto Triggering is used, the exact time of the triggering event can be indeterministic. Special care must
be taken when updating the ADMUX Register, in order to control which conversion will be affected by the
new settings.
If both ADATE and ADEN is written to one, an interrupt event can occur at any time. If the ADMUX
Register is changed in this period, the user cannot tell if the next conversion is based on the old or the
new settings. ADMUX can be safely updated in the following ways:
1. When ADATE or ADEN is cleared.
2. During conversion, minimum one ADC clock cycle after the trigger event.
3. After a conversion, before the interrupt flag used as trigger source is cleared.
When updating ADMUX in one of these conditions, the new settings will affect the next ADC conversion.
Special care should be taken when changing differential channels. Once a differential channel has been
selected, the gain stage may take as much as 125μs to stabilize to the new value. Thus conversions
should not be started within the first 125μs after selecting a new differential channel. Alternatively,
conversion results obtained within this period should be discarded.
The same settling time should be observed for the first differential conversion after changing ADC
reference (by changing the REFS1:0 bits in ADMUX).
If the JTAG Interface is enabled, the function of ADC channels on PORTF7:4 is overridden. Refer to table
Port F Pins Alternate Functions in section Alternate Functions of Port F.
Related Links
Alternate Functions of Port F on page 108
28.5.1. ADC Input Channels
When changing channel selections, the user should observe the following guidelines to ensure that the
correct channel is selected:
•In Single Conversion mode, always select the channel before starting the conversion. The channel
selection may be changed one ADC clock cycle after writing one to ADSC. However, the simplest
method is to wait for the conversion to complete before changing the channel selection.
•In Free Running mode, always select the channel before starting the first conversion. The channel
selection may be changed one ADC clock cycle after writing one to ADSC. However, the simplest
method is to wait for the first conversion to complete, and then change the channel selection. Since
the next conversion has already started automatically, the next result will reflect the previous
channel selection. Subsequent conversions will reflect the new channel selection.
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When switching to a differential gain channel, the first conversion result may have a poor accuracy due to
the required settling time for the automatic offset cancellation circuitry. The user should preferably
disregard the first conversion result.
28.5.2. ADC Voltage Reference
The reference voltage for the ADC (VREF) indicates the conversion range for the ADC. Single ended
channels that exceed VREF will result in codes close to 0x3FF. VREF can be selected as either AVCC,
internal 2.56V reference, or external AREF pin.
AVCC is connected to the ADC through a passive switch. The internal 2.56V reference is generated from
the internal bandgap reference (VBG) through an internal amplifier. In either case, the external AREF pin
is directly connected to the ADC, and the reference voltage can be made more immune to noise by
connecting a capacitor between the AREF pin and ground. VREF can also be measured at the AREF pin
with a high impedance voltmeter. Note that VREF is a high impedance source, and only a capacitive load
should be connected in a system.
If the user has a fixed voltage source connected to the AREF pin, the user may not use the other
reference voltage options in the application, as they will be shorted to the external voltage. If no external
voltage is applied to the AREF pin, the user may switch between AVCC and 2.56V as reference selection.
The first ADC conversion result after switching reference voltage source may be inaccurate, and the user
is advised to discard this result.
If differential channels are used, the selected reference should not be closer to AVCC than indicated in
table ADC Characteristics, Differential Channels in ADC Characteristics
28.6. ADC Noise Canceler
The ADC features a noise canceler that enables conversion during sleep mode to reduce noise induced
from the CPU core and other I/O peripherals. The noise canceler can be used with ADC Noise Reduction
and Idle mode. To make use of this feature, the following procedure should be used:
1. Make sure that the ADC is enabled and is not busy converting. Single Conversion mode must be
selected and the ADC conversion complete interrupt must be enabled.
2. Enter ADC Noise Reduction mode (or Idle mode). The ADC will start a conversion once the CPU
has been halted.
3. If no other interrupts occur before the ADC conversion completes, the ADC interrupt will wake up
the CPU and execute the ADC Conversion Complete interrupt routine. If another interrupt wakes up
the CPU before the ADC conversion is complete, that interrupt will be executed, and an ADC
Conversion Complete interrupt request will be generated when the ADC conversion completes. The
CPU will remain in active mode until a new sleep command is executed.
Note:  The ADC will not be automatically turned off when entering other sleep modes than Idle mode and
ADC Noise Reduction mode. The user is advised to write zero to ADCRSA.ADEN before entering such
sleep modes to avoid excessive power consumption. If the ADC is enabled in such sleep modes and the
user wants to perform differential conversions, the user is advised to switch the ADC off and on after
waking up from sleep to prompt an extended conversion to get a valid result.
28.6.1. Analog Input Circuitry
The analog input circuitry for single ended channels is illustrated below. An analog source applied to
ADCn is subjected to the pin capacitance and input leakage of that pin, regardless of whether that
channel is selected as input for the ADC. When the channel is selected, the source must drive the S/H
capacitor through the series resistance (combined resistance in the input path).
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The ADC is optimized for analog signals with an output impedance of approximately 10kΩ or less. If such
a source is used, the sampling time will be negligible. If a source with higher impedance is used, the
sampling time will depend on how long time the source needs to charge the S/H capacitor, with can vary
widely. The user is recommended to only use low impedance sources with slowly varying signals, since
this minimizes the required charge transfer to the S/H capacitor.
If differential gain channels are used, the input circuitry looks somewhat different, although source
impedances of a few hundred kΩ or less is recommended.
Signal components higher than the Nyquist frequency (fADC/2) should not be present for either kind of
channels, to avoid distortion from unpredictable signal convolution. The user is advised to remove high
frequency components with a low-pass filter before applying the signals as inputs to the ADC.
Figure 28-8 Analog Input Circuitry
ADCn
IIH
1..100k Ω
CS/H
= 14pF
IIL
VCC/2
28.6.2. Analog Noise Canceling Techniques
Digital circuitry inside and outside the device generates EMI which might affect the accuracy of analog
measurements. If conversion accuracy is critical, the noise level can be reduced by applying the following
techniques:
1. Keep analog signal paths as short as possible. Make sure analog tracks run over the ground plane,
and keep them well away from high-speed switching digital tracks.
2. The AVCC pin on the device should be connected to the digital VCC supply voltage via an LC
network as shown in the figure below.
3. Use the ADC noise canceler function to reduce induced noise from the CPU.
4. If any ADC port pins are used as digital outputs, it is essential that these do not switch while a
conversion is in progress.
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VCC Atmel
Figure 28-9 ADC Power Connections
VCC
GND
100nF
(ADC0) P F0
(ADC7) P F7
(ADC1) P F1
(ADC2) P F2
(ADC3) P F3
(ADC4) P F4
(ADC5) P F5
(ADC6) P F6
AREF
GND
AVCC
52
53
54
55
56
57
58
59
60
6161
6262
6363
6464
1
51
PEN
(AD0) PA0
10µH
28.6.3. Offset Compensation Schemes
The gain stage has a built-in offset cancellation circuitry that nulls the offset of differential measurements
as much as possible. The remaining offset in the analog path can be measured directly by selecting the
same channel for both differential inputs. This offset residue can be then subtracted in software from the
measurement results. Using this kind of software based offset correction, offset on any channel can be
reduced below one LSB.
28.6.4. ADC Accuracy Definitions
An n-bit single-ended ADC converts a voltage linearly between GND and VREF in 2n steps (LSBs). The
lowest code is read as 0, and the highest code is read as 2n-1.
Several parameters describe the deviation from the ideal behavior:
•Offset: The deviation of the first transition (0x000 to 0x001) compared to the ideal transition (at 0.5
LSB). Ideal value: 0 LSB.
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Figure 28-10 Offset Error
Output Code
V
REF Input Voltage
Ideal ADC
Actual ADC
Offset
Error
•Gain error: After adjusting for offset, the gain error is found as the deviation of the last transition
(0x3FE to 0x3FF) compared to the ideal transition (at 1.5 LSB below maximum). Ideal value: 0 LSB.
Figure 28-11 Gain Error
Output Code
V
REF Input Voltage
Ideal ADC
Actual ADC
Gain
Error
•Integral Non-linearity (INL): After adjusting for offset and gain error, the INL is the maximum
deviation of an actual transition compared to an ideal transition for any code. Ideal value: 0 LSB.
Figure 28-12 Integral Non-linearity (INL)
Output Code
V
REF Input Voltage
Ideal ADC
Actual ADC
INL
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•Differential Non-linearity (DNL): The maximum deviation of the actual code width (the interval
between two adjacent transitions) from the ideal code width (1 LSB). Ideal value: 0 LSB.
Figure 28-13 Differential Non-linearity (DNL)
Output Code
0x3FF
0x000
0V
REF Input Voltage
DNL
1 LSB
•Quantization Error: Due to the quantization of the input voltage into a finite number of codes, a
range of input voltages (1 LSB wide) will code to the same value. Always ±0.5 LSB.
•Absolute accuracy: The maximum deviation of an actual (unadjusted) transition compared to an
ideal transition for any code. This is the compound effect of offset, gain error, differential error, non-
linearity, and quantization error. Ideal value: ±0.5 LSB.
28.7. ADC Conversion Result
After the conversion is complete (ADCSRA.ADIF is high), the conversion result can be found in the ADC
Result Registers (ADCL, ADCH).
For single ended conversion, the result is
ADC = IN 1024
REF
where VIN is the voltage on the selected input pin, and VREF the selected voltage reference (see Table
28-3 ADC Voltage Reference Selection on page 325 and Table 28-4 Input Channel and Gain Selections
on page 326). 0x000 represents analog ground, and 0x3FF represents the selected reference voltage
minus one LSB.
If differential channels are used, the result is
ADC = (VPOS– VNEG ) GAIN 512
REF
where VPOS is the voltage on the positive input pin, VNEG the voltage on the negative input pin, GAIN the
selected gain factor, and VREF the selected voltage reference. The result is presented in two’s
complement form, from 0x200 (-512d) through 0x1FF (+511d). Note that if the user wants to perform a
quick polarity check of the results, it is sufficient to read the MSB of the result (ADC9 in ADCH). If this bit
is one, the result is negative, and if this bit is zero, the result is positive. The next figure shows the
decoding of the differential input range.
The table below shows the resulting output codes if the differential input channel pair (ADCn - ADCm) is
selected with a gain of GAIN and a reference voltage of VREF.
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Figure 28-14 Differential Measurement Range
0
Output Code
0x1FF
0x000
VREF/GAIN Differentia l Input
Volta ge (Volts)
0x3FF
0x200
- VREF/GAIN
Table 28-2 Correlation Between Input Voltage and Output Codes
VADCn Read Code Corresponding decimal value
VADCm + VREF /GAIN 0x1FF 511
VADCm + 511/512 VREF /GAIN 0x1FF 511
VADCm + 511/512 VREF /GAIN 0x1FE 510
:. :. :.
VADCm + 1/512 VREF /GAIN 0x001 1
VADCm 0x000 0
VADCm - 1/512 VREF /GAIN 0x3FF -1
:. :. :.
VADCm - 511/512 VREF /GAIN 0x201 -511
VADCm - VREF /GAIN 0x200 -512
Example:
ADMUX = 0xED (ADC3 - ADC2, 10x gain, 2.56V reference, left adjusted result)
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Voltage on ADC3 is 300mV, voltage on ADC2 is 500mV.
ADCR = 512 × 10 × (300 - 500) / 2560 = -400 = 0x270
ADCL will thus read 0x00, and ADCH will read 0x9C. Writing zero to ADLAR right adjusts the result:
ADCL = 0x70, ADCH = 0x02.
28.8. Register Description
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28.8.1. ADMUX – ADC Multiplexer Selection Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  ADMUX
Offset:  0x07
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x27
Bit 7 6 5 4 3 2 1 0
REFS1 REFS0 ADLAR MUX4 MUX3 MUX2 MUX1 MUX0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bits 7:6 – REFSn: Reference Selection [n = 1:0]
These bits select the voltage reference for the ADC. If these bits are changed during a conversion, the
change will not go in effect until this conversion is complete (ADIF in ADCSRA is set). The internal
voltage reference options may not be used if an external reference voltage is being applied to the AREF
pin.
Table 28-3 ADC Voltage Reference Selection
REFS[1:0] Voltage Reference Selection
00 AREF, Internal Vref turned off
01 AVCC with external capacitor at AREF pin
10 Reserved
11 Internal 2.56V Voltage Reference with external capacitor at AREF pin
Bit 5 – ADLAR: ADC Left Adjust Result
The ADLAR bit affects the presentation of the ADC conversion result in the ADC Data Register. Write one
to ADLAR to left adjust the result. Otherwise, the result is right adjusted. Changing the ADLAR bit will
affect the ADC Data Register immediately, regardless of any ongoing conversions. For a complete
description of this bit, see ADCL and ADCH.
Bits 4:0 – MUXn: Analog Channel Selection [n = 4:0]
The value of these bits selects which combination of analog inputs are connected to the ADC. These bits
also select the gain for the differential channels. Refer to table below for details. If these bits are changed
during a conversion, the change will not go in effect until this conversion is complete (ADIF in ADCSRA is
set).
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Table 28-4 Input Channel and Gain Selections
MUX[4:0] Single Ended Input Positive Differential
Input
Negative Differential
Input
Gain
00000 ADC0
N/A
00001 ADC1
00010 ADC2
00011 ADC3
00100 ADC4
00101 ADC5
00110 ADC6
00111 ADC7
01000(1) Reserved ADC0 ADC0 10x
01001 Reserved ADC1 ADC0 10x
01010(1)
N/A
ADC0 ADC0 200x
01011 ADC1 ADC0 200x
01100 ADC2 ADC2 10x
01101 ADC3 ADC2 10x
01110 ADC2 ADC2 200x
01111 ADC3 ADC2 200x
10000 ADC0 ADC1 1x
10001 ADC1 ADC1 1x
10010 ADC2 ADC1 1x
10011 ADC3 ADC1 1x
10100 ADC4 ADC1 1x
10101 ADC5 ADC1 1x
10110 ADC6 ADC1 1x
10111 ADC7 ADC1 1x
11000 ADC0 ADC2 1x
11001 ADC1 ADC2 1x
11010 ADC2 ADC2 1x
11011 ADC3 ADC2 1x
11100 ADC4 ADC2 1x
11101 Reserved ADC5 ADC2 1x
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MUX[4:0] Single Ended Input Positive Differential
Input
Negative Differential
Input
Gain
11110 1.22V (VBG)N/A
11111 0V (GND)
Note:  1. Can be used for offset calibration.
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28.8.2. ADCSRA – ADC Control and Status Register A
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  ADCSRA
Offset:  0x06
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x26
Bit 7 6 5 4 3 2 1 0
ADEN ADSC ADATE ADIF ADIE ADPS2 ADPS1 ADPS0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bit 7 – ADEN: ADC Enable
Writing this bit to one enables the ADC. By writing it to zero, the ADC is turned off. Turning the ADC off
while a conversion is in progress, will terminate this conversion.
Bit 6 – ADSC: ADC Start Conversion
In Single Conversion mode, write this bit to one to start each conversion. In Free Running mode, write
this bit to one to start the first conversion. The first conversion after ADSC has been written after the ADC
has been enabled, or if ADSC is written at the same time as the ADC is enabled, will take 25 ADC clock
cycles instead of the normal 13. This first conversion performs initialization of the ADC.
ADSC will read as one as long as a conversion is in progress. When the conversion is complete, it returns
to zero. Writing zero to this bit has no effect.
Bit 5 – ADATE: ADC Auto Trigger Enable
When this bit is written to one, Auto Triggering of the ADC is enabled. The ADC will start a conversion on
a positive edge of the selected trigger signal. The trigger source is selected by setting the ADC Trigger
Select bits, ADTS in ADCSRB.
Bit 4 – ADIF: ADC Interrupt Flag
This bit is set when an ADC conversion completes and the Data Registers are updated. The ADC
Conversion Complete Interrupt is executed if the ADIE bit and the I-bit in SREG are set. ADIF is cleared
by hardware when executing the corresponding interrupt Handling Vector. Alternatively, ADIF is cleared
by writing a logical one to the flag. Beware that if doing a Read-Modify-Write on ADCSRA, a pending
interrupt can be disabled. This also applies if the SBI and CBI instructions are used.
Bit 3 – ADIE: ADC Interrupt Enable
When this bit is written to one and the I-bit in SREG is set, the ADC Conversion Complete Interrupt is
activated.
Bits 2:0 – ADPSn: ADC Prescaler Select [n = 2:0]
These bits determine the division factor between the XTAL frequency and the input clock to the ADC.
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Table 28-5 ADC Prescaler Selections
ADPS[2:0] Division Factor
000 2
001 2
010 4
011 8
100 16
101 32
110 64
111 128
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28.8.3. ADCL – ADC Data Register Low (ADLAR=0)
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
When an ADC conversion is complete, the result is found in these two registers. If differential channels
are used, the result is presented in two’s complement form.
When ADCL is read, the ADC Data Register is not updated until ADCH is read. Consequently, if the result
is left adjusted and no more than 8-bit precision is required, it is sufficient to read ADCH. Otherwise,
ADCL must be read first, then ADCH.
The ADLAR bit and the MUXn bits in ADMUX affect the way the result is read from the registers. If
ADLAR is set, the result is left adjusted. If ADLAR is cleared (default), the result is right adjusted.
Name:  ADCL
Offset:  0x04
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x24
Bit 7 6 5 4 3 2 1 0
ADC7 ADC6 ADC5 ADC4 ADC3 ADC2 ADC1 ADC0
Access R R R R R R R R
Reset 0 0 0 0 0 0 0 0
Bits 7:0 – ADCn: ADC Conversion Result [n = 7:0]
These bits represent the result from the conversion. Refer to ADC Conversion Result on page 322 for
details.
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28.8.4. ADCH – ADC Data Register High (ADLAR=0)
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  ADCH
Offset:  0x05
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x25
Bit 7 6 5 4 3 2 1 0
ADC9 ADC8
Access R R
Reset 0 0
Bit 1 – ADC9: ADC Conversion Result
Refer to ADCL on page 330
Bit 0 – ADC8: ADC Conversion Result
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28.8.5. ADCL – ADC Data Register Low (ADLAR=1)
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  ADCL
Offset:  0x04
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x24
Bit 7 6 5 4 3 2 1 0
ADC1 ADC0
Access R R
Reset 0 0
Bit 7 – ADC1: ADC Conversion Result
Refer to ADCL on page 330
Bit 6 – ADC0: ADC Conversion Result
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28.8.6. ADCH – ADC Data Register High (ADLAR=1)
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  ADCH
Offset:  0x05
Reset:  0x00
Property:
 
When addressing I/O Registers as data space the offset address is 0x25
Bit 7 6 5 4 3 2 1 0
ADC9 ADC8 ADC7 ADC6 ADC5 ADC4 ADC3 ADC2
Access R R R R R R R R
Reset 0 0 0 0 0 0 0 0
Bit 7 – ADC9: ADC Conversion Result
Bit 6 – ADC8: ADC Conversion Result
Bit 5 – ADC7: ADC Conversion Result
Bit 4 – ADC6: ADC Conversion Result
Bit 3 – ADC5: ADC Conversion Result
Bit 2 – ADC4: ADC Conversion Result
Bit 1 – ADC3: ADC Conversion Result
Bit 0 – ADC2: ADC Conversion Result
Refer to ADCL on page 330
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28.8.7. ADCSRB – ADC Control and Status Register B
Name:  ADCSRB
Offset:  0x8E
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
ADTS2 ADTS1 ADTS0
Access R/W R/W R/W
Reset 0 0 0
Bits 2:0 – ADTSn: ADC Auto Trigger Source [n = 2:0]
If ADATE in ADCSRA is written to one, the value of these bits selects which source will trigger an ADC
conversion. If ADATE is cleared, the ADTS2:0 settings will have no effect. A conversion will be triggered
by the rising edge of the selected interrupt flag. Note that switching from a trigger source that is cleared to
a trigger source that is set, will generate a positive edge on the trigger signal. If ADEN in ADCSRA is set,
this will start a conversion. Switching to Free Running mode (ADTS[2:0]=0) will not cause a trigger event,
even if the ADC Interrupt Flag is set.
Table 28-6 ADC Auto Trigger Source Selections
ADTS[2:0] Trigger Source
000 Free Running mode
001 Analog Comparator
010 External Interrupt Request 0
011 Timer/Counter0 Compare Match
100 Timer/Counter0 Overflow
101 Timer/Counter1 Compare Match B
110 Timer/Counter1 Overflow
111 Timer/Counter1 Capture Event
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29. JTAG Interface and On-chip Debug System
29.1. Features
•JTAG (IEEE std. 1149.1 Compliant) Interface
•Boundary-scan Capabilities According to the IEEE std. 1149.1 (JTAG) Standard
•Debugger Access to:
–All Internal Peripheral Units
–Internal and External RAM
–The Internal Register File
–Program Counter
–EEPROM and Flash Memories
•Extensive On-chip Debug Support for Break Conditions, Including:
–AVR Break Instruction
–Break on Change of Program Memory Flow
–Single Step Break
–Program Memory Breakpoints on Single Address or Address Range
–Data Memory Breakpoints on Single Address or Address Range
•Programming of Flash, EEPROM, Fuses, and Lock Bits through the JTAG Interface
•On-chip Debugging Supported by Atmel Studio
29.2. Overview
The AVR IEEE std. 1149.1 compliant JTAG interface can be used for:
•Testing PCBs by using the JTAG Boundary-scan capability
•Programming the non-volatile memories, Fuses and Lock bits
•On-chip debugging
A brief description is given in the following sections. Detailed descriptions for Programming via the JTAG
interface, and using the Boundary-scan Chain can be found in the sections Programming Via the JTAG
Interface and IEEE 1149.1 (JTAG) Boundary-scan on page 340, respectively. The On-chip Debug
support is considered being private JTAG instructions, and distributed within ATMEL and to selected third
party vendors only.
Figure 29-1 Block Diagram on page 336 shows the JTAG interface and the On-chip Debug system. The
TAP Controller is a state machine controlled by the TCK and TMS signals. The TAP Controller selects
either the JTAG Instruction Register or one of several Data Registers as the scan chain (Shift Register)
between the TDI – input and TDO – output. The Instruction Register holds JTAG instructions controlling
the behavior of a Data Register.
The ID-Register, Bypass Register, and the Boundary-scan Chain are the data registers used for board-
level testing. The JTAG Programming Interface (actually consisting of several physical and virtual Data
Registers) is used for serial programming via the JTAG interface. The Internal Scan Chain and Break
Point Scan Chain are used for On-chip debugging only.
Related Links
Programming Via the JTAG Interface on page 400
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29.3. TAP – Test Access Port
The JTAG interface is accessed through four of the AVR’s pins. In JTAG terminology, these pins
constitute the Test Access Port – TAP. These pins are:
•TMS: Test mode select. This pin is used for navigating through the TAP-controller state machine.
•TCK: Test clock. JTAG operation is synchronous to TCK.
•TDI: Test Data In. Serial input data to be shifted in to the Instruction Register or Data Register
(Scan Chains).
•TDO: Test Data Out. Serial output data from Instruction Register or Data Register.
The IEEE std. 1149.1 also specifies an optional TAP signal; TRST – Test ReSeT – which is not provided.
When the JTAGEN fuse is unprogrammed, these four TAP pins are normal port pins and the TAP
controller is in reset. When programmed and the JTD bit in MCUCSR is cleared, the TAP input signals
are internally pulled high and the JTAG is enabled for Boundary-scan and programming. In this case, the
TAP output pin (TDO) is left floating in states where the JTAG TAP controller is not shifting data, and must
therefore be connected to a pull-up resistor or other hardware having pull-ups (for instance the TDI-input
of the next device in the scan chain). The device is shipped with this fuse programmed.
For the On-chip Debug system, in addition to the JTAG interface pins, the RESET pin is monitored by the
debugger to be able to detect External Reset sources. The debugger can also pull the RESET pin low to
reset the whole system, assuming only open collectors on the Reset line are used in the application.
Figure 29-1 Block Diagram
TAP
CONTROLLER
TDI
TDO
TCK
TMS
FLAS H
MEMORY
AVR CP U
DIGITAL
PERIPHERAL
UNITS
JTAG / AVR CORE
COMMUNICATION
INTERFACE
BREAKPOINT
UNIT FLOW CONTROL
UNIT
OCD STATUS
AND CONTROL
INTERNAL
SCAN
CHAIN
M
U
X
INS TRUCTION
REGISTER
ID
REGISTER
BYP ASS
REGISTER
JTAG P ROGRAMMING
INTERFACE
PC
Instruction
Addres s
Data
BREAKPOINT
SCAN CHAIN
ADDRESS
DECODER
ANALOG
PERIPHERIAL
UNITS
I/O P ORT 0
I/O P ORT n
BOUNDARY S CAN CHAIN
Analog inputs
Control & Clock line s
DEVICE BOUNDARY
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l C Test-Loglc-R Atmel
Figure 29-2 TAP Controller State Diagram
Tes t-Logic-Re s e t
Run-Test/Idle
Shift-DR
Exit1-DR
Pa use -DR
Exit2-DR
Update -DR
Se le ct-IR S can
Ca pture-IR
Shift-IR
Exit1-IR
Pa use -IR
Exit2-IR
Update -IR
Se le ct-DR S can
Ca pture-DR
0
1
01 1 1
0 0
0 0
1 1
10
1
1
0
1
0
0
10
1
1
0
1
0
0
00
11
29.4. TAP Controller
The TAP controller is a 16-state finite state machine that controls the operation of the Boundary-scan
circuitry, JTAG programming circuitry, or On-chip Debug system. The state transitions depicted in Figure
29-2 TAP Controller State Diagram on page 337 depend on the signal present on TMS (shown adjacent
to each state transition) at the time of the rising edge at TCK. The initial state after a Power-on Reset is
Test-Logic-Reset.
As a definition in this document, the LSB is shifted in and out first for all Shift Registers.
Assuming Run-Test/Idle is the present state, a typical scenario for using the JTAG interface is:
•At the TMS input, apply the sequence 1, 1, 0, 0 at the rising edges of TCK to enter the Shift
Instruction Register – Shift-IR state. While in this state, shift the 4 bits of the JTAG instructions into
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the JTAG instruction register from the TDI input at the rising edge of TCK. The TMS input must be
held low during input of the 3 LSBs in order to remain in the Shift-IR state. The MSB of the
instruction is shifted in when this state is left by setting TMS high. While the instruction is shifted in
from the TDI pin, the captured IR-state 0x01 is shifted out on the TDO pin. The JTAG Instruction
selects a particular Data Register as path between TDI and TDO and controls the circuitry
surrounding the selected Data Register.
•Apply the TMS sequence 1, 1, 0 to re-enter the Run-Test/Idle state. The instruction is latched onto
the parallel output from the Shift Register path in the Update-IR state. The Exit-IR, Pause-IR, and
Exit2-IR states are only used for navigating the state machine.
•At the TMS input, apply the sequence 1, 0, 0 at the rising edges of TCK to enter the Shift Data
Register – Shift-DR state. While in this state, upload the selected Data Register (selected by the
present JTAG instruction in the JTAG Instruction Register) from the TDI input at the rising edge of
TCK. In order to remain in the Shift-DR state, the TMS input must be held low during input of all bits
except the MSB. The MSB of the data is shifted in when this state is left by setting TMS high. While
the Data Register is shifted in from the TDI pin, the parallel inputs to the Data Register captured in
the Capture-DR state is shifted out on the TDO pin.
•Apply the TMS sequence 1, 1, 0 to re-enter the Run-Test/Idle state. If the selected Data Register
has a latched parallel-output, the latching takes place in the Update-DR state. The Exit-DR, Pause-
DR, and Exit2-DR states are only used for navigating the state machine.
As shown in the state diagram, the Run-Test/Idle state need not be entered between selecting JTAG
instruction and using Data Registers, and some JTAG instructions may select certain functions to be
performed in the Run- Test/Idle, making it unsuitable as an Idle state.
Note:  1. Independent of the initial state of the TAP Controller, the Test-Logic-Reset state can always be
entered by holding TMS high for 5 TCK clock periods.
For detailed information on the JTAG specification, refer to the literature listed in Bibliography on page
340.
29.5. Using the Boundary-scan Chain
A complete description of the Boundary-scan capabilities are given in the section IEEE 1149.1 (JTAG)
Boundary-scan on page 340.
29.6. Using the On-chip Debug System
As shown in Figure 29-1 Block Diagram on page 336, the hardware support for On-chip Debugging
consists mainly of:
•A scan chain on the interface between the internal AVR CPU and the internal peripheral units
•Break point unit
•Communication interface between the CPU and JTAG system
All read or modify/write operations needed for implementing the Debugger are done by applying AVR
instructions via the internal AVR CPU Scan Chain. The CPU sends the result to an I/O memory mapped
location which is part of the communication interface between the CPU and the JTAG system.
The Break point Unit implements Break on Change of Program Flow, Single Step Break, two Program
Memory Break points, and two combined break points. Together, the four break points can be configured
as either:
•4 Single Program Memory break points
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•3 Single Program Memory break points + 1 single Data Memory break point
•2 Single Program Memory break points + 2 single Data Memory break points
•2 Single Program Memory break points + 1 Program Memory break point with mask (“range break
point”)
•2 Single Program Memory break points + 1 Data Memory break point with mask (“range break
point”)
A debugger, like the Atmel Studio®, may however use one or more of these resources for its internal
purpose, leaving less flexibility to the end-user.
A list of the On-chip Debug specific JTAG instructions is given in On-chip Debug Specific JTAG
Instructions on page 339.
The JTAGEN fuse must be programmed to enable the JTAG Test Access Port. In addition, the OCDEN
fuse must be programmed and no Lock bits must be set for the On-chip Debug system to work. As a
security feature, the On-chip Debug system is disabled when any Lock bits are set. Otherwise, the On-
chip Debug system would have provided a back-door into a secured device.
The Atmel Studio enables the user to fully control execution of programs on an AVR device with On-chip
Debug capability, AVR In-Circuit Emulator, or the built-in AVR Instruction Set Simulator. Atmel Studio
supports source level execution of Assembly programs assembled with Atmel Corporation’s AVR
Assembler and C programs compiled with third party vendors’ compilers.
For a full description of the Atmel Studio, please refer to the Atmel Studio User Guide found in the
Online Help in Atmel Studio. Only highlights are presented in this document.
All necessary execution commands are available in Atmel Studio, both on source level and on
disassembly level. The user can execute the program, single step through the code either by tracing into
or stepping over functions, step out of functions, place the cursor on a statement and execute until the
statement is reached, stop the execution, and reset the execution target. In addition, the user can have
an unlimited number of code break points (using the BREAK instruction) and up to two data memory
break points, alternatively combined as a mask (range) break point.
29.7. On-chip Debug Specific JTAG Instructions
The On-chip debug support is considered being private JTAG instructions, and distributed within ATMEL
and to selected third-party vendors only. Instruction opcodes are listed for reference.
PRIVATE0; 0x8
Private JTAG instruction for accessing On-chip Debug system.
PRIVATE1; 0x9
Private JTAG instruction for accessing On-chip Debug system.
PRIVATE2; 0xA
Private JTAG instruction for accessing On-chip Debug system.
PRIVATE3; 0xB
Private JTAG instruction for accessing On-chip Debug system.
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29.8. Using the JTAG Programming Capabilities
Programming of AVR parts via JTAG is performed via the four-pin JTAG port, TCK, TMS, TDI, and TDO.
These are the only pins that need to be controlled/observed to perform JTAG programming (in addition to
power pins). It is not required to apply 12V externally. The JTAGEN fuse must be programmed and the
JTD bit in the MCUCSR Register must be cleared to enable the JTAG Test Access Port.
The JTAG programming capability supports:
•Flash programming and verifying
•EEPROM programming and verifying
•Fuse programming and verifying
•Lock bit programming and verifying
The Lock bit security is exactly as in Parallel Programming mode. If the Lock bits LB1 or LB2 are
programmed, the OCDEN Fuse cannot be programmed unless first doing a chip erase. This is a security
feature that ensures no back-door exists for reading out the content of a secured device.
The details on programming through the JTAG interface and programming specific JTAG instructions are
given in the section Programming Via the JTAG Interface.
Related Links
Programming Via the JTAG Interface on page 400
29.9. Bibliography
For more information about general Boundary-scan, the following literature can be consulted:
•IEEE: IEEE Std 1149.1-1990. IEEE Standard Test Access Port and Boundary-scan Architecture,
IEEE, 1993
•Colin Maunder: The Board Designers Guide to Testable Logic Circuits, Addison-Wesley, 1992
29.10. IEEE 1149.1 (JTAG) Boundary-scan
29.10.1. Features
•JTAG (IEEE std. 1149.1 Compliant) Interface
•Boundary-scan Capabilities According to the JTAG Standard
•Full Scan of all Port Functions as well as Analog Circuitry having Off-chip Connections
•Supports the Optional IDCODE Instruction
•Additional Public AVR_RESET Instruction to Reset the AVR
29.10.2. System Overview
The Boundary-scan Chain has the capability of driving and observing the logic levels on the digital I/O
pins, as well as the boundary between digital and analog logic for analog circuitry having off-chip
connections. At system level, all ICs having JTAG capabilities are connected serially by the TDI/TDO
signals to form a long Shift Register. An external controller sets up the devices to drive values at their
output pins, and observe the input values received from other devices. The controller compares the
received data with the expected result. In this way, Boundary-scan provides a mechanism for testing
interconnections and integrity of components on Printed Circuits Boards by using the four TAP signals
only.
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The four IEEE 1149.1 defined mandatory JTAG instructions IDCODE, BYPASS, SAMPLE/PRELOAD, and
EXTEST, as well as the AVR specific public JTAG instruction AVR_RESET can be used for testing the
Printed Circuit Board. Initial scanning of the data register path will show the ID-code of the device, since
IDCODE is the default JTAG instruction. It may be desirable to have the AVR device in reset during test
mode. If not reset, inputs to the device may be determined by the scan operations, and the internal
software may be in an undetermined state when exiting the test mode. Entering Reset, the outputs of any
Port Pin will instantly enter the high impedance state, making the HIGHZ instruction redundant. If needed,
the BYPASS instruction can be issued to make the shortest possible scan chain through the device. The
device can be set in the Reset state either by pulling the external RESET pin low, or issuing the
AVR_RESET instruction with appropriate setting of the Reset Data Register.
The EXTEST instruction is used for sampling external pins and loading output pins with data. The data
from the output latch will be driven out on the pins as soon as the EXTEST instruction is loaded into the
JTAG IR-register. Therefore, the SAMPLE/PRELOAD should also be used for setting initial values to the
scan ring, to avoid damaging the board when issuing the EXTEST instruction for the first time. SAMPLE/
PRELOAD can also be used for taking a snapshot of the external pins during normal operation of the
part.
The JTAGEN fuse must be programmed and the JTD bit in the I/O register MCUCSR must be cleared to
enable the JTAG Test Access Port.
When using the JTAG interface for Boundary-scan, using a JTAG TCK clock frequency higher than the
internal chip frequency is possible. The chip clock is not required to run.
29.11. Data Registers
The data registers relevant for Boundary-scan operations are:
•Bypass Register
•Device Identification Register
•Reset Register
•Boundary-scan Chain
29.11.1. Bypass Register
The Bypass Register consists of a single Shift Register stage. When the Bypass Register is selected as
path between TDI and TDO, the register is reset to 0 when leaving the Capture-DR controller state. The
Bypass Register can be used to shorten the scan chain on a system when the other devices are to be
tested.
29.11.2. Device Identification Register
The figure below shows the structure of the Device Identification Register.
Figure 29-3 The format of the Device Identification Register
Version Part Number Manufacturer ID 1
4 bits 16 bits 11 bits 1-bit
0
LSB
MSB
31 28 27 12 11 1
Bit
Device ID
29.11.2.1. Version
Version is a 4-bit number identifying the revision of the component. The JTAG version number follows the
revision of the device, and wraps around at revision P (0xF). Revision A and Q is 0x0, revision B and R is
0x1 and so on.
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29.11.2.2. Part Number
The part number is a 16-bit code identifying the component. The JTAG Part Number for ATmega64A is
listed in the table below.
Table 29-1 AVR JTAG Part Number
Part Number JTAG Part Number (Hex)
ATmega64A 0x9602
29.11.2.3. Manufacturer ID
The Manufacturer ID is a 11-bit code identifying the manufacturer. The JTAG manufacturer ID for ATMEL
is listed in the table below.
Table 29-2 Manufacturer ID
Manufacturer JTAG Manufacturer ID (Hex)
ATMEL 0x01F
29.11.3. Reset Register
The Reset Register is a Test Data Register used to reset the part. Since the AVR tri-states Port Pins
when reset, the Reset Register can also replace the function of the unimplemented optional JTAG
instruction HIGHZ.
A high value in the Reset Register corresponds to pulling the External Reset low. The part is reset as long
as there is a high value present in the Reset Register. Depending on the Fuse settings for the clock
options, the part will remain reset for a Reset Time-Out Period (refer to Clock Sources) after releasing the
Reset Register. The output from this Data Register is not latched, so the Reset will take place
immediately, as shown in the figure below.
Figure 29-4 Reset Register
D Q
From
TDI
ClockDR · AVR_RES ET
To
TDO
From Other Inte rnal a nd
Exte rnal Res e t S ource s
Interna l Res e t
Related Links
Clock Sources on page 53
29.11.4. Boundary-scan Chain
The Boundary-scan Chain has the capability of driving and observing the logic levels on the digital I/O
pins, as well as the boundary between digital and analog logic for analog circuitry having off-chip
connections. Refer to Boundary-scan Chain on page 344 for a complete description.
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29.12. Boundry-scan Specific JTAG Instructions
The Instruction Register is 4-bit wide, supporting up to 16 instructions. Listed below are the JTAG
instructions useful for Boundary-scan operation. Note that the optional HIGHZ instruction is not
implemented, but all outputs with tri-state capability can be set in high-impedant state by using the
AVR_RESET instruction, since the initial state for all port pins is tri-state.
As a definition in this data sheet, the LSB is shifted in and out first for all Shift Registers.
The OPCODE for each instruction is shown behind the instruction name in hex format. The text describes
which data register is selected as path between TDI and TDO for each instruction.
29.12.1. EXTEST; 0x0
Mandatory JTAG instruction for selecting the Boundary-scan Chain as Data Register for testing circuitry
external to the AVR package. For port-pins, Pull-up Disable, Output Control, Output Data, and Input Data
are all accessible in the scan chain. For Analog circuits having off-chip connections, the interface
between the analog and the digital logic is in the scan chain. The contents of the latched outputs of the
Boundary-scan chain is driven out as soon as the JTAG IR-register is loaded with the EXTEST
instruction.
The active states are:
•Capture-DR: Data on the external pins are sampled into the Boundary-scan Chain.
•Shift-DR: The Internal Scan Chain is shifted by the TCK input.
•Update-DR: Data from the scan chain is applied to output pins.
29.12.2. IDCODE; 0x1
Optional JTAG instruction selecting the 32-bit ID Register as Data Register. The ID Register consists of a
version number, a device number and the manufacturer code chosen by JEDEC. This is the default
instruction after power-up.
The active states are:
•Capture-DR: Data in the IDCODE Register is sampled into the Boundary-scan Chain.
•Shift-DR: The IDCODE scan chain is shifted by the TCK input.
29.12.3. SAMPLE_PRELOAD; 0x2
Mandatory JTAG instruction for pre-loading the output latches and taking a snap-shot of the input/output
pins without affecting the system operation. However, the output latches are not connected to the pins.
The Boundary-scan Chain is selected as Data Register.
The active states are:
•Capture-DR: Data on the external pins are sampled into the Boundary-scan Chain.
•Shift-DR: The Boundary-scan Chain is shifted by the TCK input.
•Update-DR: Data from the Boundary-scan Chain is applied to the output latches. However, the
output latches are not connected to the pins.
29.12.4. AVR_RESET; 0xC
The AVR specific public JTAG instruction for forcing the AVR device into the Reset mode or releasing the
JTAG Reset source. The TAP controller is not reset by this instruction. The one bit Reset Register is
selected as Data Register. Note that the Reset will be active as long as there is a logic 'one' in the Reset
Chain. The output from this chain is not latched.
The active states are:
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•Shift-DR: The Reset Register is shifted by the TCK input.
29.12.5. BYPASS; 0xF
Mandatory JTAG instruction selecting the Bypass Register for Data Register.
The active states are:
•Capture-DR: Loads a logic “0” into the Bypass Register.
•Shift-DR: The Bypass Register cell between TDI and TDO is shifted.
29.13. Boundary-scan Chain
The Boundary-scan chain has the capability of driving and observing the logic levels on the digital I/O
pins, as well as the boundary between digital and analog logic for analog circuitry having off-chip
connections.
29.13.1. Scanning the Digital Port Pins
The first figure below shows the Boundary-scan Cell for a bi-directional port pin with pull-up function. The
cell consists of a standard Boundary-scan cell for the Pull-up Enable – PUExn – function, and a bi-
directional pin cell that combines the three signals, Output Control – OCxn, Output Data – ODxn, and
Input Data – IDxn, into only a two-stage Shift Register. The port and pin indexes are not used in the
following description
The Boundary-scan logic is not included in the figures in the Data Sheet. Figure 29-6 General Port Pin
Schematic diagram on page 346 shows a simple digital Port Pin as described in the section I/O Ports.
The Boundary-scan details from the first figure below replaces the dashed box in Figure 29-6 General
Port Pin Schematic diagram on page 346.
When no alternate port function is present, the Input Data – ID corresponds to the PINxn Register value
(but ID has no synchronizer), Output Data corresponds to the PORT Register, Output Control
corresponds to the Data Direction – DD Register, and the Pull-up Enable – PUExn – corresponds to logic
expression PUD · DDxn · PORTxn.
Digital alternate port functions are connected outside the dotted box in Figure 29-6 General Port Pin
Schematic diagram on page 346 to make the scan chain read the actual pin value. For Analog function,
there is a direct connection from the external pin to the analog circuit, and a scan chain is inserted on the
interface between the digital logic and the analog circuitry.
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SmflDR Ta Km can EXTEST Vcc Pnllup Enable mm no DQ OurpulCammHOCV D Q G Oulpm Dam mm iv ©3313 um. um um Atmel
Figure 29-5 Boundary-scan Cell for Bi-directional Port Pin with Pull-Up Function.
D Q D Q
G
0
1
0
1
D Q D Q
G
0
1
0
1
0
1
0
1
D Q D Q
G
0
1
Port P in (PXn)
VccEXTES TTo Next Ce llShiftDR
Output Control (OC)
Pullup Ena ble (P UE)
Output Da ta (OD)
Input Da ta (ID)
From Las t Ce ll Upda te DRClockDR
FF2 LD2
FF1 LD1
LD0FF0
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Figure 29-6 General Port Pin Schematic diagram
CLK
RPx
RRx
WPx
RDx
WDx
PUD
SYNCHRONIZER
WDx: WRITE DDRx
WPx: WRITE PORTx
RRx: READ P ORTx REGISTER
RPx: READ P ORTx P IN
PUD: P ULLUP DISABLE
CLK : I/O CLOCK
RDx: READ DDRx
D
L
Q
Q
RES ET
RES ET
Q
Q
D
Q
QD
CLR
PORTxn
Q
QD
CLR
DDxn
PINxn
DATA BUS
SLEEP
SLEEP : SLEEP CONTROL
Pxn
I/O
I/O
Se e Boundary-Sca n de s cription
for de ta ils!
PUExn
OCxn
ODxn
IDxn
PUExn: P ULLUP ENABLE for pin Pxn
OCxn: OUTP UT CONTROL for pin Pxn
ODxn: OUTP UT DATA to pin Pxn
IDxn: INPUT DATA from pin Pxn
Related Links
I/O Ports on page 92
29.13.2. Boundary-scan and the Two-wire Interface
The two Two-wire Interface pins SCL and SDA have one additional control signal in the scan-chain; Two-
wire Interface Enable – TWIEN. As shown in the figure below, the TWIEN signal enables a tri-state buffer
with slew-rate control in parallel with the ordinary digital port pins. A general scan cell as shown in Figure
29-11 General Boundary-scan Cell used for Signals for Comparator and ADC on page 349 is attached to
the TWIEN signal.
Note: 
1. A separate scan chain for the 50ns spike filter on the input is not provided. The ordinary scan
support for digital port pins suffice for connectivity tests. The only reason for having TWIEN in the
scan path, is to be able to disconnect the slew-rate control buffer when doing boundary-scan.
2. Make sure the OC and TWIEN signals are not asserted simultaneously, as this will lead to drive
contention.
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Atmel ZIP—<1—>—‘ I ;' Zr
Figure 29-7 Additional Scan Signal for the Two-wire Interface
PUExn
OCxn
ODxn
TWIEN
IDxn
Slew-rate limited
SRC
Pxn
29.13.3. Scanning the RESET Pin
The RESET pin accepts 5V active low logic for standard Reset operation, and 12V active high logic for
High Voltage Parallel programming. An observe-only cell as shown in the figure below is inserted both for
the 5V Reset signal; RSTT, and the 12V Reset signal; RSTHV.
Figure 29-8 Observe-only Cell
0
1
D Q
From
pre vious
ce ll
ClockDR
ShiftDR
To
ne xt
ce ll
From s ys te m pin To s ys te m logic
FF1
29.13.4. Scanning the Clock Pins
The AVR devices have many clock options selectable by fuses. These are: Internal RC Oscillator,
External RC, External Clock, (High Frequency) Crystal Oscillator, Low-frequency Crystal Oscillator, and
Ceramic Resonator.
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The figure below shows how each Oscillator with external connection is supported in the scan chain. The
Enable signal is supported with a general boundary-scan cell, while the Oscillator/Clock output is
attached to an observe-only cell. In addition to the main clock, the Timer Oscillator is scanned in the same
way. The output from the internal RC Oscillator is not scanned, as this Oscillator does not have external
connections.
Figure 29-9 Boundary-scan Cells for Oscillators and Clock Options
0
1
D Q
From
Previous
Cell
ClockDR
ShiftDR
To
next
cell
To S ys tem Logic
FF1
0
1
D Q D Q
G
0
1
From
Previous
Cell
ClockDR UpdateDR
ShiftDR
To
Next
Cell EXTEST
From Digital Logic
XTAL1/TOSC1 XTAL2/TOS C2
Oscilla tor
ENABLE OUTP UT
The following table summaries the scan registers for the external clock pin XTAL1, oscillators with XTAL1/
XTAL2 connections as well as 32kHz Timer Oscillator.
Table 29-3 Scan Signals for the Oscillators(1)(2)(3)
Enable signal Scanned Clock Line Clock Option Scanned Clock Line when not
Used
EXTCLKEN EXTCLK (XTAL1) External Clock 0
OSCON OSCCK External Crystal
External Ceramic Resonator
0
RCOSCEN RCCK External RC 1
OSC32EN OSC32CK Low Freq. External Crystal 0
TOSKON TOSCK 32kHz Timer Oscillator 0
Note: 
1. Do not enable more than one clock source as main clock at a time.
2. Scanning an Oscillator output gives unpredictable results as there is a frequency drift between the
Internal Oscillator and the JTAG TCK clock. If possible, scanning an external clock is preferred.
3. The clock configuration is programmed by fuses. As a fuse does not change run-time, the clock
configuration is considered fixed for a given application. The user is advised to scan the same clock
option as to be used in the final system. The enable signals are supported in the scan chain
because the system logic can disable clock options in sleep modes, thereby disconnecting the
Oscillator pins from the scan path if not provided. The INTCAP fuses are not supported in the scan-
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Atmel AINo AIN1 XE vcc ACD 4F fl To Next Cell pdme
chain, so the boundary scan chain can not make a XTAL Oscillator requiring internal capacitors to
run unless the fuse is correctly programmed.
29.13.5. Scanning the Analog Comparator
The relevant Comparator signals regarding Boundary-scan are shown in the first figure below. The
Boundary-scan cell from the second figure below is attached to each of these signals. The signals are
described in Table 29-4 Boundary-scan Signals for the Analog Comparator on page 350.
The Comparator need not be used for pure connectivity testing, since all analog inputs are shared with a
digital port pin as well.
Figure 29-10 Analog comparator
ACBG
BANDGAP
REFERENCE
ADC MULTIPLEXER
OUTP UT
ACME
AC_IDLE
ACO
ADCEN
Figure 29-11 General Boundary-scan Cell used for Signals for Comparator and ADC
0
1
D Q D Q
G
0
1
From
Previous
Ce ll
ClockDR Upda te DR
ShiftDR
To
Next
Ce ll EXTES T
To Ana log Circuitry/
To Digital Logic
From Digital Logic/
From Analog Ciruitry
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Table 29-4 Boundary-scan Signals for the Analog Comparator
Signal
Name
Direction as
Seen from the
Comparator
Description Recommended Input
when not in Use
Output values when
Recommended Inputs
are Used
AC_IDLE Input Turns off Analog
comparator when
true
1 Depends upon μC code
being executed
ACO Output Analog Comparator
Output
Will become input to
μC code being
executed
0
ACME Input Uses output signal
from ADC mux
when true
0 Depends upon μC code
being executed
ACBG Input Bandgap Reference
enable
0 Depends upon μC code
being executed
29.13.6. Scanning the ADC
The figure below shows a block diagram of the ADC with all relevant control and observe signals. The
Boundary-scan cell from Figure 29-8 Observe-only Cell on page 347 is attached to each of these signals.
The ADC need not be used for pure connectivity testing, since all analog inputs are shared with a digital
port pin as well.
Figure 29-12 Analog to Digital Converter
10-bit DAC +
-
AREF
PRECH
DACO UT
COMP
MUXEN_7
ADC_7
MUXEN_6
ADC_6
MUXEN_5
ADC_5
MUXEN_4
ADC_4
MUXEN_3
ADC_3
MUXEN_2
ADC_2
MUXEN_1
ADC_1
MUXEN_0
ADC_0
NEGS EL_2
ADC_2
NEGS EL_1
ADC_1
NEGS EL_0
ADC_0
EXTCH
+
-
+
-
10x 20x
G10 G20
ST
ACLK
AMPEN
2.56V
ref
IREFEN
AREF
VCCREN
DAC_9..0
ADCEN
HOLD
PRECH
GNDEN
PAS S EN
ACTEN
COMP
SCTEST ADCBGEN
To Compa rator
1.22V
ref AREF
The signals are described briefly in the following table.
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Table 29-5 Boundary-scan Signals for the ADC
Signal Name Direction as
Seen from
the ADC
Description Recommend
ed Input
when not in
Use
Output
Values when
Recommend
ed Inputs
are Used,
and CPU is
not Using
the ADC
COMP Output Comparator Output 0 0
ACLK Input Clock signal to gain stages implemented as
Switch-cap filters
0 0
ACTEN Input Enable path from gain stages to the
comparator
0 0
ADCBGEN Input Enable Band-gap reference as negative
input to comparator
0 0
ADCEN Input Power-on signal to the ADC 0 0
AMPEN Input Power-on signal to the gain stages 0 0
DAC_9 Input Bit 9 of digital value to DAC 1 1
DAC_8 Input Bit 8 of digital value to DAC 0 0
DAC_7 Input Bit 7 of digital value to DAC 0 0
DAC_6 Input Bit 6 of digital value to DAC 0 0
DAC_5 Input Bit 5 of digital value to DAC 0 0
DAC_4 Input Bit 4 of digital value to DAC 0 0
DAC_3 Input Bit 3 of digital value to DAC 0 0
DAC_2 Input Bit 2 of digital value to DAC 0 0
DAC_1 Input Bit 1 of digital value to DAC 0 0
DAC_0 Input Bit 0 of digital value to DAC 0 0
EXTCH Input Connect ADC channels 0 - 3 to by-pass
path around gain stages
1 1
G10 Input Enable 10x gain 0 0
G20 Input Enable 20x gain 0 0
GNDEN Input Ground the negative input to comparator
when true
0 0
HOLD Input Sample & Hold signal. Sample analog
signal when low. Hold signal when high. If
gain stages are used, this signal must go
active when ACLK is high.
1 1
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Signal Name Direction as
Seen from
the ADC
Description Recommend
ed Input
when not in
Use
Output
Values when
Recommend
ed Inputs
are Used,
and CPU is
not Using
the ADC
IREFEN Input Enables Band-gap reference as AREF
signal to DAC
0 0
MUXEN_7 Input Input Mux bit 7 0 0
MUXEN_6 Input Input Mux bit 6 0 0
MUXEN_5 Input Input Mux bit 5 0 0
MUXEN_4 Input Input Mux bit 4 0 0
MUXEN_3 Input Input Mux bit 3 0 0
MUXEN_2 Input Input Mux bit 2 0 0
MUXEN_1 Input Input Mux bit 1 0 0
MUXEN_0 Input Input Mux bit 0 1 1
NEGSEL_2 Input Input Mux for negative input for differential
signal, bit 2
0 0
NEGSEL_1 Input Input Mux for negative input for differential
signal, bit 1
0 0
NEGSEL_0 Input Input Mux for negative input for differential
signal, bit 0
0 0
PASSEN Input Enable pass-gate of gain stages. 1 1
PRECH Input Precharge output latch of comparator.
(Active low)
1 1
SCTEST Input Switch-cap TEST enable. Output from x10
gain stage send out to Port Pin having
ADC_4
0 0
ST Input Output of gain stages will settle faster if this
signal is high first two ACLK periods after
AMPEN goes high.
0 0
VCCREN Input Selects Vcc as the ACC reference voltage. 0 0
Note:  1. Incorrect setting of the switches in Figure 29-12 Analog to Digital Converter on page 350 will
make signal contention and may damage the part. There are several input choices to the S&H circuitry on
the negative input of the output comparator in Figure 29-12 Analog to Digital Converter on page 350.
Make sure only one path is selected from either one ADC pin, Bandgap reference source, or Ground.
If the ADC is not to be used during scan, the recommended input values from the table above should be
used. The user is recommended not to use the Differential Gain stages during scan. Switch-Cap based
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gain stages require fast operation and accurate timing which is difficult to obtain when used in a scan
chain. Details concerning operations of the differential gain stage is therefore not provided.
The AVR ADC is based on the analog circuitry shown in Figure 29-12 Analog to Digital Converter on
page 350 with a successive approximation algorithm implemented in the digital logic. When used in
Boundary-scan, the problem is usually to ensure that an applied analog voltage is measured within some
limits. This can easily be done without running a successive approximation algorithm: apply the lower limit
on the digital DAC[9:0] lines, make sure the output from the comparator is low, then apply the upper limit
on the digital DAC[9:0] lines, and verify the output from the comparator to be high.
The ADC need not be used for pure connectivity testing, since all analog inputs are shared with a digital
port pin as well.
When using the ADC, remember the following:
•The Port Pin for the ADC channel in use must be configured to be an input with pull-up disabled to
avoid signal contention.
•In normal mode, a dummy conversion (consisting of 10 comparisons) is performed when enabling
the ADC. The user is advised to wait at least 200ns after enabling the ADC before controlling/
observing any ADC signal, or perform a dummy conversion before using the first result.
•The DAC values must be stable at the midpoint value 0x200 when having the HOLD signal low
(Sample mode).
As an example, consider the task of verifying a 1.5V ±5% input signal at ADC channel 3 when the power
supply is 5.0V and AREF is externally connected to VCC.
The lower limit is: 1024 ⋅ 1,5V ⋅ 0,95 ⁄ 5V = 291 = 0x123
The upper limit is: 1024 ⋅ 1,5V ⋅ 1,05 ⁄ 5V = 323 = 0x143
The recommended values from Table 29-5 Boundary-scan Signals for the ADC on page 351 are used
unless other values are given in the algorithm in the following table. Only the DAC and Port Pin values of
the Scan Chain are shown. The column “Actions” describes what JTAG instruction to be used before
filling the Boundary-scan Register with the succeeding columns. The verification should be done on the
data scanned out when scanning in the data on the same row in the table.
Table 29-6 Algorithm for Using the ADC
Step Actions ADCEN DAC MUXEN HOLD PRECH PA3.
Data
PA3.
Control
PA3.
Pullup_
Enable
1 SAMPLE_P
RELOAD
1 0x200 0x08 1 1 0 0 0
2 EXTEST 1 0x200 0x08 0 1 0 0 0
3 1 0x200 0x08 1 1 0 0 0
4 1 0x123 0x08 1 1 0 0 0
5 1 0x123 0x08 1 0 0 0 0
6 Verify the
COMP bit
scanned out to
be 0
1 0x200 0x08 1 1 0 0 0
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Step Actions ADCEN DAC MUXEN HOLD PRECH PA3.
Data
PA3.
Control
PA3.
Pullup_
Enable
7 1 0x200 0x08 0 1 0 0 0
8 1 0x200 0x08 1 1 0 0 0
9 1 0x143 0x08 1 1 0 0 0
10 1 0x143 0x08 1 0 0 0 0
11 Verify the
COMP bit
scanned out to
be 1
1 0x200 0x08 1 1 0 0 0
Using this algorithm, the timing constraint on the HOLD signal constrains the TCK clock frequency. As the
algorithm keeps HOLD high for five steps, the TCK clock frequency has to be at least five times the
number of scan bits divided by the maximum hold time, thold,max
29.14. ATmega64A Boundary-scan Order
The table below shows the Scan order between TDI and TDO when the Boundary-scan Chain is selected
as data path. Bit 0 is the LSB; the first bit scanned in, and the first bit scanned out. The scan order follows
the pin-out order as far as possible. Therefore, the bits of Port A are scanned in the opposite bit order of
the other ports.
Exceptions from the rules are the scan chains for the analog circuits, which constitute the most significant
bits of the scan chain regardless of which physical pin they are connected to. In Figure 29-5 Boundary-
scan Cell for Bi-directional Port Pin with Pull-Up Function. on page 345, PXn. Data corresponds to FF0,
PXn. Control corresponds to FF1, and PXn. Pullup_enable corresponds to FF2. Bit 2, 3, 4, and 5 of Port
C is not in the scan chain, since these pins constitute the TAP pins when the JTAG is enabled.
Table 29-7 ATmega64A Boundary-scan Order
Bit Number Signal Name Module
204 AC_IDLE Comparator
203 ACO
202 ACME
201 AINBG
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Bit Number Signal Name Module
200 COMP ADC
199 PRIVATE_SIGNAL1(1)
198 ACLK
197 ACTEN
196 PRIVATE_SIGNAL1(2)
195 ADCBGEN
194 ADCEN
193 AMPEN
192 DAC_9
191 DAC_8
190 DAC_7
189 DAC_6
188 DAC_5
187 DAC_4
186 DAC_3
185 DAC_2
184 DAC_1
183 DAC_0
182 EXTCH
181 G10
180 G20
179 GNDEN
178 HOLD
177 IREFEN
176 MUXEN_7
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Bit Number Signal Name Module
175 MUXEN_6 ADC
174 MUXEN_5
173 MUXEN_4
172 MUXEN_3
171 MUXEN_2
170 MUXEN_1
169 MUXEN_0
168 NEGSEL_2
167 NEGSEL_1
166 NEGSEL_0
165 PASSEN
164 PRECH
163 SCTEST
162 ST
161 VCCREN
160 PEN Programming enable (observe only)
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Bit Number Signal Name Module
159 PE0.Data Port E
158 PE0.Control
157 PE0.Pullup_Enable
156 PE1.Data
155 PE1.Control
154 PE1.Pullup_Enable
153 PE2.Data
152 PE2.Control
151 PE2.Pullup_Enable
150 PE3.Data
149 PE3.Control
148 PE3.Pullup_Enable
147 PE4.Data
146 PE4.Control
145 PE4.Pullup_Enable
144 PE5.Data
143 PE5.Control
142 PE5.Pullup_Enable
141 PE6.Data
140 PE6.Control
139 PE6.Pullup_Enable Port E
138 PE7.Data
137 PE7.Control
136 PE7.Pullup_Enable
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Bit Number Signal Name Module
135 PB0.Data Port B
134 PB0.Control
133 PB0.Pullup_Enable
132 PB1.Data
131 PB1.Control
130 PB1.Pullup_Enable
129 PB2.Data
128 PB2.Control
127 PB2.Pullup_Enable
126 PB3.Data
125 PB3.Control
124 PB3.Pullup_Enable
123 PB4.Data
122 PB4.Control
121 PB4.Pullup_Enable
120 PB5.Data
119 PB5.Control
118 PB5.Pullup_Enable
117 PB6.Data
116 PB6.Control
115 PB6.Pullup_Enable
114 PB7.Data
113 PB7.Control
112 PB7.Pullup_Enable
111 PG3.Data Port G
110 PG3.Control
109 PG3.Pullup_Enable
108 PG4.Data
107 PG4.Control
106 PG4.Pullup_Enable
105 TOSC 32kHz Timer Oscillator
104 TOSCON
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Bit Number Signal Name Module
103 RSTT Reset Logic
(Observe-only)
102 RSTHV
101 EXTCLKEN Enable signals for main Clock/Oscillators
100 OSCON
99 RCOSCEN
98 OSC32EN
97 EXTCLK (XTAL1) Clock input and Oscillators for the main clock
(Observe-only)
96 OSCCK
95 RCCK
94 OSC32CK
93 TWIEN TWI
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Bit Number Signal Name Module
92 PD0.Data Port D
91 PD0.Control
90 PD0.Pullup_Enable
89 PD1.Data
88 PD1.Control
87 PD1.Pullup_Enable
86 PD2.Data
85 PD2.Control
84 PD2.Pullup_Enable
83 PD3.Data
82 PD3.Control
81 PD3.Pullup_Enable
80 PD4.Data
79 PD4.Control
78 PD4.Pullup_Enable
77 PD5.Data
76 PD5.Control
75 PD5.Pullup_Enable
74 PD6.Data
73 PD6.Control
72 PD6.Pullup_Enable
71 PD7.Data
70 PD7.Control
69 PD7.Pullup_Enable
68 PG0.Data Port G
67 PG0.Control Port G
66 PG0.Pullup_Enable
65 PG1.Data
64 PG1.Control
63 PG1.Pullup_Enable
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Bit Number Signal Name Module
62 PC0.Data Port C
61 PC0.Control
60 PC0.Pullup_Enable
59 PC1.Data
58 PC1.Control
57 PC1.Pullup_Enable
56 PC2.Data
55 PC2.Control
54 PC2.Pullup_Enable
53 PC3.Data
52 PC3.Control
51 PC3.Pullup_Enable
50 PC4.Data
49 PC4.Control
48 PC4.Pullup_Enable
47 PC5.Data
46 PC5.Control
45 PC5.Pullup_Enable
44 PC6.Data
43 PC6.Control
42 PC6.Pullup_Enable
41 PC7.Data
40 PC7.Control
39 PC7.Pullup_Enable
38 PG2.Data Port G
37 PG2.Control
36 PG2.Pullup_Enable
35 PA7.Data Port A
34 PA7.Control
33 PA7.Pullup_Enable
32 PA6.Data
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Bit Number Signal Name Module
31 PA6.Control Port A
30 PA6.Pullup_Enable
29 PA5.Data
28 PA5.Control
27 PA5.Pullup_Enable
26 PA4.Data
25 PA4.Control
24 PA4.Pullup_Enable
23 PA3.Data
22 PA3.Control
21 PA3.Pullup_Enable
20 PA2.Data
19 PA2.Control
18 PA2.Pullup_Enable
17 PA1.Data
16 PA1.Control
15 PA1.Pullup_Enable
14 PA0.Data
13 PA0.Control
12 PA0.Pullup_Enable
11 PF3.Data Port F
10 PF3.Control
9 PF3.Pullup_Enable
8 PF2.Data
7 PF2.Control
6 PF2.Pullup_Enable
5 PF1.Data
4 PF1.Control
3 PF1.Pullup_Enable
2 PF0.Data
1 PF0.Control
0 PF0.Pullup_Enable
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Note: 
1. PRIVATE_SIGNAL1 should always scanned in as zero.
2. PRIVATE_SIGNAL2 should always scanned in as zero.
29.15. Boundary-scan Description Language Files
Boundary-scan Description Language (BSDL) files describe Boundary-scan capable devices in a
standard format used by automated test-generation software. The order and function of bits in the
Boundary-scan Data Register are included in this description.
29.16. Register Description
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29.16.1. OCDR – On-chip Debug Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
Name:  OCDR
Offset:  0x22
Reset:  0x20
Property:
 
When addressing I/O Registers as data space the offset address is 0x42
Bit 7 6 5 4 3 2 1 0
IDRD/OCDR7 OCDR6 OCDR5 OCDR4 OCDR3 OCDR2 OCDR1 OCDR0
Access R/W R/W R/W R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0 0
Bit 7 – IDRD/OCDR7: USART Receive Complete
The OCDR Register provides a communication channel from the running program in the microcontroller
to the debugger. The CPU can transfer a byte to the debugger by writing to this location. At the same
time, an internal flag; I/O Debug Register Dirty – IDRD – is set to indicate to the debugger that the
register has been written. When the CPU reads the OCDR Register the 7 LSB will be from the OCDR
Register, while the MSB is the IDRD bit. The debugger clears the IDRD bit when it has read the
information.
In some AVR devices, this register is shared with a standard I/O location. In this case, the OCDR Register
can only be accessed if the OCDEN fuse is programmed, and the debugger enables access to the OCDR
Register. In all other cases, the standard I/O location is accessed.
•Bit 7 is MSB
•Bit 1 is LSB
Refer to the debugger documentation for further information on how to use this register.
Bits 6:0 – OCDRn: On-chip Debug Register n [n = 6:0]
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29.16.2. MCUCSR – MCU Control and Status Register
When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When
addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset
addresses. The device is a complex microcontroller with more peripheral units than can be supported
within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space
from 0x60 in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
The MCU Control and Status Register contains control bits for general MCU functions, and provides
information on which reset source caused an MCU Reset.
Name:  MCUCSR
Offset:  0x34
Reset:  0x20
Property:
 
When addressing I/O Registers as data space the offset address is 0x54
Bit 7 6 5 4 3 2 1 0
JTD JTRF
Access R/W R/W
Reset 0 0
Bit 7 – JTD: JTAG Interface Disable
When this bit is zero, the JTAG interface is enabled if the JTAGEN fuse is programmed. If this bit is one,
the JTAG interface is disabled. In order to avoid unintentional disabling or enabling of the JTAG interface,
a timed sequence must be followed when changing this bit: The application software must write this bit to
the desired value twice within four cycles to change its value.
If the JTAG interface is left unconnected to other JTAG circuitry, the JTD bit should be set to one. The
reason for this is to avoid static current at the TDO pin in the JTAG interface.
Bit 4 – JTRF: JTAG Reset Flag
This bit is set if a Reset is being caused by a logic one in the JTAG Reset Register selected by the JTAG
instruction AVR_RESET. This bit is reset by a Power-on Reset, or by writing a logic zero to the flag.
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30. BTLDR - Boot Loader Support – Read-While-Write Self-Programming
30.1. Features
•Read-While-Write Self-Programming
•Flexible Boot Memory Size
•High Security (Separate Boot Lock Bits for a Flexible Protection)
•Separate Fuse to Select Reset Vector
•Optimized Page(1) Size
•Code Efficient Algorithm
•Efficient Read-Modify-Write Support
Note:  1. A page is a section in the Flash consisting of several bytes (Refer to table Number of Words in
a Page and number of Pages in the Flash in Signal Names) used during programming. The page
organization does not affect normal operation.
Related Links
Signal Names on page 394
30.2. Overview
In this device, the Boot Loader Support provides a real Read-While-Write Self-Programming mechanism
for downloading and uploading program code by the MCU itself. This feature allows flexible application
software updates controlled by the MCU using a Flash-resident Boot Loader program. The Boot Loader
program can use any available data interface and associated protocol to read code and write (program)
that code into the Flash memory, or read the code from the program memory. The program code within
the Boot Loader section has the capability to write into the entire Flash, including the Boot Loader
memory. The Boot Loader can thus even modify itself, and it can also erase itself from the code if the
feature is not needed anymore. The size of the Boot Loader memory is configurable with fuses and the
Boot Loader has two separate sets of Boot Lock bits which can be set independently. This gives the user
a unique flexibility to select different levels of protection.
30.3. Application and Boot Loader Flash Sections
The Flash memory is organized in two main sections, the Application section and the Boot Loader
section. The size of the different sections is configured by the BOOTSZ Fuses. These two sections can
have different level of protection since they have different sets of Lock bits.
30.3.1. Application Section
The Application section is the section of the Flash that is used for storing the application code. The
protection level for the Application section can be selected by the application Boot Lock bits (Boot Lock
bits 0). The Application section can never store any Boot Loader code since the SPM instruction is
disabled when executed from the Application section.
30.3.2. BLS – Boot Loader Section
While the Application section is used for storing the application code, the Boot Loader software must be
located in the BLS since the SPM instruction can initiate a programming when executing from the BLS
only. The SPM instruction can access the entire Flash, including the BLS itself. The protection level for
the Boot Loader section can be selected by the Boot Loader Lock bits (Boot Lock bits 1).
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30.4. Read-While-Write and No Read-While-Write Flash Sections
Whether the CPU supports Read-While-Write or if the CPU is halted during a Boot Loader software
update is dependent on which address that is being programmed. In addition to the two sections that are
configurable by the BOOTSZ Fuses as described above, the Flash is also divided into two fixed sections,
the Read-While-Write (RWW) section and the No Read-While-Write (NRWW) section. The limit between
the RWW- and NRWW sections is given in the Boot Loader Parameters section and Figure 30-2 Memory
Sections on page 369. The main difference between the two sections is:
•When erasing or writing a page located inside the RWW section, the NRWW section can be read
during the operation
•When erasing or writing a page located inside the NRWW section, the CPU is halted during the
entire operation
The user software can never read any code that is located inside the RWW section during a Boot Loader
software operation. The syntax “Read-While-Write section” refers to which section that is being
programmed (erased or written), not which section that actually is being read during a Boot Loader
software update.
Related Links
ATmega64A Boot Loader Parameters on page 379
30.4.1. RWW – Read-While-Write Section
If a Boot Loader software update is programming a page inside the RWW section, it is possible to read
code from the Flash, but only code that is located in the NRWW section. During an on-going
programming, the software must ensure that the RWW section never is being read. If the user software is
trying to read code that is located inside the RWW section (i.e. by a call/jmp/lpm or an interrupt) during
programming, the software might end up in an unknown state. To avoid this, the interrupts should either
be disabled or moved to the Boot Loader section. The Boot Loader section is always located in the
NRWW section. The RWW Section Busy bit (RWWSB) in the Store Program Memory Control Register
(SPMCSR) will be read as logical one as long as the RWW section is blocked for reading. After a
programming is completed, the RWWSB must be cleared by software before reading code located in the
RWW section. Please refer to SPMCSR on page 381 in this chapter for details on how to clear RWWSB.
30.4.2. NRWW – No Read-While-Write Section
The code located in the NRWW section can be read when the Boot Loader software is updating a page in
the RWW section. When the Boot Loader code updates the NRWW section, the CPU is halted during the
entire Page Erase or Page Write operation.
Table 30-1 Read-While-Write Features
Which Section does the Z-
pointer Address during the
Programming?
Which Section can be read
during Programming?
CPU Halted? Read-While-Write
Supported?
RWW Section NRWW Section No Yes
NRWW Section None Yes No
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Figure 30-1 Read-While-Write vs. No Read-While-Write
Read-While-Write
(RWW) Section
No Read-While-Write
(NRWW) Section
Z-pointer
Addresses RWW
Section
Z-pointer
Addresses NRWW
Section
CPU is Halted
During the Operation
Code Located in
NRWW Section
Can be Read During
the Operation
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ngnm Memory ngnm Vlemory 5.33 2:35.2363 5:3 2:32:35: 2 5.3,“ 2:33:263 =23,“ 2.3.3.5363: uz
Figure 30-2 Memory Sections
0x0000
Flashend
Program Memory
BOOTSZ = '11'
Application Flash Section
Boot Loader Flash Section
Flashend
Program Memory
BOOTSZ = '10'
0x0000
Program Memory
BOOTSZ = '01'
Program Memory
BOOTSZ = '00'
Application Flash Section
Boot Loader Flash Section
0x0000
Flashend
Application Flash Section
Flashend
End RWW
Start NRWW
Application Flash Section
Boot Loader Flash Section
Boot Loader Flash Section
End RWW
Start NRWW
End RWW
Start NRWW
0x0000
End RWW, End Application
Start NRWW, Start Boot Loader
Application Flash SectionApplication Flash Section
Application Flash Section
Read-While-Write SectionNo Read-While-Write Section Read-While-Write SectionNo Read-While-Write Section
Read-While-Write SectionNo Read-While-Write SectionRead-While-Write SectionNo Read-While-Write Section
End Application
Start Boot Loader
End Application
Start Boot Loader
End Application
Start Boot Loader
30.5. Boot Loader Lock Bits
If no Boot Loader capability is needed, the entire Flash is available for application code. The Boot Loader
has two separate sets of Boot Lock bits which can be set independently. This gives the user a unique
flexibility to select different levels of protection.
The user can select:
•To protect the entire Flash from a software update by the MCU
•To protect only the Boot Loader Flash section from a software update by the MCU
•To protect only the Application Flash section from a software update by the MCU
•Allow software update in the entire Flash
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See tables below for further details. The Boot Lock bits can be set in software and in Serial or Parallel
Programming mode, but they can be cleared by a Chip Erase command only. The general Write Lock
(Lock Bit mode 2) does not control the programming of the Flash memory by SPM instruction. Similarly,
the general Read/Write Lock (Lock Bit mode 3) does not control reading nor writing by LPM/SPM, if it is
attempted.
Table 30-2 Boot Lock Bit0 Protection Modes (Application Section)(1)
BLB0
Mode
BLB02 BLB01 Protection
1 1 1 No restrictions for SPM or LPM accessing the Application section.
2 1 0 SPM is not allowed to write to the Application section.
3 0 0 SPM is not allowed to write to the Application section, and LPM executing
from the Boot Loader section is not allowed to read from the Application
section. If Interrupt Vectors are placed in the Boot Loader section,
interrupts are disabled while executing from the Application section.
4 0 1 LPM executing from the Boot Loader section is not allowed to read from
the Application section. If Interrupt Vectors are placed in the Boot Loader
section, interrupts are disabled while executing from the Application
section.
Note:  1. “1” means unprogrammed, “0” means programmed.
Table 30-3 Boot Lock Bit1 Protection Modes (Boot Loader Section)(1)
BLB1
Mode
BLB12 BLB11 Protection
1 1 1 No restrictions for SPM or LPM accessing the Boot Loader section.
2 1 0 SPM is not allowed to write to the Boot Loader section.
3 0 0 SPM is not allowed to write to the Boot Loader section, and LPM executing
from the Application section is not allowed to read from the Boot Loader
section. If Interrupt Vectors are placed in the Application section, interrupts
are disabled while executing from the Boot Loader section.
4 0 1 LPM executing from the Application section is not allowed to read from the
Boot Loader section. If Interrupt Vectors are placed in the Application
section, interrupts are disabled while executing from the Boot Loader
section.
Note:  1. “1” means unprogrammed, “0” means programmed.
30.6. Entering the Boot Loader Program
Entering the Boot Loader takes place by a jump or call from the application program. This may be initiated
by a trigger such as a command received via USART, or SPI interface. Alternatively, the Boot Reset Fuse
can be programmed so that the Reset Vector is pointing to the Boot Flash start address after a reset. In
this case, the Boot Loader is started after a reset. After the application code is loaded, the program can
start executing the application code. The fuses cannot be changed by the MCU itself. This means that
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Bu ZH (R31) ZH (R31) B|1 Atmel 15 14 13 12 11 10 9 8 215 214 Z13 212 211 210 2.9 28 27 ZG Z5 24 23 22 11 0 7 G 5 4 3 2 1 0
once the Boot Reset Fuse is programmed, the Reset Vector will always point to the Boot Loader Reset
and the fuse can only be changed through the serial or parallel programming interface.
Table 30-4 Boot Reset Fuse(1)
BOOTRST Reset Address
1 Reset Vector = Application Reset (address 0x0000)
0 Reset Vector = Boot Loader Reset, as described in the Boot Loader Parameters
Note:  1. '1' means unprogrammed, '0' means programmed.
30.7. Addressing the Flash During Self-Programming
The Z-pointer is used to address the SPM commands.
Since the Flash is organized in pages, the Program Counter can be treated as having two different
sections. One section, consisting of the least significant bits, is addressing the words within a page, while
the most significant bits are addressing the pages. This is shown in the following figure. The Page Erase
and Page Write operations are addressed independently. Therefore it is of major importance that the Boot
Loader software addresses the same page in both the Page Erase and Page Write operation. Once a
programming operation is initiated, the address is latched and the Z-pointer can be used for other
operations.
The only SPM operation that does not use the Z-pointer is Setting the Boot Loader Lock bits. The content
of the Z-pointer is ignored and will have no effect on the operation. The LPM instruction does also use the
Z-pointer to store the address. Since this instruction addresses the Flash byte-by-byte, also the LSB (bit
Z0) of the Z-pointer is used.
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BIT 15 ZPCMSB ZPAGEMSB 1 o Atmel
Figure 30-3 Addressing the Flash During SPM(1)
PROGRAM MEMORY
0115
Z - REGISTER
BIT
0
ZPAGEMSB
WORD ADDRESS
WITHIN A PAGE
PAGE ADDRESS
WITHIN THE FLASH
ZPCMSB
INSTRUCTION WORD
PAGE PCWORD[PAGEMSB:0]:
00
01
02
PAGEEND
PAGE
PCWORDPCPAGE
PCMSB PAGEMSB
PROGRAM
COUNTER
Note: 
1. The different variables used in the figure are listed in Table 30-7 Read-While-Write Limit,
ATmega64A(1) on page 379.
2. PCPAGE and PCWORD are listed in table Number of Words in a Page and number of Pages in the
Flash in the Signal Names section.
30.8. Self-Programming the Flash
The program memory is updated in a page by page fashion. Before programming a page with the data
stored in the temporary page buffer, the page must be erased. The temporary page buffer is filled one
word at a time using SPM and the buffer can be filled either before the Page Erase command or between
a Page Erase and a Page Write operation:
Alternative 1, fill the buffer before a Page Erase:
•Fill temporary page buffer
•Perform a Page Erase
•Perform a Page Write
Alternative 2, fill the buffer after Page Erase:
•Perform a Page Erase
•Fill temporary page buffer
•Perform a Page Write
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If only a part of the page needs to be changed, the rest of the page must be stored (for example in the
temporary page buffer) before the erase, and then be rewritten. When using alternative 1, the Boot
Loader provides an effective Read-Modify-Write feature which allows the user software to first read the
page, do the necessary changes, and then write back the modified data. If alternative 2 is used, it is not
possible to read the old data while loading since the page is already erased. The temporary page buffer
can be accessed in a random sequence. It is essential that the page address used in both the Page
Erase and Page Write operation is addressing the same page. Please refer to Simple Assembly Code
Example for a Boot Loader on page 376 for an assembly code example.
30.8.1. Performing Page Erase by SPM
To execute page erase, set up the address in the Z-pointer, write “X0000011” to SPMCSR and execute
SPM within four clock cycles after writing SPMCSR. The data in R1 and R0 is ignored. The page address
must be written to PCPAGE in the Z-register. Other bits in the Z-pointer must be written to zero during this
operation.
•Page Erase to the RWW section: The NRWW section can be read during the Page Erase.
•Page Erase to the NRWW section: The CPU is halted during the operation.
Note:  If an interrupt occurs in the timed sequence the four cycle access cannot be guaranteed. In order
to ensure atomic operation disable interrupts before writing to SPMCSR.
30.8.2. Filling the Temporary Buffer (Page Loading)
To write an instruction word, set up the address in the Z-pointer and data in R1:R0, write “00000001” to
SPMCSR and execute SPM within four clock cycles after writing SPMCSR. The content of PCWORD in
the Z-register is used to address the data in the temporary buffer. The temporary buffer will auto-erase
after a page write operation or by writing the RWWSRE bit in SPMCSR. It is also erased after a System
Reset. Note that it is not possible to write more than one time to each address without erasing the
temporary buffer.
Note:  If the EEPROM is written in the middle of an SPM Page Load operation, all data loaded will be
lost.
30.8.3. Performing a Page Write
To execute page write, set up the address in the Z-pointer, write “X0000101” to SPMCSR and execute
SPM within four clock cycles after writing SPMCSR. The data in R1 and R0 is ignored. The page address
must be written to PCPAGE. Other bits in the Z-pointer must be written to zero during this operation.
•Page Write to the RWW section: The NRWW section can be read during the Page Write
•Page Write to the NRWW section: The CPU is halted during the operation
30.8.4. Using the SPM Interrupt
If the SPM interrupt is enabled, the SPM interrupt will generate a constant interrupt when the SPMEN bit
in SPMCSR is cleared (SPMCSR.SPMEN). This means that the interrupt can be used instead of polling
the SPMCSR Register in software. When using the SPM interrupt, the Interrupt Vectors should be moved
to the Boot Loader Section (BLS) section to avoid that an interrupt is accessing the RWW section when it
is blocked for reading. How to move the interrupts is described in Interrupts chapter.
Related Links
Interrupts on page 77
30.8.5. Consideration While Updating Boot Loader Section (BLS)
Special care must be taken if the user allows the Boot Loader Section (BLS) to be updated by leaving
Boot Lock bit11 unprogrammed. An accidental write to the Boot Loader itself can corrupt the entire Boot
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Loader, and further software updates might be impossible. If it is not necessary to change the Boot
Loader software itself, it is recommended to program the Boot Lock bit11 to protect the Boot Loader
software from any internal software changes.
30.8.6. Prevent Reading the RWW Section During Self-Programming
During Self-Programming (either Page Erase or Page Write), the RWW section is always blocked for
reading. The user software itself must prevent that this section is addressed during the self programming
operation. The RWWSB in the SPMCSR (SPMCSR.RWWSB) will be set as long as the RWW section is
busy. During Self-Programming the Interrupt Vector table should be moved to the BLS as described in
Interrupts chapter, or the interrupts must be disabled. Before addressing the RWW section after the
programming is completed, the user software must clear the SPMCSR.RWWSB by writing the
SPMCSR.RWWSRE. Refer to Simple Assembly Code Example for a Boot Loader on page 376 for an
example.
Related Links
Interrupts on page 77
30.8.7. Setting the Boot Loader Lock Bits by SPM
To set the Boot Loader Lock bits and general Lock Bits, write the desired data to R0, write “0x0001001” to
SPMCSR and execute SPM within four clock cycles after writing SPMCSR. The only accessible lock bits
are the Boot Lock bits that may prevent the Application and Boot Loader section from any software
update by the MCU.
Bit 7 6 5 4 3 2 1 0
Rd – – – – – – LB2 LB1
BLB01
BLB02
BLB11
BLB12
1
1 1 1
The tables in Boot Loader Lock Bits on page 369 show how the different settings of the Boot Loader bits
affect the Flash access.
If bits 5:2 in R0 are cleared (zero), the corresponding Lock bit will be programmed if an SPM instruction is
executed within four cycles after BLBSET and SPMEN are set in SPMCSR. The Z-pointer don’t care
during this operation, but for future compatibility it is recommended to load the Z-pointer with 0x0001
(same as used for reading the Lock bits). For future compatibility it is also recommended to set bits 7, 6, 1
and 0 in R0 to “1” when writing the Lock bits. When programming the Lock bits the entire Flash can be
read during the operation.
30.8.8. EEPROM Write Prevents Writing to SPMCSR
An EEPROM write operation will block all software programming to Flash. Reading the Fuses and Lock
bits from software will also be prevented during the EEPROM write operation. It is recommended that the
user checks the status bit (EEWE) in the EECR Register (EECR.EEWE) and verifies that the bit is
cleared before writing to the SPMCSR Register.
30.8.9. Reading the Fuse and Lock Bits from Software
It is possible to read both the Fuse and Lock Bits from software. To read the Lock Bits, load the Z-pointer
with 0x0001 and set the BLBSET and SPMEN bits in SPMCSR. When an LPM instruction is executed
within three CPU cycles after the BLBSET and SPMEN bits are set in SPMCSR, the value of the Lock
Bits will be loaded in the destination register. The BLBSET and SPMEN bits will auto-clear upon
completion of reading the Lock Bits or if no LPM instruction is executed within three CPU cycles or no
SPM instruction is executed within four CPU cycles. When BLBSET and SPMEN are cleared, LPM will
work as described in the Instruction set Manual.
Bit 7 6 5 4 3 2 1 0
Rd – – – – – – LB2 LB1
BLB01
BLB02
BLB11
BLB12
–
– LB2 LB1
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The algorithm for reading the Fuse Low bits is similar to the one described above for reading the Lock
Bits. To read the Fuse Low bits, load the Z-pointer with 0x0000 and set the BLBSET and SPMEN bits in
SPMCSR. When an LPM instruction is executed within three cycles after the BLBSET and SPMEN bits
are set in the SPMCSR, the value of the Fuse Low bits (FLB) will be loaded in the destination register as
shown below. Refer to table Fuse Low Byte in section Fuse Bits for a detailed description and mapping of
the fuse low bits.
Bit 7 6 5 4 3 2 1 0
Rd FLB7 FLB6 FLB5 FLB4 FLB3 FLB2 FLB1 FLB0
Similarly, when reading the Fuse High bits, load 0x0003 in the Z-pointer. When an LPM instruction is
executed within three cycles after the BLBSET and SPMEN bits are set in the SPMCSR, the value of the
Fuse High bits (FHB) will be loaded in the destination register as shown below. Refer to table Fuse High
Byte in section Fuse Bits for detailed description and mapping of the fuse high bits.
Bit 7 6 5 4 3 2 1 0
Rd FHB7 FHB6 FHB5 FHB4 FHB3 FHB2 FHB1 FHB0
When reading the Extended Fuse bits, load 0x0002 in the Z-pointer. When an LPM instruction is executed
within three cycles after the BLBSET and SPMEN bits are set in the SPMCSR, the value of the Extended
Fuse bits (EFB) will be loaded in the destination register as shown below. Refer to table Extended Fuse
Byte in section Fuse Bits for detailed description and mapping of the Fuse High bits.
Bit 7 6 5 4 3 2 1 0
Rd EFB1 EFB0
Fuse and Lock bits that are programmed read as '0'. Fuse and Lock bits that are unprogrammed, will be
read as '1'.
Related Links
Fuse Bits on page 384
30.8.10. Preventing Flash Corruption
During periods of low VCC, the Flash program can be corrupted because the supply voltage is too low for
the CPU and the Flash to operate properly. These issues are the same as for board level systems using
the Flash, and the same design solutions should be applied.
A Flash program corruption can be caused by two situations when the voltage is too low. First, a regular
write sequence to the Flash requires a minimum voltage to operate correctly. Secondly, the CPU itself can
execute instructions incorrectly, if the supply voltage for executing instructions is too low.
Flash corruption can easily be avoided by following these design recommendations (one is sufficient):
1. If there is no need for a Boot Loader update in the system, program the Boot Loader Lock bits to
prevent any Boot Loader software updates.
2. Keep the AVR RESET active (low) during periods of insufficient power supply voltage. This can be
done by enabling the internal Brown-out Detector (BOD) if the operating voltage matches the
detection level. If not, an external low VCC reset protection circuit can be used. If a reset occurs
while a write operation is in progress, the write operation will be completed provided that the power
supply voltage is sufficient.
3. Keep the AVR core in Power-down sleep mode during periods of low VCC. This will prevent the
CPU from attempting to decode and execute instructions, effectively protecting the SPMCSR
Register and thus the Flash from unintentional writes.
30.8.11. Programming Time for Flash when Using SPM
The calibrated RC Oscillator is used to time Flash accesses. The following table shows the typical
programming time for Flash accesses from the CPU.
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Table 30-5 SPM Programming Time(1)
Symbol Min. Programming Time Max. Programming Time
Flash write (Page Erase, Page Write, and write Lock bits
by SPM)
3.7ms 4.5ms
Note:  1. Minimum and maximum programming time is per individual operation.
30.8.12. Simple Assembly Code Example for a Boot Loader
;-the routine writes one page of data from RAM to Flash
; the first data location in RAM is pointed to by the Y pointer
; the first data location in Flash is pointed to by the Z-
pointer
;-error handling is not included
;-the routine must be placed inside the Boot space
; (at least the Do_spm sub routine). Only code inside NRWW
section can
; be read during Self-Programming (Page Erase and Page Write).
;-registers used: r0, r1, temp1 (r16), temp2 (r17), looplo
(r24),
; loophi (r25), spmcsrval (r20)
; storing and restoring of registers is not included in the
routine
; register usage can be optimized at the expense of code size
;-It is assumed that either the interrupt table is moved to
the Boot
; loader section or that the interrupts are disabled.
.equ PAGESIZEB = PAGESIZE*2 ;PAGESIZEB is page size in BYTES,
not words
.org SMALLBOOTSTART
Write_page:
; Page Erase
ldi spmcsrval, (1<<PGERS) | (1<<SPMEN)
call Do_spm
; re-enable the RWW section
ldi spmcsrval, (1<<RWWSRE) | (1<<SPMEN)
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call Do_spm
; transfer data from RAM to Flash page buffer
ldi looplo, low(PAGESIZEB) ;init loop variable
ldi loophi, high(PAGESIZEB) ;not required for
PAGESIZEB<=256
Wrloop:
ld r0, Y+
ld r1, Y+
ldi spmcsrval, (1<<SPMEN)
call Do_spm
adiw ZH:ZL, 2
sbiw loophi:looplo, 2 ;use subi for PAGESIZEB<=256
brne Wrloop
; execute Page Write
subi ZL, low(PAGESIZEB) ;restore pointer
sbci ZH, high(PAGESIZEB) ;not required for PAGESIZEB<=256
ldi spmcsrval, (1<<PGWRT) | (1<<SPMEN)
call Do_spm
; re-enable the RWW section
ldi spmcrval, (1<<RWWSRE) | (1<<SPMEN)
call Do_spm
; read back and check, optional
ldi looplo, low(PAGESIZEB) ;init loop variable
ldi loophi, high(PAGESIZEB) ;not required for
PAGESIZEB<=256
subi YL, low(PAGESIZEB) ;restore pointer
sbci YH, high(PAGESIZEB)
Rdloop:
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lpm r0, Z+
ld r1, Y+
cpse r0, r1
jmp Error
sbiw loophi:looplo, 1 ;use subi for PAGESIZEB<=256
brne Rdloop
; return to RWW section
; verify that RWW section is safe to read
Return:
lds temp1, SPMCSR
sbrs temp1, RWWSB ; If RWWSB is set, the RWW section is
not ready yet
ret
; re-enable the RWW section
ldi spmcsrval, (1<<RWWSRE) | (1<<SPMEN)
call Do_spm
rjmp Return
Do_spm:
; check for previous SPM complete
Wait_spm:
lds temp1, SPMCSR
sbrc temp1, SPMEN
rjmp Wait_spm
; input: spmcsrval determines SPM action
; disable interrupts if enabled, store status
in temp2, SREG
cli
; check that no EEPROM write access is present
Wait_ee:
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sbic EECR, EEWE
rjmp Wait_ee
; SPM timed sequence
sts SPMCSR, spmcsrval
spm
; restore SREG (to enable interrupts if originally
enabled)
out SREG, temp2
ret
30.8.13. ATmega64A Boot Loader Parameters
In the following tables, the parameters used in the description of the self programming are given.
Table 30-6 Boot Size Configuration, ATmega64A
BOOTSZ1 BOOTSZ0 Boot
Size
Pages Application
Flash Section
Boot
Loader
Flash
Section
End
Application
Section
Boot Reset
Address
(Start Boot
Loader
Section)
1 1 512
words
4 0x0000 -
0x7DFF
0x7E00 -
0x7FFF
0x7DFF 0x7E00
1 0 1024
words
8 0x0000 -
0x7BFF
0x7C00 -
0x7FFF
0x7BFF 0x7C00
0 1 2048
words
16 0x0000 -
0x77FF
0x7800 -
0x7FFF
0x77FF 0x7800
0 0 4096
words
32 0x0000 -
0x6FFF
0x7000 -
0x7FFF
0x6FFF 0x7000
Note:  The different BOOTSZ Fuse configurations are shown in Figure 30-2 Memory Sections on page
369.
Table 30-7 Read-While-Write Limit, ATmega64A(1)
Section Pages Address
Read-While-Write section (RWW) 224 0x0000 - 0x6FFF
No Read-While-Write section (NRWW) 32 0x7000 - 0x7FFF
Note:  1. For details about these two sections, see NRWW – No Read-While-Write Section on page 367
and RWW – Read-While-Write Section on page 367.
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Table 30-8 Explanation of Different Variables Used in Figure and the Mapping to the Z-pointer, ATmega64A(3)
Variable Corresponding Z-
value(1)
Description(2)
PCMSB 14 Most significant bit in the program counter. (The program
counter is 15 bits PC[14:0])
PAGEMSB 6 Most significant bit which is used to address the words
within one page (128 words in a page requires 7 bits PC
[6:0]).
ZPCMSB Z15(1) Bit in Z-register that is mapped to PCMSB. Because Z0
is not used, the ZPCMSB equals PCMSB + 1.
ZPAGEMSB Z7 Bit in Z-register that is mapped to PAGEMSB. Because
Z0 is not used, the ZPAGEMSB equals PAGEMSB + 1.
PCPAGE PC[14:7] Z15(1):Z8 Program counter page address: Page select, for page
erase and page write
PCWORD PC[6:0] Z7:Z1 Program counter word address: Word select, for filling
temporary buffer (must be zero during page write
operation)
Note: 
1. Z0: should be zero for all SPM commands, byte select for the LPM instruction.
2. See Addressing the Flash During Self-Programming on page 371 for details about the use of Z-
pointer during self-programming.
30.9. Register Description
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30.9.1. SPMCSR – Store Program Memory Control and Status Register
The Store Program Memory Control and Status Register contains the control bits needed to control the
Boot Loader operations.
Name:  SPMCSR
Offset:  0x68
Reset:  0x00
Property:
 
–
Bit 7 6 5 4 3 2 1 0
SPMIE RWWSB RWWSRE BLBSET PGWRT PGERS SPMEN
Access R/W R R/W R/W R/W R/W R/W
Reset 0 0 0 0 0 0 0
Bit 7 – SPMIE: SPM Interrupt Enable
When the SPMIE bit is written to one, and the I-bit in the Status Register is set (one), the SPM ready
interrupt will be enabled. The SPM ready Interrupt will be executed as long as the SPMEN bit in the
SPMCSR Register is cleared.
Bit 6 – RWWSB: Read-While-Write Section Busy
When a Self-Programming (Page Erase or Page Write) operation to the RWW section is initiated, the
RWWSB will be set (one) by hardware. When the RWWSB bit is set, the RWW section cannot be
accessed. The RWWSB bit will be cleared if the RWWSRE bit is written to one after a Self-Programming
operation is completed. Alternatively the RWWSB bit will automatically be cleared if a page load operation
is initiated.
Bit 4 – RWWSRE: Read-While-Write Section Read Enable
When programming (Page Erase or Page Write) to the RWW section, the RWW section is blocked for
reading (the RWWSB will be set by hardware). To re-enable the RWW section, the user software must
wait until the programming is completed (SPMEN will be cleared). Then, if the RWWSRE bit is written to
one at the same time as SPMEN, the next SPM instruction within four clock cycles re-enables the RWW
section. The RWW section cannot be re-enabled while the Flash is busy with a Page Erase or a Page
Write (SPMEN is set). If the RWWSRE bit is written while the Flash is being loaded, the Flash load
operation will abort and the data loaded will be lost.
Bit 3 – BLBSET: Boot Lock Bit Set
If this bit is written to one at the same time as SPMEN, the next SPM instruction within four clock cycles
sets Boot Lock bits, according to the data in R0. The data in R1 and the address in the Z-pointer are
ignored. The BLBSET bit will automatically be cleared upon completion of the Lock bit set, or if no SPM
instruction is executed within four clock cycles.
An LPM instruction within three cycles after BLBSET and SPMEN are set in the SPMCSR Register
(SPMCSR.BLBSET and SPMCSR.SPMEN), will read either the Lock bits or the Fuse bits (depending on
Z0 in the Z-pointer) into the destination register. Refer to Reading the Fuse and Lock Bits from Software
on page 374.
Bit 2 – PGWRT: Page Write
If this bit is written to one at the same time as SPMEN, the next SPM instruction within four clock cycles
executes Page Write, with the data stored in the temporary buffer. The page address is taken from the
high part of the Z-pointer. The data in R1 and R0 are ignored. The PGWRT bit will auto-clear upon
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completion of a Page Write, or if no SPM instruction is executed within four clock cycles. The CPU is
halted during the entire Page Write operation if the NRWW section is addressed.
Bit 1 – PGERS: Page Erase
If this bit is written to one at the same time as SPMEN, the next SPM instruction within four clock cycles
executes Page Erase. The page address is taken from the high part of the Z-pointer. The data in R1 and
R0 are ignored. The PGERS bit will auto-clear upon completion of a Page Erase, or if no SPM instruction
is executed within four clock cycles. The CPU is halted during the entire Page Write operation if the
NRWW section is addressed.
Bit 0 – SPMEN: Store Program Memory Enable
This bit enables the SPM instruction for the next four clock cycles. If written to one together with either
RWWSRE, BLBSET, PGWRT or PGERS, the following SPM instruction will have a special meaning, see
description above. If only SPMEN is written, the following SPM instruction will store the value in R1:R0 in
the temporary page buffer addressed by the Z-pointer. The LSB of the Z-pointer is ignored. The SPMEN
bit will auto-clear upon completion of an SPM instruction, or if no SPM instruction is executed within four
clock cycles. During Page Erase and Page Write, the SPMEN bit remains high until the operation is
completed.
Writing any other combination than “0x10001”, “0x01001”, “0x00101”, “0x00011” or “0x00001” in the lower
five bits will have no effect.
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31. Memory Programming
31.1. Program and Data Memory Lock Bits
The ATmega64A provides six Lock bits. These can be left unprogrammed ('1') or can be programmed ('0')
to obtain the additional features listed in table Lock Bit Protection Modes below. The Lock Bits can only
be erased to “1” with the Chip Erase command.
Table 31-1 Lock Bit Byte
Lock Bit Byte Bit No. Description Default Value(1)
7 – 1 (unprogrammed)
6 – 1 (unprogrammed)
BLB12 5 Boot Lock bit 1 (unprogrammed)
BLB11 4 Boot Lock bit 1 (unprogrammed)
BLB02 3 Boot Lock bit 1 (unprogrammed)
BLB01 2 Boot Lock bit 1 (unprogrammed)
LB2 1 Lock bit 1 (unprogrammed)
LB1 0 Lock bit 1 (unprogrammed)
Note:  1. “1” means unprogrammed, “0” means programmed.
Table 31-2 Lock Bit Protection Modes(2)
Memory Lock Bits Protection Type
LB Mode LB2 LB1
1 1 1 No memory lock features enabled.
2 1 0 Further programming of the Flash and EEPROM is disabled in Parallel
and Serial Programming mode. The Fuse bits are locked in both Serial
and Parallel Programming mode.(1)
3 0 0 Further programming and verification of the Flash and EEPROM is
disabled in parallel and SPI/JTAG Serial Programming mode. The Fuse
Bits are locked in both Serial and Parallel Programming modes.(1)
BLB0
Mode
BLB02 BLB01
1 1 1 No restrictions for SPM or (E)LPM accessing the Application section.
2 1 0 SPM is not allowed to write to the Application section.
3 0 0 SPM is not allowed to write to the Application section, and LPM executing
from the Boot Loader section is not allowed to read from the Application
section. If Interrupt Vectors are placed in the Boot Loader section,
interrupts are disabled while executing from the Application section.
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Memory Lock Bits Protection Type
LB Mode LB2 LB1
4 0 1 LPM executing from the Boot Loader section is not allowed to read from
the Application section. If Interrupt Vectors are placed in the Boot Loader
section, interrupts are disabled while executing from the Application
section.
BLB1
Mode
BLB12 BLB11
1 1 1 No restrictions for SPM or LPM accessing the Boot Loader section.
2 1 0 SPM is not allowed to write to the Boot Loader section.
3 0 0 SPM is not allowed to write to the Boot Loader section, and LPM
executing from the Application section is not allowed to read from the Boot
Loader section. If Interrupt Vectors are placed in the Application section,
interrupts are disabled while executing from the Boot Loader section.
4 0 1 LPM executing from the Application section is not allowed to read from the
Boot Loader section. If Interrupt Vectors are placed in the Application
section, interrupts are disabled while executing from the Boot Loader
section.
Note: 
1. Program the Fuse Bits before programming the Lock Bits.
2. “1” means unprogrammed, “0” means programmed.
31.2. Fuse Bits
The ATmega64A has three fuse bytes. The tables of this section describe briefly the functionality of all the
fuses and how they are mapped into the fuse bytes. Note that the fuses are read as logical zero, “0”, if
they are programmed.
Table 31-3 Extended Fuse Byte
Extended Fuse Byte Bit No. Description Default Value
– 7 – 1
– 6 – 1
– 5 – 1
– 4 – 1
– 3 – 1
– 2 – 1
M103C(1) 1 ATmega103 compatibility mode 0 (programmed)
WDTON(2) 0 Watchdog Timer always on 1 (unprogrammed)
Note: 
1. See ATmega103 and ATmega64A Compatibility for details.
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2. See WDTCR - Watchdog Timer Control Register for details.
Table 31-4 Fuse High Byte
Fuse High Byte Bit No. Description Default Value
OCDEN(4) 7 Enable OCD 1 (unprogrammed, OCD
disabled)
JTAGEN(5) 6 Enable JTAG 0 (programmed, JTAG enabled)
SPIEN(1) 5 Enable Serial Program and Data
Downloading
0 (programmed, SPI prog.
enabled)
CKOPT(2) 4 Oscillator options 1 (unprogrammed)
EESAVE 3 EEPROM memory is preserved through the
Chip Erase
1 (unprogrammed, EEPROM
not preserved)
BOOTSZ1 2 Select Boot Size (see table Boot Size
Configuration in section ATmega64A Boot
Loader Parameters for details)
0 (programmed)(3)
BOOTSZ0 1 Select Boot Size (see table Boot Size
Configuration in section ATmega64A Boot
Loader Parameters for details)
0 (programmed)(3)
BOOTRST 0 Select Reset Vector 1 (unprogrammed)
Note: 
1. The SPIEN Fuse is not accessible in SPI Serial Programming mode.
2. The CKOPT Fuse functionality depends on the setting of the CKSEL bits, see Clock Sources for
details.
3. The default value of BOOTSZ1:0 results in maximum Boot Size. See table Boot Size Configuration
in section ATmega64A Boot Loader Parameters.
4. Never ship a product with the OCDEN Fuse programmed regardless of the setting of lock bits and
the JTAGEN Fuse. A programmed OCDEN Fuse enables some parts of the clock system to be
running in all sleep modes. This may increase the power consumption.
5. If the JTAG interface is left unconnected, the JTAGEN fuse should if possible be disabled. This to
avoid static current at the TDO pin in the JTAG interface.
Table 31-5 Fuse Low Byte
Fuse Low Byte Bit No. Description Default Value
BODLEVEL 7 Brown out detector trigger level 1 (unprogrammed)
BODEN 6 Brown out detector enable 1 (unprogrammed, BOD disabled)
SUT1 5 Select start-up time 1 (unprogrammed)(1)
SUT0 4 Select start-up time 0 (programmed)(1)
CKSEL3 3 Select Clock source 0 (programmed)(2)
CKSEL2 2 Select Clock source 0 (programmed)(2)
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Fuse Low Byte Bit No. Description Default Value
CKSEL1 1 Select Clock source 0 (programmed)(2)
CKSEL0 0 Select Clock source 1 (unprogrammed)(2)
Note: 
1. The default value of SUT1:0 results in maximum start-up time. See table Start-up Times for the
Internal Calibrated RC Oscillator Clock Selection in section Calibrated Internal RC Oscillator for
details.
2. The default setting of CKSEL3:0 results in Internal RC Oscillator @ 1MHz. See table Device
Clocking Options Select in section Clock Sources for details.
The status of the Fuse bits is not affected by Chip Erase. Note that the Fuse bits are locked if Lock bit1
(LB1) is programmed. Program the Fuse bits before programming the Lock bits.
Related Links
ATmega64A Boot Loader Parameters on page 379
Calibrated Internal RC Oscillator on page 56
ATmega103 and ATmega64A Compatibility on page 13
Clock Sources on page 53
31.2.1. Latching of Fuses
The fuse values are latched when the device enters programming mode and changes of the fuse values
will have no effect until the part leaves Programming mode. This does not apply to the EESAVE Fuse
which will take effect once it is programmed. The fuses are also latched on Power-up in Normal mode.
31.3. Signature Bytes
All Atmel microcontrollers have a three-byte signature code which identifies the device. This code can be
read in both serial and parallel mode, also when the device is locked. The three bytes reside in a
separate address space.
For the ATmega64A the signature bytes are given in the following table.
Table 31-6 Device and JTAG ID
Part Signature Bytes Address JTAG
0x000 0x001 0x002 Part Number Manufacture ID
ATmega64A 0x1E 0x96 0x02 9602 0x1F
31.4. Calibration Byte
The ATmega64A stores four different calibration values for the internal RC Oscillator. These bytes resides
in the signature row High byte of the addresses 0x0000, 0x0001, 0x0002, and 0x0003 for 1, 2, 4, and
8MHz respectively. During Reset, the 1MHz value is automatically loaded into the OSCCAL Register. If
other frequencies are used, the calibration value has to be loaded manually, see OSCCAL – Oscillator
Calibration Register for details.
Related Links
OSCCAL on page 60
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31.5. Page Size
Table 31-7 Number of Words in a Page and number of Pages in the Flash
Flash Size Page Size PCWORD Number of Pages PCPAGE PCMSB
32K words (64K bytes) 128 words PC[6:0] 256 PC[14:7] 14
Table 31-8 Number of Words in a Page and number of Pages in the EEPROM
EEPROM Size Page Size PCWORD Number of Pages PCPAGE EEAMSB
2K bytes 8 bytes EEA[2:0] 256 EEA[8:2] 10
31.6. Parallel Programming
31.6.1. Enter Programming Mode
The following algorithm puts the device in Parallel Programming mode:
1. Apply 4.5 - 5.5V between VCC and GND, and wait at least 100µs.
2. Set RESET to “0” and toggle XTAL1 at least 6 times
3. Set the Prog_enable pins listed in Table 31-10 Pin Values Used to Enter Programming Mode on
page 395 to “0000” and wait at least 100ns.
4. Apply 11.5 - 12.5V to RESET. Any activity on Prog_enable pins within 100ns after +12V has been
applied to RESET, will cause the device to fail entering Programming mode.
Note, if External Crystal or External RC configuration is selected, it may not be possible to apply qualified
XTAL1 pulses. In such cases, the following algorithm should be followed:
1. Set Prog_enable pins listed in Table 31-10 Pin Values Used to Enter Programming Mode on page
395 to “0000”.
2. Apply 4.5 - 5.5V between VCC and GND simultaneously as 11.5 - 12.5V is applied to RESET.
3. Wait 100μs.
4. Re-program the fuses to ensure that External Clock is selected as clock source (CKSEL3:0 =
0b0000). If Lock bits are programmed, a Chip Erase command must be executed before changing
the fuses.
5. Exit Programming mode by power the device down or by bringing RESET pin to 0b0.
6. Entering Programming mode with the original algorithm, as described above.
31.6.2. Considerations for Efficient Programming
The loaded command and address are retained in the device during programming. For efficient
programming, the following should be considered.
•The command needs only be loaded once when writing or reading multiple memory locations.
•Skip writing the data value 0xFF, that is the contents of the entire EEPROM (unless the EESAVE
Fuse is programmed) and Flash after a Chip Erase.
•Address high byte needs only be loaded before programming or reading a new 256 word window in
Flash or 256byte EEPROM. This consideration also applies to Signature bytes reading.
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31.6.3. Chip Erase
The Chip Erase will erase the Flash and EEPROM memories plus Lock bits. The Lock bits are not reset
until the program memory has been completely erased. The Fuse bits are not changed. A Chip Erase
must be performed before the Flash and/or EEPROM are reprogrammed.
Note:  The EEPRPOM memory is preserved during Chip Erase if the EESAVE Fuse is programmed.
Load Command “Chip Erase”:
1. Set XA1, XA0 to “10”. This enables command loading.
2. Set BS1 to “0”.
3. Set DATA to “1000 0000”. This is the command for Chip Erase.
4. Give XTAL1 a positive pulse. This loads the command.
5. Give WR a negative pulse. This starts the Chip Erase. RDY/BSY goes low.
6. Wait until RDY/BSY goes high before loading a new command.
31.6.4. Programming the Flash
The Flash is organized in pages. When programming the Flash, the program data is latched into a page
buffer. This allows one page of program data to be programmed simultaneously. The following procedure
describes how to program the entire Flash memory:
Step A. Load Command “Write Flash”.
1. Set XA1, XA0 to “10”. This enables command loading.
2. Set BS1 to “0”.
3. Set DATA to “0001 0000”. This is the command for Write Flash.
4. Give XTAL1 a positive pulse. This loads the command.
Step B. Load Address Low Byte.
1. Set XA1, XA0 to “00”. This enables address loading.
2. Set BS1 to “0”. This selects low address.
3. Set DATA = Address low byte (0x00 - 0xFF).
4. Give XTAL1 a positive pulse. This loads the address low byte.
Step C. Load Data Low Byte.
1. Set XA1, XA0 to “01”. This enables data loading.
2. Set DATA = Data low byte (0x00 - 0xFF).
3. Give XTAL1 a positive pulse. This loads the data byte.
Step D. Load Data High Byte.
1. Set BS1 to “1”. This selects high data byte.
2. Set XA1, XA0 to “01”. This enables data loading.
3. Set DATA = Data high byte (0x00 - 0xFF).
4. Give XTAL1 a positive pulse. This loads the data byte.
Step E. Latch Data.
1. Set BS1 to “1”. This selects high data byte.
2. Give PAGEL a positive pulse. This latches the data bytes. (Refer to figure Programming the Flash
Waveforms in this section for signal waveforms)
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PCMSB Atmel PAGEMSE
Step F. Repeat B through E until the entire buffer is filled or until all data within the page is loaded.
While the lower bits in the address are mapped to words within the page, the higher bits address the
pages within the FLASH. This is illustrated in the following figure, Addressing the Flash Which is
Organized in Pages, in this section. Note that if less than eight bits are required to address words in the
page (pagesize < 256), the most significant bit(s) in the address low byte are used to address the page
when performing a Page Write.
Step G. Load Address High byte.
1. Set XA1, XA0 to “00”. This enables address loading.
2. Set BS1 to “1”. This selects high address.
3. Set DATA = Address high byte (0x00 - 0xFF).
4. Give XTAL1 a positive pulse. This loads the address high byte.
Step H. Program Page.
1. Set BS1 = “0”
2. Give WR a negative pulse. This starts programming of the entire page of data. RDY/BSY goes low.
3. Wait until RDY/BSY goes high (Refer to figure Programming the Flash Waveforms in this section).
Step I. Repeat B through H until the entire Flash is programmed or until all data has been
programmed.
Step J. End Page Programming.
1. 1. Set XA1, XA0 to “10”. This enables command loading.
2. Set DATA to “0000 0000”. This is the command for No Operation.
3. Give XTAL1 a positive pulse. This loads the command, and the internal write signals are reset.
Figure 31-1 Addressing the Flash Which is Organized in Pages
PROGRAM MEMORY
WORD ADDRESS
WITHIN A PAGE
PAGE ADDRESS
WITHIN THE FLASH
INSTRUCTION WORD
PAGE PCWORD[PAGEMSB:0]:
00
01
02
PAGEEND
PAGE
PCWORDPCPAGE
PCMSB PAGEMSB
PROGRAM
COUNTER
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Note:  PCPAGE and PCWORD are listed in the section Page Size.
Figure 31-2 Programming the Flash Waveform
RDY/BSY
WR
OE
RESET +12V
PAGEL
BS2
0x10 ADDR. LOW ADDR. HIGH
DATA DATA LOW DATA HIGH ADDR. LOW DATA LOW DATA HIGH
XA1
XA0
BS1
XTAL1
XX XX XX
A B C D E B C D E G H
F
Note:  “XX” is don’t care. The letters refer to the programming description above.
31.6.5. Programming the EEPROM
The EEPROM is organized in pages. When programming the EEPROM, the program data is latched into
a page buffer. This allows one page of data to be programmed simultaneously. The programming
algorithm for the EEPROM data memory is as follows (For details on Command, Address and Data
loading, refer to Programming the Flash on page 388):
1. Step A: Load Command “0001 0001”.
2. Step G: Load Address High Byte (0x00 - 0xFF).
3. Step B: Load Address Low Byte (0x00 - 0xFF).
4. Step C: Load Data (0x00 - 0xFF).
5. Step E: Latch data (give PAGEL a positive pulse).
6. Step K:Repeat 3 through 5 until the entire buffer is filled.
7. Step L: Program EEPROM page
7.1. Set BS1 to “0”.
7.2. Give WR a negative pulse. This starts programming of the EEPROM page. RDY/BSY goes
low.
7.3. Wait until to RDY/BSY goes high before programming the next page. Refer to the figure
below for signal waveforms.
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Figure 31-3 Programming the EEPROM Waveforms
RDY/BSY
WR
OE
RESET +12V
PAGEL
BS2
0x11 ADDR. HIGH
DATA ADDR. LOW DATA ADDR. LOW DATA XX
XA1
XA0
BS1
XTAL1
XX
A G B C E B C E L
K
31.6.6. Reading the Flash
The algorithm for reading the Flash memory is as follows (Please refer to Programming the Flash on
page 388 in this chapter for details on Command and Address loading):
1. Step A: Load Command “0000 0010”.
2. Step G: Load Address High Byte (0x00 - 0xFF).
3. Step B: Load Address Low Byte (0x00 - 0xFF).
4. Set OE to “0”, and BS1 to “0”. The Flash word low byte can now be read at DATA.
5. Set BS1 to “1”. The Flash word high byte can now be read at DATA.
6. Set OE to “1”.
31.6.7. Reading the EEPROM
The algorithm for reading the EEPROM memory is as follows (Please refer to Programming the Flash on
page 388 for details on Command and Address loading):
1. Step A: Load Command “0000 0011”.
2. Step G: Load Address High Byte (0x00 - 0xFF).
3. Step B: Load Address Low Byte (0x00 - 0xFF).
4. Set OE to “0”, and BS1 to “0”. The EEPROM Data byte can now be read at DATA.
5. Set OE to “1”.
31.6.8. Programming the Fuse Low Bits
The algorithm for programming the Fuse Low bits is as follows (Please refer to Programming the Flash on
page 388 for details on Command and Data loading):
1. Step A: Load Command “0100 0000”.
2. Step C: Load Data Low Byte. Bit n = “0” programs and bit n = “1” erases the Fuse bit.
3. Set BS1 and BS2 to “0”.
4. Give WR a negative pulse and wait for RDY/BSY to go high.
31.6.9. Programming the Fuse High Bits
The algorithm for programming the Fuse High bits is as follows (Please refer to Programming the Flash
on page 388 for details on Command and Data loading):
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1. Step A: Load Command “0100 0000”.
2. Step C: Load Data Low Byte. Bit n = “0” programs and bit n = “1” erases the Fuse bit.
3. Set BS1 to “1” and BS2 to “0”. This selects high data byte.
4. Give WR a negative pulse and wait for RDY/BSY to go high.
5. Set BS1 to “0”. This selects low data byte.
31.6.10. Programming the Extended Fuse Bits
The algorithm for programming the Extended Fuse bits is as follows (Please refer to Programming the
Flash for details on Command and Data loading):
1. Step A: Load Command “0100 0000”.
2. Step C: Load Data Low Byte. Bit n = “0” programs and bit n = “1” erases the Fuse bit.
3. Set BS1 to “0” and BS2 to “1”. This selects extended data byte.
4. Give WR a negative pulse and wait for RDY/BSY to go high.
5. Set BS2 to “0”. This selects low data byte.
Figure 31-4 Programming the FUSES Waveforms
RDY/BSY
WR
OE
RESET +12V
PAGEL
0x40
DATA DATA XX
XA1
XA0
BS1
XTAL1
A C
0x40 DATA XX
A C
Write Fuse Low byte Write Fuse high byte
0x40 DATA XX
A C
Write Extended Fuse byte
BS2
31.6.11. Programming the Lock Bits
The algorithm for programming the Lock bits is as follows (Please refer to Programming the Flash on
page 388 for details on Command and Data loading):
1. Step A: Load Command “0010 0000”.
2. Step C: Load Data Low Byte. Bit n = “0” programs the Lock bit.
3. Give WR a negative pulse and wait for RDY/BSY to go high.
The Lock bits can only be cleared by executing Chip Erase.
31.6.12. Reading the Fuse and Lock Bits
The algorithm for reading the Fuse and Lock bits is as follows (Please refer to Programming the Flash for
details on Command loading):
1. Step A: Load Command “0000 0100”.
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2. Set OE to “0”, BS2 to “0” and BS1 to “0”. The status of the Fuse Low bits can now be read at DATA
(“0” means programmed).
3. Set OE to “0”, BS2 to “1” and BS1 to “1”. The status of the Fuse High bits can now be read at DATA
(“0” means programmed).
4. Set OE to “0”, BS2 to “1”, and BS1 to “0”. The status of the Extended Fuse bits can now be read at
DATA (“0” means programmed).
5. Set OE to “0”, BS2 to “0” and BS1 to “1”. The status of the Lock bits can now be read at DATA (“0”
means programmed).
6. Set OE to “1”.
Figure 31-5 Mapping Between BS1, BS2 and the Fuse and Lock Bits During Read
Lock Bits 0
1
BS2
Fuse High Byte
0
1
BS1
DATA
Fuse Low Byte 0
1
BS2
Extended Fuse Byte
31.6.13. Reading the Signature Bytes
The algorithm for reading the Signature bytes is as follows (Please refer to Programming the Flash on
page 388 for details on Command and Address loading):
1. Step A: Load Command “0000 1000”.
2. Step B: Load Address Low Byte (0x00 - 0x02).
3. Set OE to “0”, and BS1 to “0”. The selected Signature byte can now be read at DATA.
4. Set OE to “1”.
31.6.14. Reading the Calibration Byte
The algorithm for reading the Calibration byte is as follows (Please refer to Programming the Flash on
page 388 for details on Command and Address loading):
1. Step A: Load Command “0000 1000”.
2. Step B: Load Address Low byte, (0x00 - 0x03).
3. Set OE to “0”, and BS1 to “1”. The Calibration byte can now be read at DATA.
4. Set OE to “1”.
31.6.15. Parallel Programming Characteristics
For characteristics of the Parallel Programming, please refer to Parallel Programming Characteristics.
Related Links
Parallel Programming Characteristics on page 421
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WW 47 O_E —> W 4» BSl —> XAO 4b XAl —> PAGEL 4> +12V —> —> _l_|_—> PDl PD2 PD3 PD4 PDS PDG PAO XTAL GND <—> ATA Atmel
31.7. Parallel Programming Parameters, Pin Mapping, and Commands
This section describes how to parallel program and verify Flash Program memory, EEPROM Data
memory, Memory Lock bits, and Fuse bits in the device. Pulses are assumed to be at least 250ns unless
otherwise noted.
31.7.1. Signal Names
In this section, some pins of this device are referenced by signal names describing their functionality
during parallel programming, refer to the following figure and table Pin Name Mapping in this section.
Pins not described in the following table are referenced by pin names.
The XA1/XA0 pins determine the action executed when the XTAL1 pin is given a positive pulse. The bit
coding is shown in Table 31-11 XA1 and XA0 Coding on page 395.
When pulsing WR or OE, the command loaded determines the action executed. The different Commands
are shown in Table 31-12 Command Byte Bit Coding on page 395.
Figure 31-6 Parallel Programming
VCC
+5V
GND
XTAL1
PD1
PD2
PD3
PD4
PD5
PD6
DATA
RESET
PB7-PB0
PD7
+12 V
BS1
XA0
XA1
OE
RDY/BSY
WR
PAGEL
PA0
BS2
AVCC
+5V
Table 31-9 Pin Name Mapping
Signal Name in
Programming Mode
Pin Name I/O Function
RDY/BSY PD1 O 0: Device is busy programming, 1: Device is ready for new
command
OE PD2 I Output Enable (Active low)
WR PD3 I Write Pulse (Active low)
BS1 PD4 I Byte Select 1 (“0” selects Low byte, “1” selects High byte)
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Signal Name in
Programming Mode
Pin Name I/O Function
XA0 PD5 I XTAL Action Bit 0
XA1 PD6 I XTAL Action Bit 1
PAGEL PD7 I Program memory and EEPROM Data Page Load
BS2 PA0 I Byte Select 2 (“0” selects Low byte, “1” selects second
High byte)
DATA PB7-0 I/O Bi-directional Data bus (Output when OE is low)
Table 31-10 Pin Values Used to Enter Programming Mode
Pin Symbol Value
PAGEL Prog_enable[3] 0
XA1 Prog_enable[2] 0
XA0 Prog_enable[1] 0
BS1 Prog_enable[0] 0
Table 31-11 XA1 and XA0 Coding
XA1 XA0 Action when XTAL1 is Pulsed
0 0 Load Flash or EEPROM Address (High or low address byte determined by BS1)
0 1 Load Data (High or Low data byte for Flash determined by BS1)
1 0 Load Command
1 1 No Action, Idle
Table 31-12 Command Byte Bit Coding
Command Byte Command Executed
1000 0000 Chip Erase
0100 0000 Write Fuse bits
0010 0000 Write Lock bits
0001 0000 Write Flash
0001 0001 Write EEPROM
0000 1000 Read Signature Bytes and Calibration byte
0000 0100 Read Fuse and Lock bits
0000 0010 Read Flash
0000 0011 Read EEPROM
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31.8. Serial Downloading
Both the Flash and EEPROM memory arrays can be programmed using the serial SPI bus while RESET
is pulled to GND. The serial interface consists of pins SCK, MOSI (input) and MISO (output). After
RESET is set low, the Programming Enable instruction needs to be executed first before program/erase
operations can be executed.
Note:  The pin mapping for SPI programming is listed in the following section. Not all parts use the SPI
pins dedicated for the internal SPI interface. Throughout the description about Serial downloading, MOSI
and MISO are used to describe the serial data in and serial data out respectively. For ATmega64A, these
pins are mapped to PDI and PDO.
31.9. Serial Programming Pin Mapping
Even though the SPI Programming interface re-uses the SPI I/O module, there is one important
difference: The MOSI/MISO pins that are mapped to PB2 and PB3 in the SPI I/O module are not used in
the Programming interface. Instead, PE0 and PE1 are used for data in SPI Programming mode as shown
in the following table.
Table 31-13 Pin Mapping SPI Serial Programming
Symbol Pins I/O Description
MOSI (PDI) PE0 I Serial Data in
MISO (PDO) PE1 O Serial Data out
SCK PB1 I Serial Clock
Figure 31-7 Serial Programming and Verify(1)
VCC
GND
XTAL1
AVCC
+2.7 - 5.5V (2)
PDI
PDO
SCK
RESET
+2.7 - 5.5V
PE0
PE1
PB1
Note: 
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1. If the device is clocked by the Internal Oscillator, it is no need to connect a clock source to the
XTAL1 pin.
2. VCC - 0.3 < AVCC < VCC + 0.3V, however, AVCC should always be within 2.7 - 5.5V.
When programming the EEPROM, an auto-erase cycle is built into the self-timed programming operation
(in the Serial mode ONLY) and there is no need to first execute the Chip Erase instruction. The Chip
Erase operation turns the content of every memory location in both the Program and EEPROM arrays
into 0xFF.
Depending on CKSEL Fuses, a valid clock must be present. The minimum low and high periods for the
Serial Clock (SCK) input are defined as follows:
Low: > 2 CPU clock cycles for fck < 12MHz, 3 CPU clock cycles for fck ≥ 12MHz
High: > 2 CPU clock cycles for fck < 12MHz, 3 CPU clock cycles for fck ≥ 12MHz
31.9.1. SPI Serial Programming Algorithm
When writing serial data to the ATmega64A, data is clocked on the rising edge of SCK.
When reading data from the ATmega64A, data is clocked on the falling edge of SCK. Refer to Figure
31-8 Serial Programming Waveforms on page 399 for timing details.
To program and verify the ATmega64A in the SPI Serial Programming mode, the following sequence is
recommended (See four byte instruction formats in Figure 31-8 Serial Programming Waveforms on page
399):
1. Power-up sequence:
Apply power between VCC and GND while RESET and SCK are set to “0”. In some systems, the
programmer can not guarantee that SCK is held low during power-up. In this case, RESET must be
given a positive pulse of at least two CPU clock cycles duration after SCK has been set to “0”.
As an alternative to using the RESET signal, PEN can be held low during Power-on Reset while
SCK is set to “0”. In this case, only the PEN value at Power-on Reset is important. If the
programmer cannot guarantee that SCK is held low during power-up, the PEN method cannot be
used. The device must be powered down in order to commence normal operation when using this
method.
2. Wait for at least 20ms and enable SPI Serial Programming by sending the Programming Enable
serial instruction to pin MOSI.
3. The SPI Serial Programming instructions will not work if the communication is out of
synchronization. When in sync. the second byte (0x53), will echo back when issuing the third byte
of the Programming Enable instruction. Whether the echo is correct or not, all 4 bytes of the
instruction must be transmitted. If the 0x53 did not echo back, give RESET a positive pulse and
issue a new Programming Enable command.
4. The Flash is programmed one page at a time (see Page Size). The memory page is loaded one
byte at a time by supplying the 7 LSB of the address and data together with the Load Program
Memory Page instruction. To ensure correct loading of the page, the data low byte must be loaded
before data high byte is applied for given address. The Program Memory Page is stored by loading
the Write Program Memory Page instruction with the 8MSB of the address. If polling is not used,
the user must wait at least tWD_FLASH before issuing the next page. (See Table 31-14 Minimum Wait
Delay Before Writing the Next Flash or EEPROM Location, VCC = 5V ± 10% on page 398).
Note:  1. If other commands than polling (read) are applied before any write operation (Flash,
EEPROM, Lock bits, Fuses) is completed, may result in incorrect programming.
5. The EEPROM array is programmed one byte at a time by supplying the address and data together
with the appropriate Write instruction. An EEPROM memory location is first automatically erased
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before new data is written. If polling is not used, the user must wait at least tWD_EEPROM before
issuing the next byte. (See Table 31-14 Minimum Wait Delay Before Writing the Next Flash or
EEPROM Location, VCC = 5V ± 10% on page 398). In a chip erased device, no 0xFFs in the data
file(s) need to be programmed.
6. Any memory location can be verified by using the Read instruction which returns the content at the
selected address at serial output MISO.
7. At the end of the programming session, RESET can be set high to commence normal operation.
8. Power-off sequence (if needed):
–Set RESET to “1”.
–Turn VCC power off.
Note:  If other commands that polling (read) are applied before any write operation (FLASH, EEPROM,
Lock bits, Fuses) is completed, may result in incorrect programming.
Related Links
Page Size on page 387
31.9.2. Data Polling Flash
When a page is being programmed into the Flash, reading an address location within the page being
programmed will give the value 0xFF. At the time the device is ready for a new page, the programmed
value will read correctly. This is used to determine when the next page can be written. Note that the entire
page is written simultaneously and any address within the page can be used for polling. Data polling of
the Flash will not work for the value 0xFF, so when programming this value, the user will have to wait for
at least tWD_FLASH before programming the next page. As a chip-erased device contains 0xFF in all
locations, programming of addresses that are meant to contain 0xFF, can be skipped. See table in next
section for tWD_FLASH value.
31.9.3. Data Polling EEPROM
When a new byte has been written and is being programmed into EEPROM, reading the address location
being programmed will give the value 0xFF. At the time the device is ready for a new byte, the
programmed value will read correctly. This is used to determine when the next byte can be written. This
will not work for the value 0xFF, but the user should have the following in mind: As a chip-erased device
contains 0xFF in all locations, programming of addresses that are meant to contain 0xFF, can be skipped.
This does not apply if the EEPROM is programmed without chip-erasing the device. In this case, data
polling cannot be used for the value 0xFF, and the user will have to wait at least tWD_EEPROM before
programming the next byte. See table below for tWD_EEPROM value.
Table 31-14 Minimum Wait Delay Before Writing the Next Flash or EEPROM Location, VCC = 5V ± 10%
Symbol Minimum Wait Delay
tWD_FUSE 4.5ms
tWD_FLASH 4.5ms
tWD_EEPROM 9ms
tWD_ERASE 9ms
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:x HHHHHHHH TTTTTTTT Atmel
Figure 31-8 Serial Programming Waveforms
MSB
MSB
LSB
LSB
SERIAL CLOCK INPUT
(SCK)
SERIAL DATA INPUT
(MOSI)
(MISO)
SAMPLE
SERIAL DATA OUTPUT
Table 31-15 Serial Programming Instruction Set
Instruction Format
Instruction Byte 1 Byte 2 Byte 3 Byte 4 Operation
Programming
Enable
1010 1100 0101 0011 xxxx xxxx xxxx xxxx Enable SPI Serial Programming
after RESET goes low.
Chip Erase 1010 1100 100x xxxx xxxx xxxx xxxx xxxx Chip Erase EEPROM and Flash.
Read Program
Memory
0010 H000 aaaa aaaa bbbb bbbb oooo oooo Read H (high or low) data o from
Program memory at word address
a:b.
Load Program
Memory Page
0100 H000 xxxx xxxx xbbb bbbb iiii iiii Write H (high or low) data i to
Program memory page at word
address b. Data Low byte must be
loaded before Data High byte is
applied within the same address.
Write Program
Memory Page
0100 1100 aaaa aaaa bxxx xxxx xxxx xxxx Write Program memory Page at
address a:b.
Read EEPROM
Memory
1010 0000 xxxx aaaa bbbb bbbb oooo oooo Read data o from EEPROM
memory at address a:b.
Write EEPROM
Memory
1100 0000 xxxx aaaa bbbb bbbb iiii iiii Write data i to EEPROM memory at
address a:b.
Read Lock Bits 0101 1000 0000 0000 xxxx xxxx xxoo oooo Read Lock Bits. “0” = programmed,
“1” = unprogrammed. See Table
Lock Bit Byte for details.
Write Lock Bits 1010 1100 111x xxxx xxxx xxxx 11ii iiii Write Lock Bits. Set bits = “0” to
program Lock Bits. See Table Lock
Bit Byte for details.
Read Signature
Byte
0011 0000 xxxx xxxx xxxx xxbb oooo oooo Read Signature Byte o at address
b.
Write Fuse Bits 1010 1100 1010 0000 xxxx xxxx iiii iiii Set bits = “0” to program, “1” to
unprogram. See table Fuse Low
Byte for details.
Write Fuse High
Bits
1010 1100 1010 1000 xxxx xxxx iiii iiii Set bits = “0” to program, “1” to
unprogram. See table Fuse High
Byte for details.
Write Extended
Fuse bits
1010 1100 1010 0100 xxxx xxxx xxxx xxii Set bits = “0” to program, “1” to
unprogram. See table Fuse Low
Byte for details.
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Instruction Format
Instruction Byte 1 Byte 2 Byte 3 Byte 4 Operation
Read Fuse Bits 0101 0000 0000 0000 xxxx xxxx oooo oooo Read Fuse Bits. “0” = programmed,
“1” = unprogrammed. See table
Fuse Low Byte for details.
Read Extended
Fuse bits
0101 0000 0000 1000 xxxx xxxx oooo oooo Read Extended Fuse bits. “0” =
programmed, “1” = unprogrammed.
See table Fuse Low Byte for
details.
Read Fuse High
Bits
0101 1000 0000 1000 xxxx xxxx oooo oooo Read Fuse high bits. “0” =
programmed, “1” = unprogrammed.
See table Fuse High Byte for
details.
Read Calibration
Byte
0011 1000 xxxx xxxx 0000 00bb oooo oooo Read Calibration Byte o at address
b.
Note: 
a = address high bits
b = address low bits
H = 0 – Low byte, 1 – High byte
o = data out
i = data in
x = don’t care
31.9.4. SPI Serial Programming Characteristics
For characteristics of the SPI module, see SPI Timing Characteristics.
Related Links
SPI Timing Characteristics on page 422
SPI Timing Characteristics on page 422
31.10. Programming Via the JTAG Interface
Programming through the JTAG interface requires control of the four JTAG specific pins: TCK, TMS, TDI,
and TDO. Control of the Reset and clock pins is not required.
To be able to use the JTAG interface, the JTAGEN fuse must be programmed. The device is default
shipped with the Fuse programmed. In addition, the JTD bit in MCUCSR must be cleared. Alternatively, if
the JTD bit is set, the external reset can be forced low. Then, the JTD bit will be cleared after two chip
clocks, and the JTAG pins are available for programming. This provides a means of using the JTAG pins
as normal port pins in running mode while still allowing In-System Programming via the JTAG interface.
Note that this technique can not be used when using the JTAG pins for Boundary-scan or On-chip Debug.
In these cases the JTAG pins must be dedicated for this purpose.
As a definition in this data sheet, the LSB is shifted in and out first of all Shift Registers.
31.10.1. Programming Specific JTAG Instructions
The instruction register is 4-bit wide, supporting up to 16 instructions. The JTAG instructions useful for
Programming are listed below.
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The OPCODE for each instruction is shown behind the instruction name in hex format. The text describes
which data register is selected as path between TDI and TDO for each instruction.
The Run-Test/Idle state of the TAP controller is used to generate internal clocks. It can also be used as an
idle state between JTAG sequences. The state machine sequence for changing the instruction word is
shown in the figure below.
Figure 31-9 State Machine Sequence for Changing the Instruction Word
Test-Logic-Re s e t
Run-Te s t/Idle
Shift-DR
Exit1-DR
Paus e -DR
Exit2-DR
Update-DR
Se lect-IR S can
Ca pture -IR
Shift-IR
Exit1-IR
Paus e -IR
Exit2-IR
Update-IR
Se lect-DR Scan
Ca pture -DR
0
1
01 1 1
0 0
0 0
1 1
10
1
1
0
1
0
0
10
1
1
0
1
0
0
00
11
31.10.2. AVR_RESET (0xC)
The AVR specific public JTAG instruction for setting the AVR device in the Reset mode or taking the
device out from the Reset mode. The TAP controller is not reset by this instruction. The one bit Reset
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Register is selected as Data Register. Note that the reset will be active as long as there is a logic 'one' in
the Reset Chain. The output from this chain is not latched.
The active states are:
•Shift-DR: The Reset Register is shifted by the TCK input.
31.10.3. PROG_ENABLE (0x4)
The AVR specific public JTAG instruction for enabling programming via the JTAG port. The 16-bit
Programming Enable Register is selected as data register. The active states are the following:
•Shift-DR: the programming enable signature is shifted into the data register.
•Update-DR: the programming enable signature is compared to the correct value, and Programming
mode is entered if the signature is valid.
31.10.4. PROG_COMMANDS (0x5)
The AVR specific public JTAG instruction for entering programming commands via the JTAG port. The 15-
bit Programming Command Register is selected as data register. The active states are the following:
•Capture-DR: the result of the previous command is loaded into the data register.
•Shift-DR: the data register is shifted by the TCK input, shifting out the result of the previous
command and shifting in the new command.
•Update-DR: the programming command is applied to the Flash inputs.
•Run-Test/Idle: one clock cycle is generated, executing the applied command.
31.10.5. PROG_PAGELOAD (0x6)
The AVR specific public JTAG instruction to directly load the Flash data page via the JTAG port. The
1024-bit Virtual Flash Page Load Register is selected as data register. This is a virtual scan chain with
length equal to the number of bits in one Flash page. Internally the Shift Register is 8-bit. Unlike most
JTAG instructions, the Update-DR state is not used to transfer data from the Shift Register. The data are
automatically transferred to the Flash page buffer byte by byte in the Shift-DR state by an internal state
machine. This is the only active state:
•Shift-DR: Flash page data are shifted in from TDI by the TCK input, and automatically loaded into
the Flash page one byte at a time.
Note:  1. The JTAG instruction PROG_PAGELOAD can only be used if the AVR device is the first device
in JTAG scan chain. If the AVR cannot be the first device in the scan chain, the byte-wise programming
algorithm must be used.
31.10.6. PROG_PAGEREAD (0x7)
The AVR specific public JTAG instruction to read one full Flash data page via the JTAG port. The 1032-bit
Virtual Flash Page Read Register is selected as data register. This is a virtual scan chain with length
equal to the number of bits in one Flash page plus 8. Internally the Shift Register is 8-bit. Unlike most
JTAG instructions, the Capture-DR state is not used to transfer data to the Shift Register. The data are
automatically transferred from the Flash page buffer byte by byte in the Shift-DR state by an internal state
machine. This is the only active state:
•Shift-DR: Flash data are automatically read one byte at a time and shifted out on TDO by the TCK
input. The TDI input is ignored.
Note:  1. The JTAG instruction PROG_PAGEREAD can only be used if the AVR device is the first device
in JTAG scan chain. If the AVR cannot be the first device in the scan chain, the byte-wise programming
algorithm must be used.
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31.10.7. Data Registers
The data registers are selected by the JTAG instruction registers described in section Programming
Specific JTAG Instructions on page 400. The data registers relevant for programming operations are:
•Reset Register
•Programming Enable Register
•Programming Command Register
•Virtual Flash Page Load Register
•Virtual Flash Page Read Register
31.10.8. Reset Register
The Reset Register is a Test Data Register used to reset the part during programming. It is required to
reset the part before entering programming mode.
A high value in the Reset Register corresponds to pulling the external Reset low. The part is reset as long
as there is a high value present in the Reset Register. Depending on the Fuse settings for the clock
options, the part will remain reset for a Reset Time-Out Period (refer to Clock Sources) after releasing the
Reset Register. The output from this Data Register is not latched, so the reset will take place immediately,
as shown in figure Reset Register.
Related Links
Reset Register on page 342
Clock Sources on page 53
31.10.9. Programming Enable Register
The Programming Enable Register is a 16-bit register. The contents of this register is compared to the
programming enable signature, binary code 1010_0011_0111_0000. When the contents of the register is
equal to the programming enable signature, programming via the JTAG port is enabled. The Register is
reset to 0 on Power-on Reset, and should always be reset when leaving Programming mode.
Figure 31-10 Programming Enable Register
TDI
TDO
D
A
T
A
=D Q
ClockDR & P ROG_ENABLE
Programming e nable
$A370
31.10.10. Programming Command Register
The Programming Command Register is a 15-bit register. This register is used to serially shift in
programming commands, and to serially shift out the result of the previous command, if any. The JTAG
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Programming Instruction Set is shown in the following table. The state sequence when shifting in the
programming commands is illustrated in State Machine Sequence for Changing/Reading the Data Word
further down in this section.
Figure 31-11 Programming Command Register
TDI
TDO
S
T
R
O
B
E
S
A
D
D
R
E
S
S
/
D
A
T
A
Flash
EEPROM
Fuse s
Lock Bits
Table 31-16 JTAG Programming Instruction Set
a = address high bits, b = address low bits, H = 0 - Low byte, 1 - High Byte, o = data out, i = data in, x =
don’t care
Instruction TDI sequence TDO sequence Notes
1a. Chip erase 0100011_10000000
0110001_10000000
0110011_10000000
0110011_10000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
1b. Poll for chip erase complete 0110011_10000000 xxxxxox_xxxxxxxx (2)
2a. Enter Flash Write 0100011_00010000 xxxxxxx_xxxxxxxx
2b. Load Address High Byte 0000111_aaaaaaaa xxxxxxx_xxxxxxxx (9)
2c. Load Address Low Byte 0000011_bbbbbbbb xxxxxxx_xxxxxxxx
2d. Load Data Low Byte 0010011_iiiiiiii xxxxxxx_xxxxxxxx
2e. Load Data High Byte 0010111_iiiiiiii xxxxxxx_xxxxxxxx
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Instruction TDI sequence TDO sequence Notes
2f. Latch Data 0110111_00000000
1110111_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(1)
2g. Write Flash Page 0110111_00000000
0110101_00000000
0110111_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(1)
2h. Poll for Page Write complete 0110111_00000000 xxxxxox_xxxxxxxx (2)
3a. Enter Flash Read 0100011_00000010 xxxxxxx_xxxxxxxx
3b. Load Address High Byte 0000111_aaaaaaaa xxxxxxx_xxxxxxxx (9)
3c. Load Address Low Byte 0000011_bbbbbbbb xxxxxxx_xxxxxxxx
3d. Read Data Low and High Byte 0110010_00000000
0110110_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_oooooooo
xxxxxxx_oooooooo
low byte
high byte
4a. Enter EEPROM Write 0100011_00010001 xxxxxxx_xxxxxxxx
4b. Load Address High Byte 0000111_aaaaaaaa xxxxxxx_xxxxxxxx (9)
4c. Load Address Low Byte 0000011_bbbbbbbb xxxxxxx_xxxxxxxx
4d. Load Data Byte 0010011_iiiiiiii xxxxxxx_xxxxxxxx
4e. Latch Data 0110111_00000000
1110111_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(1)
4f. Write EEPROM Page 0110011_00000000
0110001_00000000
0110011_00000000
0110011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(1)
4g. Poll for Page Write complete 0110011_00000000 xxxxxox_xxxxxxxx (2)
5a. Enter EEPROM Read 0100011_00000011 xxxxxxx_xxxxxxxx
5b. Load Address High Byte 0000111_aaaaaaaa xxxxxxx_xxxxxxxx (9)
5c. Load Address Low Byte 0000011_bbbbbbbb xxxxxxx_xxxxxxxx
5d. Read Data Byte 0110011_bbbbbbbb
0110010_00000000
0110011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_oooooooo
6a. Enter Fuse Write 0100011_01000000 xxxxxxx_xxxxxxxx
6b. Load Data Low Byte(6) 0010011_iiiiiiii xxxxxxx_xxxxxxxx (3)
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Instruction TDI sequence TDO sequence Notes
6c. Write Fuse Extended byte 0111011_00000000
0111001_00000000
0111011_00000000
0111011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(1)
6d. Poll for Fuse Write complete 0110111_00000000 xxxxxox_xxxxxxxx (2)
6e. Load Data Low Byte(7) 0010011_iiiiiiii xxxxxxx_xxxxxxxx (3)
6f. Write Fuse High byte 0110111_00000000
0110101_00000000
0110111_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(1)
6g. Poll for Fuse Write complete 0110111_00000000 xxxxxox_xxxxxxxx (2)
6h. Load Data Low Byte(7) 0010011_iiiiiiii xxxxxxx_xxxxxxxx (3)
6i. Write Fuse Low byte 0110011_00000000
0110001_00000000
0110011_00000000
0110011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(1)
6j. Poll for Fuse Write complete 0110011_00000000 xxxxxox_xxxxxxxx (2)
7a. Enter Lock bit Write 0100011_00100000 xxxxxxx_xxxxxxxx
7b. Load Data Byte(9) 0010011_11iiiiii xxxxxxx_xxxxxxxx (4)
7c. Write Lock bits 0110011_00000000
0110001_00000000
0110011_00000000
0110011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(1)
7d. Poll for Lock bit Write complete 0110011_00000000 xxxxxox_xxxxxxxx (2)
8a. Enter Fuse/Lock bit Read 0100011_00000100 xxxxxxx_xxxxxxxx
8b. Read Extended Fuse Byte(6) 0111010_00000000
0111011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_oooooooo
8c. Read Fuse High Byte(7) 0111110_00000000
0111111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_oooooooo
8d. Read Fuse Low Byte(8) 0110010_00000000
0110011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_oooooooo
8e. Read Lock bits(9) 0110110_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxoooooo
(5)
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Instruction TDI sequence TDO sequence Notes
8f. Read Fuses and Lock bits 0111010_00000000
0111110_00000000
0110010_00000000
0110110_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_oooooooo
xxxxxxx_oooooooo
xxxxxxx_oooooooo
xxxxxxx_oooooooo
(5)
fuse ext. byte
fuse high byte
fuse low byte
lock bits
9a. Enter Signature Byte Read 0100011_00001000 xxxxxxx_xxxxxxxx
9b. Load Address Byte 0000011_bbbbbbbb xxxxxxx_xxxxxxxx
9c. Read Signature Byte 0110010_00000000
0110011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_oooooooo
10a. Enter Calibration Byte Read 0100011_00001000 xxxxxxx_xxxxxxxx
10b. Load Address Byte 0000011_bbbbbbbb xxxxxxx_xxxxxxxx
10c. Read Calibration Byte 0110110_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_oooooooo
Note: 
1. This command sequence is not required if the seven MSB are correctly set by the previous
command sequence (which is normally the case).
2. Repeat until o = “1”.
3. Set bits to “0” to program the corresponding fuse, “1” to unprogram the Fuse.
4. Set bits to “0” to program the corresponding lock bit, “1” to leave the Lock bit unchanged.
5. “0” = programmed, “1” = unprogrammed.
6. The bit mapping for Fuses Extended byte is listed in Table 31-3 Extended Fuse Byte on page 384
7. The bit mapping for Fuses High byte is listed in Table 31-4 Fuse High Byte on page 385
8. The bit mapping for Fuses Low byte is listed in Table 31-5 Fuse Low Byte on page 385
9. The bit mapping for Lock bits byte is listed in Table 31-1 Lock Bit Byte on page 383
10. Address bits exceeding PCMSB and EEAMSB (Command Byte Bit Coding and Page Size) are
don’t care
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Figure 31-12 State Machine Sequence for Changing/Reading the Data Word
Test-Logic-Res e t
Run-Te s t/Idle
Shift-DR
Exit1-DR
Pause -DR
Exit2-DR
Update -DR
Se le ct-IR S can
Ca pture-IR
Shift-IR
Exit1-IR
Pause -IR
Exit2-IR
Update -IR
Se le ct-DR S can
Ca pture-DR
0
1
01 1 1
0 0
0 0
1 1
10
1
1
0
1
0
0
10
1
1
0
1
0
0
00
11
Related Links
Page Size on page 387
31.10.11. Virtual Flash Page Load Register
The Virtual Flash Page Load Register is a virtual scan chain with length equal to the number of bits in one
Flash page. Internally the Shift Register is 8-bit, and the data are automatically transferred to the Flash
page buffer byte by byte. Shift in all instruction words in the page, starting with the LSB of the first
instruction in the page and ending with the MSB of the last instruction in the page. This provides an
efficient way to load the entire Flash page buffer before executing Page Write.
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Figure 31-13 Virtual Flash Page Load Register
TDI
TDO
D
A
T
A
Flash
EEPROM
Fuse s
Lock Bits
STROBES
ADDRESS
Sta te
ma chine
31.10.12. Virtual Flash Page Read Register
The Virtual Flash Page Read Register is a virtual scan chain with length equal to the number of bits in
one Flash page plus 8. Internally the Shift Register is 8-bit, and the data are automatically transferred
from the Flash data page byte by byte. The first eight cycles are used to transfer the first byte to the
internal Shift Register, and the bits that are shifted out during these 8 cycles should be ignored. Following
this initialization, data are shifted out starting with the LSB of the first instruction in the page and ending
with the MSB of the last instruction in the page. This provides an efficient way to read one full Flash page
to verify programming.
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Figure 31-14 Virtual Flash Page Read Register
TDI
TDO
D
A
T
A
Flash
EEPROM
Fuse s
Lock Bits
STROBES
ADDRESS
Sta te
ma chine
31.10.13. Programming Algorithm
All references below of type “1a”, “1b”, and so on, refer to Table 31-16 JTAG Programming Instruction
Set on page 404.
31.10.14. Entering Programming Mode
1. Enter JTAG instruction AVR_RESET and shift 1 in the Reset Register.
2. Enter instruction PROG_ENABLE and shift 1010_0011_0111_0000 in the Programming Enable
Register.
31.10.15. Leaving Programming Mode
1. Enter JTAG instruction PROG_COMMANDS.
2. Disable all programming instructions by using no operation instruction 11a.
3. Enter instruction PROG_ENABLE and shift 0000_0000_0000_0000 in the programming Enable
Register.
4. Enter JTAG instruction AVR_RESET and shift 0 in the Reset Register.
31.10.16. Performing Chip Erase
1. Enter JTAG instruction PROG_COMMANDS.
2. Start chip erase using programming instruction 1a.
3. Poll for chip erase complete using programming instruction 1b, or wait for tWLRH_CE (refer to table
Command Byte Bit Coding in section Parallel Programming Parameters, Pin Mapping, and
Commands).
Related Links
Parallel Programming Characteristics on page 421
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31.10.17. Programming the Flash
Before programming the Flash a Chip Erase must be performed. See Performing Chip Erase on page
410.
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable Flash write using programming instruction 2a.
3. Load address high byte using programming instruction 2b.
4. Load address low byte using programming instruction 2c.
5. Load data using programming instructions 2d, 2e and 2f.
6. Repeat steps 4 and 5 for all instruction words in the page.
7. Write the page using programming instruction 2g.
8. Poll for Flash write complete using programming instruction 2h, or wait for tWLRH (refer to table
Parallel Programming Characteristics, VCC = 5V ±10% in chapter Parallel Programming
Characteristics).
9. Repeat steps 3 to 7 until all data have been programmed.
A more efficient data transfer can be achieved using the PROG_PAGELOAD instruction:
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable Flash write using programming instruction 2a.
3. Load the page address using programming instructions 2b and 2c. PCWORD (refer to Table 31-12 
Command Byte Bit Coding on page 395) is used to address within one page and must be written as
0.
4. Enter JTAG instruction PROG_PAGELOAD.
5. Load the entire page by shifting in all instruction words in the page, starting with the LSB of the first
instruction in the page and ending with the MSB of the last instruction in the page.
6. Enter JTAG instruction PROG_COMMANDS.
7. Write the page using programming instruction 2g.
8. Poll for Flash write complete using programming instruction 2h, or wait for tWLRH (refer to table
Parallel Programming Characteristics, VCC = 5V ±10% in chapter Parallel Programming
Characteristics).
9. Repeat steps 3 to 8 until all data have been programmed.
Related Links
Parallel Programming Characteristics on page 421
Parallel Programming Characteristics on page 421
31.10.18. Reading the Flash
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable Flash read using programming instruction 3a.
3. Load address using programming instructions 3b and 3c.
4. Read data using programming instruction 3d.
5. Repeat steps 3 and 4 until all data have been read.
A more efficient data transfer can be achieved using the PROG_PAGEREAD instruction:
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable Flash read using programming instruction 3a.
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3. Load the page address using programming instructions 3b and 3c. PCWORD (refer to table
Command Byte Bit Coding in section Parallel Programming Parameters, Pin Mapping, and
Commands) is used to address within one page and must be written as 0.
4. Enter JTAG instruction PROG_PAGEREAD.
5. Read the entire page by shifting out all instruction words in the page, starting with the LSB of the
first instruction in the page and ending with the MSB of the last instruction in the page. Remember
that the first 8 bits shifted out should be ignored.
6. Enter JTAG instruction PROG_COMMANDS.
7. Repeat steps 3 to 6 until all data have been read.
Related Links
Parallel Programming Characteristics on page 421
31.10.19. Programming the EEPROM
Before programming the EEPROM a Chip Erase must be performed. See Performing Chip Erase on page
410.
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable EEPROM write using programming instruction 4a.
3. Load address high byte using programming instruction 4b.
4. Load address low byte using programming instruction 4c.
5. Load data using programming instructions 4d and 4e.
6. Repeat steps 4 and 5 for all data bytes in the page.
7. Write the data using programming instruction 4f.
8. Poll for EEPROM write complete using programming instruction 4g, or wait for tWLRH (refer to table
Parallel Programming Characteristics, VCC = 5V ±10% in chapter Parallel Programming
Characteristics).
9. Repeat steps 3 to 8 until all data have been programmed.
Note that the PROG_PAGELOAD instruction can not be used when programming the EEPROM
Related Links
Parallel Programming Characteristics on page 421
Parallel Programming Characteristics on page 421
31.10.20. Reading the EEPROM
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable EEPROM read using programming instruction 5a.
3. Load address using programming instructions 5b and 5c.
4. Read data using programming instruction 5d.
5. Repeat steps 3 and 4 until all data have been read.
Note that the PROG_PAGEREAD instruction can not be used when reading the EEPROM
31.10.21. Programming the Fuses
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable Fuse write using programming instruction 6a.
3. Load data byte using programming instructions 6b. A bit value of “0” will program the corresponding
fuse, a “1” will unprogram the fuse.
4. Write Extended Fuse byte using programming instruction 6c.
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5. Poll for Fuse write complete using programming instruction 6d, or wait for tWLRH (refer to table
Parallel Programming Characteristics, VCC = 5V ±10% in chapter Parallel Programming
Characteristics).
6. Load data byte using programming instructions 6e. A bit value of “0” will program the corresponding
fuse, a “1” will unprogram the fuse.
7. Write Fuse high byte using programming instruction 6f.
8. Poll for Fuse write complete using programming instruction 6g, or wait for tWLRH (refer to table
Parallel Programming Characteristics, VCC = 5V ±10% in chapter Parallel Programming
Characteristics).
9. Load data byte using programming instructions 6h. A “0” will program the fuse, a “1” will unprogram
the fuse.
10. Write Fuse low byte using programming instruction 6i.
11. Poll for Fuse write complete using programming instruction 6j, or wait for tWLRH (refer to table
Parallel Programming Characteristics, VCC = 5V ±10% in chapter Parallel Programming
Characteristics).
Related Links
Parallel Programming Characteristics on page 421
Parallel Programming Characteristics on page 421
31.10.22. Programming the Lock Bits
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable Lock bit write using programming instruction 7a.
3. Load data using programming instructions 7b. A bit value of “0” will program the corresponding lock
bit, a “1” will leave the lock bit unchanged.
4. Write Lock bits using programming instruction 7c.
5. Poll for Lock bit write complete using programming instruction 7d, or wait for tWLRH (refer to table
Parallel Programming Characteristics, VCC = 5V ±10% in chapter Parallel Programming
Characteristics).
Related Links
Parallel Programming Characteristics on page 421
31.10.23. Reading the Fuses and Lock Bits
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable Fuse/Lock bit read using programming instruction 8a.
3. –To read all Fuses and Lock bits, use programming instruction 8f.
–To only read Extended Fuse byte, use programming instruction 8b.
–To only read Fuse high byte, use programming instruction 8c.
–To only read Fuse low byte, use programming instruction 8d.
–To only read Lock bits, use programming instruction 8e.
31.10.24. Reading the Signature Bytes
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable Signature byte read using programming instruction 9a.
3. Load address 0x00 using programming instruction 9b.
4. Read first signature byte using programming instruction 9c.
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5. Repeat steps 3 and 4 with address 0x01 and address 0x02 to read the second and third signature
bytes, respectively.
31.10.25. Reading the Calibration Byte
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable Calibration byte read using programming instruction 10a.
3. Load address 0x00 using programming instruction 10b.
4. Read the calibration byte using programming instruction 10c.
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32. Electrical Characteristics – TA = -40°C to 85°C
Table 32-1 Absolute Maximum Ratings*
Operating
Temperature
-55°C to +125°C *NOTICE: Stresses beyond those listed under
“Absolute Maximum Ratings” may cause permanent
damage to the device. This is a stress rating only and
functional operation of the device at these or other
conditions beyond those indicated in the operational
sections of this specification is not implied. Exposure
to absolute maximum rating conditions for extended
periods may affect device reliability.
Storage Temperature -65°C to +150°C
Voltage on any Pin
except RESET
with respect to
Ground
-0.5V to VCC+0.5V
Voltage on RESET
with respect to
Ground
-0.5V to +13.0V
Maximum Operating
Voltage
6.0V
DC Current per I/O
Pin
40.0mA
DC Current VCC and
GND Pins
200.0 - 400.0mA
32.1. DC Characteristics
Table 32-2 TA = -40°C to 85°C, VCC = 2.7V to 5.5V (unless otherwise noted)
Symbol Parameter Condition Min Typ Max Units
VIL Input Low Voltage except XTAL1
and RESET pins
VCC = 2.7 - 5.5V -0.5 0.2 VCC(1)
V
VIH Input High Voltage except XTAL1
and RESET pins
VCC = 2.7 - 5.5V 0.6
VCC(2)
VCC + 0.5
VIL1 Input Low Voltage
XTAL1 pin
VCC = 2.7 - 5.5V -0.5 0.1 VCC(1)
VIH1 Input High Voltage
XTAL 1 pin
VCC = 2.7 - 5.5V 0.7
VCC(2)
VCC + 0.5
VIL2 Input Low Voltage
RESET pin
VCC = 2.7 - 5.5V -0.5 0.2 VCC(1)
VIH2 Input High Voltage
RESET pin
VCC = 2.7 - 5.5V 0.85
VCC(2)
VCC + 0.5
VOL Output Low Voltage(3)
(Ports A,B,C,D,E,F,G)
IOL = 20mA, VCC = 5V
IOL = 10mA, VCC = 3V
0.9
0.6
VOH Output High Voltage(4)
(Ports A,B,C,D,E,F,G)
IOH = -20mA, VCC = 5V
IOH = -10mA, VCC = 3V
4.2
2.2
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Symbol Parameter Condition Min Typ Max Units
IIL Input Leakage
Current I/O Pin
VCC = 5.5V, pin low
(absolute value)
1.0
μA
IIH Input Leakage
Current I/O Pin
VCC = 5.5V, pin high
(absolute value)
1.0
RRST Reset Pull-up Resistor 30 60
kΩRPEN PEN Pull-up Resistor 30 60
RPU I/O Pin Pull-up Resistor 20 50
ICC
Power Supply Current Active 4MHz, VCC = 3V 2.5 5
mA
Active 8MHz, VCC = 5V 8.1 20
Idle 4MHz, VCC = 3V 0.7 2
Idle 8MHz, VCC = 5V 2.8 12
Power-down mode(5) WDT enabled, VCC = 3V <10 20 μA
WDT disabled, VCC = 3V <4 10
VACIO Analog Comparator
Input Offset Voltage
VCC = 5V
Vin = VCC/2
-40 40 mV
IACLK Analog Comparator
Input Leakage Current
VCC = 5V
Vin = VCC/2
-50 50 nA
tACPD Analog Comparator
Propagation Delay
VCC = 2.7V
VCC = 5.0V
750
500 ns
Note: 
1. “Max” means the highest value where the pin is guaranteed to be read as low
2. “Min” means the lowest value where the pin is guaranteed to be read as high
3. Although each I/O port can sink more than the test conditions (20mA at VCC = 5V, 10mA at VCC =
3V) under steady state conditions (non-transient), the following must be observed:
TQFP and QFN/MLF Package:
1. The sum of all IOL, for all ports, should not exceed 400mA.
2. The sum of all IOL, for ports A0 - A7, G2, C3 - C7 should not exceed 100mA.
3. The sum of all IOL, for ports C0 - C2, G0 - G1, D0 - D7, XTAL2 should not exceed 100mA.
4. The sum of all IOL, for ports B0 - B7, G3 - G4, E0 - E7 should not exceed 100mA.
5. The sum of all IOL, for ports F0 - F7, should not exceed 100mA.
If IOL exceeds the test condition, VOL may exceed the related specification. Pins are not guaranteed
to sink current greater than the listed test condition.
4. Although each I/O port can source more than the test conditions (20mA at Vcc = 5V, 10mA at Vcc =
3V) under steady state conditions (non-transient), the following must be observed:
TQFP and QFN/MLF Package:
1. The sum of all IOH, for all ports, should not exceed 400mA.
2. The sum of all IOH, for ports A0 - A7, G2, C3 - C7 should not exceed 100mA.
3. The sum of all IOH, for ports C0 - C2, G0 - G1, D0 - D7, XTAL2 should not exceed 100mA.
4. The sum of all IOH, for ports B0 - B7, G3 - G4, E0 - E7 should not exceed 100mA.
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5. The sum of all IOH, for ports F0 - F7, should not exceed 100mA.
If IOH exceeds the test condition, VOH may exceed the related specification. Pins are not
guaranteed to source current greater than the listed test condition.
5. Minimum VCC for Power-down is 2.5V.
Related Links
External Clock on page 57
32.2. Speed Grades
Figure 32-1 Maximum Frequency vs. Vcc
2.7V 4.5V 5.5V
Sa fe Ope rating Area
16 MHz
8 MHz
32.3. Clock Characteristics
Related Links
External Clock on page 57
32.3.1. External Clock Drive Waveforms
Figure 32-2 External Clock Drive Waveforms
VIL1
VIH1
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32.3.2. External Clock Drive
Table 32-3 External Clock Drive(1)
Symbol Parameter VCC = 2.7V to 5.5V VCC = 4.5V to 5.5V Units
Min Max Min Max
1/tCLCL Oscillator Frequency 0 8 0 16 MHz
tCLCL Clock Period 125 62.5 ns
tCHCX High Time 50 25 ns
tCLCX Low Time 50 25 ns
tCLCH Rise Time 1.6 0.5 μs
tCHCL Fall Time 1.6 0.5 μs
ΔtCLCL Change in period from one clock cycle to the
next
2 2 %
Note:  1. Refer to External Clock for details.
Table 32-4 External RC Oscillator, Typical Frequencies
R [kΩ](1) C [pF] f(2)
31.5 20 650kHz
6.5 20 2.0MHz
Note: 
1. R should be in the range 3kΩ - 100kΩ, and C should be at least 20pF. The C values given in the
table includes pin capacitance. This will vary with package type.
2. The frequency will vary with package type and board layout.
32.4. System and Reset Characteristics
Table 32-5 Reset, Brown-out and Internal Voltage Reference Characteristics
Symbol Parameter Condition Min Typ Max Units
VPOT Power-on Reset Threshold Voltage (rising)(1) 1.4 2.3 V
Power-on Reset Threshold Voltage (falling) 1.3 2.3 V
VRST RESET Pin Threshold Voltage 0.2VCC 0.85VCC V
tRST Pulse width on RESET Pin 1.5 μs
VBOT Brown-out Reset Threshold Voltage(2) BODLEVEL = 0 3.6 4.0 4.2 V
BODLEVEL = 1 2.5 2.7 2.9 V
tBOD Minimum low voltage period for Brown-out
Detection
BODLEVEL = 0 2 μs
BODLEVEL = 1 2 μs
VHYST Brown-out Detector hysteresis 120 mV
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v c — 0.4V moans W Ch 1/ c — 0.4V 300ns W (3,, Atmel
Symbol Parameter Condition Min Typ Max Units
VBG Bandgap reference voltage 1.15 1.23 1.35 V
tBG Bandgap reference start-up time 40 70 μs
IBG Bandgap reference current consumption 10 μA
Note: 
1. The Power-on Reset will not work unless the supply voltage has been below VPOT (falling).
2. VBOT may be below nominal minimum operating voltage for some devices. For devices where this
is the case, the device is tested down to VCC = VBOT during the production test. This guarantees
that a Brown-out Reset will occur before VCC drops to a voltage where correct operation of the
microcontroller is no longer guaranteed. The test is performed using BODLEVEL = 0 and
BODLEVEL = 1.
32.5. Two-wire Serial Interface Characteristics
The table below describes the requirements for devices connected to the Two-wire Serial Bus. The
ATmega64A Two-wire Serial Interface meets or exceeds these requirements under the noted conditions.
Timing symbols refer to Figure 32-3 Two-wire Serial Bus Timing on page 420.
Table 32-6 Two-wire Serial Bus Requirements
Symbol Parameter Condition Min Max Units
VIL Input Low-voltage -0.5 0.3VCC V
VIH Input High-voltage 0.7VCC VCC + 0.5 V
Vhys(1) Hysteresis of Schmitt Trigger
Inputs
0.05VCC(2) – V
VOL(1) Output Low-voltage 3mA sink current 0 0.4 V
tr(1) Rise Time for both SDA and
SCL
20 + 0.1Cb(3)(2) 300 ns
tof(1) Output Fall Time from VIHmin to
VILmax
10pF < Cb < 400pF(3) 20 + 0.1Cb(3)(2) 250 ns
tSP(1) Spikes Suppressed by Input
Filter
0 50(2) ns
IiInput Current each I/O Pin 0.1VCC < Vi < 0.9VCC -10 10 μA
Ci(1) Capacitance for each I/O Pin – 10 pF
fSCL SCL Clock Frequency fCK(4) > max(16fSCL,
250kHz)(5)
0 400 kHz
Rp Value of Pull-up resistor fSCL ≤ 100kHz CC 0.4V
3mA
1000ns
fSCL > 100kHz CC 0.4V
3mA
300ns
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AAAAA
Symbol Parameter Condition Min Max Units
tHD;STA Hold Time (repeated) START
Condition
fSCL ≤ 100kHz 4.0 – μs
fSCL > 100kHz 0.6 – μs
tLOW Low Period of the SCL Clock fSCL ≤ 100kHz 4.7 – μs
fSCL > 100kHz 1.3 – μs
tHIGH High period of the SCL clock fSCL ≤ 100kHz 4.0 – μs
fSCL > 100kHz 0.6 – μs
tSU;STA Set-up time for a repeated
START condition
fSCL ≤ 100kHz 4.7 – μs
fSCL > 100kHz 0.6 – μs
tHD;DAT Data hold time fSCL ≤ 100kHz 0 3.45 μs
fSCL > 100kHz 0 0.9 μs
tSU;DAT Data setup time fSCL ≤ 100kHz 250 – ns
fSCL > 100kHz 100 – ns
tSU;STO Setup time for STOP condition fSCL ≤ 100kHz 4.0 – μs
fSCL > 100kHz 0.6 – μs
tBUF Bus free time between a STOP
and START condition
fSCL ≤ 100kHz 4.7 – μs
Note: 
1. In ATmega64A, this parameter is characterized and not 100% tested.
2. Required only for fSCL > 100kHz.
3. Cb = capacitance of one bus line in pF.
4. fCK = CPU clock frequency
5. This requirement applies to all ATmega64A Two-wire Serial Interface operation. Other devices
connected to the Two-wire Serial Bus need only obey the general fSCL requirement.
Figure 32-3 Two-wire Serial Bus Timing
tSU;S TA
tLOW
tHIGH
tLOW
tof
tHD;STA tHD;DAT tSU;DAT tSU;S TO
tBUF
SCL
SDA
tr
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mm Ammss mm) mm mm) mm mm DAM mm mm»- r—Hf—Hr—Hf—Hl—H fl‘—'W ‘#4/_\ LOAD Ammss HtAD mm kLAD mm LOAD/“muss r—Hr—Hr—HI—H L—kzi — ~ r Atmel
32.6. Parallel Programming Characteristics
Figure 32-4 Parallel Programming Timing, Including some General Timing Requirements
Data & Contol
(DATA, XA0/1, BS1, BS2)
XTAL1
tXHXL
tWLWH
tDVXH tXLDX
tPLWL
tWLRH
WR
RDY/BSY
PAGEL
tPHPL
tPLBX
tBVPH
tXLWL
tWLBX
tBVWL
WLRL
Figure 32-5 Parallel Programming Timing, Loading Sequence with Timing Requirements(1)
XTAL1
PAGEL
tPLXH
XLXH
ttXLPH
ADDR0 (Low Byte) DATA (Low Byte) DATA (High Byte) ADDR1 (Low Byte)
DATA
BS1
XA0
XA1
LOAD ADDRESS
(LOW BYTE)
LOAD DATA
(LOW BYTE)
LOAD DATA
(HIGH BYTE)
LOAD DATA
LOAD ADDRESS
(LOW BYTE)
Note:  1. The timing requirements shown in the first figure in this section (i.e., tDVXH, tXHXL, and tXLDX)
also apply to loading operation.
Figure 32-6 Parallel Programming Timing, Reading Sequence (within the same Page) with Timing
Requirements(1)
XTAL1
OE
ADDR0 (Low Byte) DATA (Low Byte) DATA (High Byte) ADDR1 (Low Byte)
DATA
BS1
XA0
XA1
LOAD ADDRESS
(LOW BYTE)
READ DATA
(LOW BYTE)
READ DATA
(HIGH BYTE)
LOAD ADDRESS
(LOW BYTE)
tBVDV
tOLDV
tXLOL
tOHDZ
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Note:  1. The timing requirements shown in the first figure in this section (i.e., tDVXH, tXHXL, and tXLDX)
also apply to reading operation.
Table 32-7 Parallel Programming Characteristics, VCC = 5V ± 10%
Symbol Parameter Min Typ Max Units
VPP Programming Enable Voltage 11.5 12.5 V
IPP Programming Enable Current 250 μA
tDVXH Data and Control Valid before XTAL1 High 67 ns
tXLXH XTAL1 Low to XTAL1 High 200 ns
tXHXL XTAL1 Pulse Width High 150 ns
tXLDX Data and Control Hold after XTAL1 Low 67 ns
tXLWL XTAL1 Low to WR Low 0 ns
tXLPH XTAL1 Low to PAGEL high 0 ns
tPLXH PAGEL low to XTAL1 high 150 ns
tBVPH BS1 Valid before PAGEL High 67 ns
tPHPL PAGEL Pulse Width High 150 ns
tPLBX BS1 Hold after PAGEL Low 67 ns
tWLBX BS2/1 Hold after WR Low 67 ns
tPLWL PAGEL Low to WR Low 67 ns
tBVWL BS1 Valid to WR Low 67 ns
tWLWH WR Pulse Width Low 150 ns
tWLRL WR Low to RDY/BSY Low 0 1 μs
tWLRH WR Low to RDY/BSY High(1) 3.7 4.5 ms
tWLRH_CE WR Low to RDY/BSY High for Chip Erase(2) 7.5 9 ms
tXLOL XTAL1 Low to OE Low 0 ns
tBVDV BS1 Valid to DATA valid 0 250 ns
tOLDV OE Low to DATA Valid 250 ns
tOHDZ OE High to DATA Tri-stated 250 ns
Note: 
1. tWLRH is valid for the Write Flash, Write EEPROM, Write Fuse Bits and Write Lock Bits commands.
2. tWLRH_CE is valid for the Chip Erase command.
32.7. SPI Timing Characteristics
See figures below for details.
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WL w \r VA VNN
Table 32-8 SPI Timing Parameters
Description Mode Min Typ Max
1 SCK period Master See Table 24-5 Relationship between SCK and
Oscillator Frequency on page 242
ns
2 SCK high/low Master 50% duty cycle
3 Rise/Fall time Master 3.6
4 Setup Master 10
5 Hold Master 10
6 Out to SCK Master 0.5 • tSCK
7 SCK to out Master 10
8 SCK to out high Master 10
9 SS low to out Slave 15
10 SCK period Slave 4 • tck
11 SCK high/low(1) Slave 2 • tck
12 Rise/Fall time Slave 1.6
13 Setup Slave 10
14 Hold Slave 10
15 SCK to out Slave 15
16 SCK to SS high Slave 20
17 SS high to tri-state Slave 10
18 SS low to SCK Salve 2 • tck
Note: 
1. In SPI Programming mode the minimum SCK high/low period is:
- 2tCLCL for fCK < 12MHz
- 3tCLCL for fCK > 12MHz
Figure 32-7 SPI interface timing requirements (Master Mode)
MOS I
(Data Output)
SCK
(CP OL = 1)
MISO
(Data Input)
SCK
(CP OL = 0)
SS
MSB LSB
LSBMSB
...
...
6 1
2 2
34 5
8
7
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SPI interface timing requirements (Slave Mode)
MISO
(Data Output)
SCK
(CP OL = 1)
MOS I
(Data Input)
SCK
(CP OL = 0)
SS
MSB LSB
LSBMSB
...
...
10
11 11
1213 14
17
15
9
X
16
18
32.8. ADC Characteristics
Table 32-9 ADC Characteristics, Single Ended Channels, -40°C – 85°C
Symbol Parameter Condition Min Typ Max Units
Resolution Single Ended Conversion 10 Bits
Absolute accuracy (Including
INL, DNL, Quantization Error,
Gain, and Offset Error)
Single Ended Conversion VREF = 4V,
VCC = 4V ADC clock = 200kHz
1.5
Single Ended Conversion VREF = 4V,
VCC = 4V ADC clock = 1MHz
3
Single Ended Conversion VREF = 4V,
VCC = 4V ADC clock = 200kHz Noise
Reduction mode
1.5 LSB
Single Ended Conversion VREF = 4V,
VCC = 4V ADC clock = 1MHz Noise
Reduction mode
3 LSB
Integral Non-linearity (INL) Single Ended Conversion
VREF = 4V, VCC = 4V
ADC clock = 200kHz
0.75 LSB
Differential Non-linearity
(DNL)
Single Ended Conversion
VREF = 4V, VCC = 4V
ADC clock = 200kHz
0.25 LSB
Gain Error Single Ended Conversion
VREF = 4V, VCC = 4V
ADC clock = 200kHz
0.75 LSB
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Symbol Parameter Condition Min Typ Max Units
Offset Error Single Ended Conversion
VREF = 4V, VCC = 4V
ADC clock = 200kHz
0.75 LSB
Clock Frequency 50 1000 kHz
Conversion Time Free Running Conversion 13 260 μs
AVCC Analog Supply Voltage VCC -
0.3(1)
VCC +
0.3(2)
V
VREF Reference Voltage 2.0 AVCC V
VIN Input voltage GND VREF V
Input bandwidth 38.5 kHz
VINT Internal Voltage Reference 2.4 2.56 2.8 V
RREF Reference Input Resistance 32 kΩ
RAIN Analog Input Resistance 100 MΩ
Note: 
1. Minimum for AVCC is 2.7V.
2. Maximum for AVCC is 5.5V.
Table 32-10 ADC Characteristics, Differential Channels
Symbol Parameter Condition Min(1) Typ(1) Max(1) Units
Resolution
Gain = 1x 10 Bits
Gain = 10x 10 Bits
Gain = 200x 10 Bits
Absolute accuracy
Gain = 1x
VREF = 4V, VCC = 5V ADC clock = 50 -
200kHz
16 LSB
Gain = 10xVREF = 4V, VCC = 5V ADC
clock = 50 - 200kHz
16 LSB
Gain = 200xVREF = 4V, VCC = 5V ADC
clock = 50 - 200kHz
8 LSB
Integral Non-linearity
(INL)
(Accuracy after
Calibration for Offset and
Gain Error)
Gain = 1x
VREF = 4V, VCC = 5V ADC clock = 50 -
200kHz
0.75 LSB
Gain = 10x VREF = 4V, VCC = 5V ADC
clock = 50 - 200kHz
0.75 LSB
Gain = 200x VREF = 4V, VCC = 5V
ADC clock = 50 - 200kHz
2.5 LSB
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Symbol Parameter Condition Min(1) Typ(1) Max(1) Units
Gain Error
Gain = 1x 1.6 %
Gain = 10x 1.6 %
Gain = 200x 0.3 %
Offset Error
Gain = 1x
VREF = 4V, VCC = 5V ADC clock = 50 -
200kHz
1.5 LSB
Gain = 10x
VREF = 4V, VCC = 5V ADC clock = 50 -
200kHz
1 LSB
Gain = 200x
VREF = 4V, VCC = 5V ADC clock = 50 -
200kHz
6 LSB
Clock Frequency 50 200 kHz
Conversion Time 13 260 μs
AVCC Analog Supply Voltage VCC -
0.3(1)
VCC +
0.3(2)
V
VREF Reference Voltage 2.0 AVCC - 0.5 V
VIN Input voltage GND VCC V
VDIFF Input Differential Voltage -VREF/
Gain
VREF/Gain V
ADC Conversion Output -511 511 LSB
Input Bandwidth 4 kHz
VINT Internal Voltage
Reference
2.3 2.56 2.7 V
RREF Reference Input
Resistance
32 kΩ
RAIN Analog Input Resistance 100 MΩ
Note: 
1. Minimum for AVCC is 2.7V.
2. Maximum for AVCC is 5.5V.
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32.9. External Data Memory Timing
Table 32-11 External Data Memory Characteristics, 4.5V - 5.5V, No Wait-state
Symbol Parameter 8MHz Oscillator Variable Oscillator Unit
Min Max Min Max
0 1/tCLCL Oscillator Frequency 0.0 16 MHz
1 tLHLL ALE Pulse Width 115 1.0tCLCL-10 ns
2 tAVLL Address Valid A to ALE Low 57.5 0.5tCLCL-5(1) ns
3a tLLAX_ST Address Hold After ALE Low,
write access
5 5 ns
3b tLLAX_LD Address Hold after ALE Low,
read access
5 5 ns
4 tAVLLC Address Valid C to ALE Low 57.5 0.5tCLCL-5(1) ns
5 tAVRL Address Valid to RD Low 115 1.0tCLCL-10 ns
6 tAVWL Address Valid to WR Low 115 1.0tCLCL-10 ns
7 tLLWL ALE Low to WR Low 47.5 67.5 0.5tCLCL-15(2) 0.5tCLCL+5(2) ns
8 tLLRL ALE Low to RD Low 47.5 67.5 0.5tCLCL-15(2) 0.5tCLCL+5(2) ns
9 tDVRH Data Setup to RD High 40 40 ns
10 tRLDV Read Low to Data Valid 75 1.0tCLCL-50 ns
11 tRHDX Data Hold After RD High 0 0 ns
12 tRLRH RD Pulse Width 115 1.0tCLCL-10 ns
13 tDVWL Data Setup to WR Low 42.5 0.5tCLCL-20(1) ns
14 tWHDX Data Hold After WR High 115 1.0tCLCL-10 ns
15 tDVWH Data Valid to WR High 125 1.0tCLCL ns
16 tWLWH WR Pulse Width 115 1.0tCLCL-10 ns
Note: 
1. This assumes 50% clock duty cycle. The half period is actually the high time of the external clock,
XTAL1.
2. This assumes 50% clock duty cycle. The half period is actually the low time of the external clock,
XTAL1.
Table 32-12 External Data Memory Characteristics, 4.5V - 5.5V, 1 Cycle Wait-state
Symbol Parameter 8MHz Oscillator Variable Oscillator Unit
Min Max Min Max
0 1/tCLCL Oscillator Frequency 0.0 16 MHz
10 tRLDV Read Low to Data Valid 200 2.0tCLCL-50 ns
12 tRLRH RD Pulse Width 240 2.0tCLCL-10 ns
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Symbol Parameter 8MHz Oscillator Variable Oscillator Unit
Min Max Min Max
15 tDVWH Data Valid to WR High 240 2.0tCLCL ns
16 tWLWH WR Pulse Width 240 2.0tCLCL-10 ns
Table 32-13 External Data Memory Characteristics, 4.5V - 5.5V, SRWn1 = 1, SRWn0 = 0
Symbol Parameter 4MHz Oscillator Variable Oscillator Unit
Min Max Min Max
0 1/tCLCL Oscillator Frequency 0.0 16 MHz
10 tRLDV Read Low to Data Valid 325 3.0tCLCL-50 ns
12 tRLRH RD Pulse Width 365 3.0tCLCL-10 ns
15 tDVWH Data Valid to WR High 375 3.0tCLCL ns
16 tWLWH WR Pulse Width 365 3.0tCLCL-10 ns
Table 32-14 External Data Memory Characteristics, 4.5V - 5.5V, SRWn1 = 1, SRWn0 = 1
Symbol Parameter 4MHz Oscillator Variable Oscillator Unit
Min Max Min Max
0 1/tCLCL Oscillator Frequency 0.0 16 MHz
10 tRLDV Read Low to Data Valid 325 3.0tCLCL-50 ns
12 tRLRH RD Pulse Width 365 3.0tCLCL-10 ns
14 tWHDX Data Hold After WR High 240 2.0tCLCL-10 ns
15 tDVWH Data Valid to WR High 375 3.0tCLCL ns
16 tWLWH WR Pulse Width 365 3.0tCLCL-10 ns
Table 32-15 External Data Memory Characteristics, 2.7V - 5.5V, No Wait-state
Symbol Parameter 4MHz Oscillator Variable Oscillator Unit
Min Max Min Max
0 1/tCLCL Oscillator Frequency 0.0 8 MHz
1 tLHLL ALE Pulse Width 235 tCLCL-15 ns
2 tAVLL Address Valid A to ALE Low 115 0.5tCLCL-10(1) ns
3a tLLAX_ST Address Hold After ALE Low,
write access
5 5 ns
3b tLLAX_LD Address Hold after ALE Low,
read access
5 5 ns
4 tAVLLC Address Valid C to ALE Low 115 0.5tCLCL-10(1) ns
5 tAVRL Address Valid to RD Low 235 1.0tCLCL-15 ns
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Symbol Parameter 4MHz Oscillator Variable Oscillator Unit
Min Max Min Max
6 tAVWL Address Valid to WR Low 235 1.0tCLCL-15 ns
7 tLLWL ALE Low to WR Low 115 130 0.5tCLCL-10(2) 0.5tCLCL+5(2) ns
8 tLLRL ALE Low to RD Low 115 130 0.5tCLCL-10(2) 0.5tCLCL+5(2) ns
9 tDVRH Data Setup to RD High 45 45 ns
10 tRLDV Read Low to Data Valid 190 1.0tCLCL-60 ns
11 tRHDX Data Hold After RD High 0 0 ns
12 tRLRH RD Pulse Width 235 1.0tCLCL-15 ns
13 tDVWL Data Setup to WR Low 105 0.5tCLCL-20(1) ns
14 tWHDX Data Hold After WR High 235 1.0tCLCL-15 ns
15 tDVWH Data Valid to WR High 250 1.0tCLCL ns
16 tWLWH WR Pulse Width 235 1.0tCLCL-15 ns
Note: 
1. This assumes 50% clock duty cycle. The half period is actually the high time of the external clock,
XTAL1.
2. This assumes 50% clock duty cycle. The half period is actually the low time of the external clock,
XTAL1.
Table 32-16 External Data Memory Characteristics, 2.7V - 5.5V, SRWn1 = 0, SRWn0 = 1
Symbol Parameter 4MHz Oscillator Variable Oscillator Unit
Min Max Min Max
0 1/tCLCL Oscillator Frequency 0.0 8 MHz
10 tRLDV Read Low to Data Valid 440 2.0tCLCL-60 ns
12 tRLRH RD Pulse Width 485 2.0tCLCL-15 ns
15 tDVWH Data Valid to WR High 500 2.0tCLCL ns
16 tWLWH WR Pulse Width 485 2.0tCLCL-15 ns
Table 32-17 External Data Memory Characteristics, 2.7V - 5.5V, SRWn1 = 1, SRWn0 = 0
Symbol Parameter 4MHz Oscillator Variable Oscillator Unit
Min Max Min Max
0 1/tCLCL Oscillator Frequency 0.0 8 MHz
10 tRLDV Read Low to Data Valid 690 3.0tCLCL-60 ns
12 tRLRH RD Pulse Width 735 3.0tCLCL-15 ns
15 tDVWH Data Valid to WR High 750 3.0tCLCL ns
16 tWLWH WR Pulse Width 735 3.0tCLCL-15 ns
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Table 32-18 External Data Memory Characteristics, 2.7V - 5.5 V, SRWn1 = 1, SRWn0 = 1
Symbol Parameter 4MHz Oscillator Variable Oscillator Unit
Min Max Min Max
0 1/tCLCL Oscillator Frequency 0.0 8 MHz
10 tRLDV Read Low to Data Valid 690 3.0tCLCL-60 ns
12 tRLRH RD Pulse Width 735 3.0tCLCL-15 ns
14 tWHDX Data Hold After WR High 485 2.0tCLCL-15 ns
15 tDVWH Data Valid to WR High 750 3.0tCLCL ns
16 tWLWH WR Pulse Width 735 3.0tCLCL-15 ns
Figure 32-8 External Memory Timing (SRWn1 = 0, SRWn0 = 0
ALE
T1 T2 T3
Write
Re a d
WR
T4
A15:8 Addres s
Prev. a ddr.
DA7:0 Addre s s Da taP rev. data XX
RD
DA7:0 (XMBK = 0) Data
Addre s s
Sys te m Clock (CLKCPU)
1
4
2
7
6
3a
3b
5
8 12
16
13
10
11
14
15
9
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Figure 32-9 External Memory Timing (SRWn1 = 0, SRWn0 = 1)
ALE
T1 T2 T3
Write
Re a d
WR
T5
A15:8 Addre s s
Prev. a ddr.
DA7:0 Addre s s Data
Prev. da ta XX
RD
DA7:0 (XMBK = 0) Data
Addre s s
Sys te m Clock (CLKCP U)
1
4
2
7
6
3a
3b
5
8 12
16
13
10
11
14
15
9
T4
Figure 32-10 External Memory Timing (SRWn1 = 1, SRWn0 = 0)
ALE
T1 T2 T3
Write
Read
WR
T6
A15:8 Addre ss
Prev. addr.
DA7:0 Addre s s DataPrev. data XX
RD
DA7:0 (XMBK = 0) Data
Address
System Clock (CLKCP U)
1
4
2
7
6
3a
3b
5
8 12
16
13
10
11
14
15
9
T4 T5
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Figure 32-11 External Memory Timing (SRWn1 = 1, SRWn0 = 1)
ALE
T1 T2 T3
Write
Rea d
WR
T7
A15:8 Addres s
Prev. a ddr.
DA7:0 Addre ss DataPrev. da ta XX
RD
DA7:0 (XMBK = 0) Da ta
Addres s
Syste m Clock (CLKCP U )
1
4
2
7
6
3a
3b
5
8 12
16
13
10
11
14
15
9
T4 T5 T6
The ALE pulse in the last period (T4-T7) is only present if the next instruction accesses the RAM (internal
or external).
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33. Electrical Characteristics – TA = -40°C to 105°C
Table 33-1 Absolute Maximum Ratings*
Operating
Temperature
-55°C to +125°C *NOTICE: Stresses beyond those listed under
“Absolute Maximum Ratings” may cause permanent
damage to the device. This is a stress rating only and
functional operation of the device at these or other
conditions beyond those indicated in the operational
sections of this specification is not implied. Exposure
to absolute maximum rating conditions for extended
periods may affect device reliability.
Storage Temperature -65°C to +150°C
Voltage on any Pin
except RESET
with respect to
Ground
-0.5V to VCC+0.5V
Voltage on RESET
with respect to
Ground
-0.5V to +13.0V
Maximum Operating
Voltage
6.0V
DC Current per I/O
Pin
40.0mA
DC Current VCC and
GND Pins
200.0 - 400.0mA
33.1. DC Characteristics
Table 33-2 TA = -40°C to 105°C, VCC = 2.7V to 5.5V (unless otherwise noted)
Symbol Parameter Condition Min Typ Max Units
VIL Input Low Voltage except XTAL1
and RESET pins
VCC = 2.7 - 5.5V -0.5 0.2 VCC(1)
V
VIH Input High Voltage except XTAL1
and RESET pins
VCC = 2.7 - 5.5V 0.6
VCC(2)
VCC + 0.5
VIL1 Input Low Voltage
XTAL1 pin
VCC = 2.7 - 5.5V -0.5 0.1 VCC(1)
VIH1 Input High Voltage
XTAL 1 pin
VCC = 2.7 - 5.5V 0.7
VCC(2)
VCC + 0.5
VIL2 Input Low Voltage
RESET pin
VCC = 2.7 - 5.5V -0.5 0.2 VCC(1)
VIH2 Input High Voltage
RESET pin
VCC = 2.7 - 5.5V 0.85
VCC(2)
VCC + 0.5
VOL Output Low Voltage(3)
(Ports A,B,C,D,E,F,G)
IOL = 20mA, VCC = 5V
IOL = 10mA, VCC = 3V
0.9
0.6
VOH Output High Voltage(4)
(Ports A,B,C,D,E,F,G)
IOH = -20mA, VCC = 5V
IOH = -10mA, VCC = 3V
4.2
2.2
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Symbol Parameter Condition Min Typ Max Units
IIL Input Leakage
Current I/O Pin
VCC = 5.5V, pin low
(absolute value)
1.0
μA
IIH Input Leakage
Current I/O Pin
VCC = 5.5V, pin high
(absolute value)
1.0
RRST Reset Pull-up Resistor 30 60
kΩRPEN PEN Pull-up Resistor 30 60
RPU I/O Pin Pull-up Resistor 20 50
ICC
Power Supply Current Active 4MHz, VCC = 3V 2.5 5
mA
Active 8MHz, VCC = 5V 8.1 20
Idle 4MHz, VCC = 3V 0.7 2
Idle 8MHz, VCC = 5V 2.8 12
Power-down mode(5) WDT enabled, VCC = 3V <10 25 μA
WDT disabled, VCC = 3V <4 10
VACIO Analog Comparator
Input Offset Voltage
VCC = 5V
Vin = VCC/2
-40 40 mV
IACLK Analog Comparator
Input Leakage Current
VCC = 5V
Vin = VCC/2
-50 50 nA
tACPD Analog Comparator
Propagation Delay
VCC = 2.7V
VCC = 5.0V
750
500 ns
Note: 
1. “Max” means the highest value where the pin is guaranteed to be read as low
2. “Min” means the lowest value where the pin is guaranteed to be read as high
3. Although each I/O port can sink more than the test conditions (20mA at VCC = 5V, 10mA at VCC =
3V) under steady state conditions (non-transient), the following must be observed:
TQFP and QFN/MLF Package:
1. The sum of all IOL, for all ports, should not exceed 400mA.
2. The sum of all IOL, for ports A0 - A7, G2, C3 - C7 should not exceed 100mA.
3. The sum of all IOL, for ports C0 - C2, G0 - G1, D0 - D7, XTAL2 should not exceed 100mA.
4. The sum of all IOL, for ports B0 - B7, G3 - G4, E0 - E7 should not exceed 100mA.
5. The sum of all IOL, for ports F0 - F7, should not exceed 100mA.
If IOL exceeds the test condition, VOL may exceed the related specification. Pins are not guaranteed
to sink current greater than the listed test condition.
4. Although each I/O port can source more than the test conditions (20mA at Vcc = 5V, 10mA at Vcc =
3V) under steady state conditions (non-transient), the following must be observed:
TQFP and QFN/MLF Package:
1. The sum of all IOH, for all ports, should not exceed 400mA.
2. The sum of all IOH, for ports A0 - A7, G2, C3 - C7 should not exceed 100mA.
3. The sum of all IOH, for ports C0 - C2, G0 - G1, D0 - D7, XTAL2 should not exceed 100mA.
4. The sum of all IOH, for ports B0 - B7, G3 - G4, E0 - E7 should not exceed 100mA.
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5. The sum of all IOH, for ports F0 - F7, should not exceed 100mA.
If IOH exceeds the test condition, VOH may exceed the related specification. Pins are not
guaranteed to source current greater than the listed test condition.
5. Minimum VCC for Power-down is 2.5V.
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// // // //
34. Typical Characteristics – TA = -40°C to 85°C
The following charts show typical behavior. These figures are not tested during manufacturing. All current
consumption measurements are performed with all I/O pins configured as inputs and with internal pull-ups
enabled. A sine wave generator with rail-to-rail output is used as clock source.
The power consumption in Power-down mode is independent of clock selection.
The current consumption is a function of several factors such as: Operating voltage, operating frequency,
loading of I/O pins, switching rate of I/O pins, code executed and ambient temperature. The dominating
factors are operating voltage and frequency.
The current drawn from capacitive loaded pins may be estimated (for one pin) as CL*VCC*f where CL =
load capacitance, VCC = operating voltage and f = average switching frequency of I/O pin.
The parts are characterized at frequencies higher than test limits. Parts are not guaranteed to function
properly at frequencies higher than the ordering code indicates.
The difference between current consumption in Power-down mode with Watchdog Timer enabled and
Power-down mode with Watchdog Timer disabled represents the differential current drawn by the
Watchdog Timer.
34.1. Active Supply Current
Figure 34-1 Active Supply Current vs. Frequency (0.1 - 1.0 MHz)
ACTIVE S UP P LY CURRENT vs . LOW FREQUENCY
0.1 - 1.0 MHz
5.5 V
5.0 V
4.5 V
4.0 V
3.6 V
3.3 V
2.7 V
0
0.5
1
1.5
2
2.5
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Fre quency (MHz)
ICC (mA)
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,4ov / 7/ éé// 27V
Figure 34-2 Active Supply Current vs. Frequency (1 - 16 MHz)
5.5 V
5.0 V
4.5 V
4.0 V
3.6 V
3.3 V
2.7 V
ACTIVE S UP P LY CURRENT vs . FREQUENCY
1 - 16 MHz
0
4
8
12
16
20
0 2 4 6 8 10 12 14 16
Fre quency (MHz)
ICC (mA)
Figure 34-3 Active Supply Current vs. VCC (Internal RC Oscillator, 1 MHz)
ACTIVE S UP P LY CURRENT vs . VCC
INTERNAL RC OS CILLATOR. 1 MHz
85 °C
25 °C
0 °C
-40 °C
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
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ACTIVE SUPPLY CURRENT vs. V \\ ACTIVE SUPPLY CURRENT vs, V / \ Atmel
Figure 34-4 Active Supply Current vs. VCC (Internal RC Oscillator, 2 MHz)
ACTIVE S UP P LY CURRENT vs . VCC
INTERNAL RC OS CILLATOR, 2 MHz
85 °C
25 °C
-40 °C
1.5
2
2.5
3
3.5
4
4.5
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
Figure 34-5 Active Supply Current vs. VCC (Internal RC Oscillator, 4 MHz)
ACTIVE S UP P LY CURRENT vs . V
CC
INTERNAL RC OS CILLATOR, 4 MHz
85 °C
25 °C
-40 °C
2
3
4
5
6
7
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
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ACTIVE SUPPLY CURRENT vs. V ACTIVE SUPPLY CURRENT vs‘ V Atmel
Figure 34-6 Active Supply Current vs. VCC (Internal RC Oscillator, 8 MHz)
ACTIVE S UP P LY CURRENT vs . VCC
INTERNAL RC OSCILLATOR, 8 MHz
85 °C
25 °C
-40 °C
4
5
6
7
8
9
10
11
12
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
Figure 34-7 Active Supply Current vs. VCC (32 kHz External Oscillator)
ACTIVE S UP P LY CURRENT vs . V
CC
EXTERNAL RC OS CILLATOR, 32 kHz
25 °C
50
60
70
80
90
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
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IDLE SUPPLY CURRENT vs‘ LOW FREQUENCY \\\\\\ \\\ / j \ \\ \\ \\ ‘\\ Atmel
34.2. Idle Supply Current
Figure 34-8 Idle Supply Current vs. Frequency (0.1 - 1.0 MHz)
IDLE S UP P LY CURRENT vs. LOW FREQUENCY
0.1 - 1.0 MHz
5.5 V
5.0 V
4.5 V
4.0 V
3.6 V
3.3 V
2.7 V
0
0.1
0.2
0.3
0.4
0.5
0.6
0.1 0.2 0.3 0.4 0,5 0.6 0.7 0.8 0.9 1
Fre quency (MHz)
ICC (mA)
Figure 34-9 Idle Supply Current vs. Frequency (1 - 16 MHz)
IDLE S UP P LY CURRENT vs. FREQUENCY
1 - 16 MHz
0
1
2
3
4
5
6
7
8
0 2 4 6 8 10 12 14 16
Fre quency (MHz)
ICC (mA)
5.5 V
5.5 V
5.5 V
5.5 V
3.6 V
3.3 V
2.7 V
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IDLE SUPPLY CURRENT vs. V / \\ IDLE SUPPLY CURRENT vs. V Atmel
Figure 34-10 Idle Supply Current vs. VCC (Internal RC Oscillator, 1 MHz)
IDLE S UP P LY CURRENT vs. VCC
INTERNAL RC OS CILLATOR, 1 MHz
85 °C
25 °C
-40 °C
0,2
0.3
0.4
0.5
0.6
0.7
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
Figure 34-11 Idle Supply Current vs. VCC (Internal RC Oscillator, 2 MHz)
IDLE S UP P LY CURRENT vs. V
CC
INTERNAL RC OS CILLATOR, 2 MHz
85 °C
25 °C
-40 °C
0,4
0.6
0.8
1
1.2
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
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IDLE SUPPLY CURRENT vs‘ V / \\ IDLE SUPPLY CURRENT vs. V Atmel
Figure 34-12 Idle Supply Current vs. VCC (Internal RC Oscillator, 4 MHz)
IDLE S UP P LY CURRENT vs. VCC
INTERNAL RC OSCILLATOR, 4 MHz
85 °C
25 °C
-40 °C
1
1.4
1.8
2.2
2.6
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
Figure 34-13 Idle Supply Current vs. VCC (Internal RC Oscillator, 8 MHz)
IDLE S UP P LY CURRENT vs. V
CC
INTERNAL RC OSCILLATOR, 8 MHz
85 °C
25 °C
-40 °C
1.5
2
2.5
3
3.5
4
4.5
5
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
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IDLE SUPPLY CURRENT vs‘ V POWER-DOWN SUPPLY CURRENT vs. V Atmel
Figure 34-14 Idle Supply Current vs. VCC (32 kHz External Oscillator)
IDLE S UP P LY CURRENT vs. V
CC
EXTERNAL RC OS CILLATOR, 32 kHz
25 °C
0
5
10
15
20
25
30
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
34.3. Power-Down Supply Current
Figure 34-15 Power-Down Supply Current vs. VCC (Watchdog Timer Disabled)
POWER-DOWN S UP P LY CURRENT vs. V
CC
WATCHDOG TIMER DISABLED
85 °C
25 °C
-40 °C
0
0.5
1
1.5
2
2.5
3
3.5
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (uA)
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POWER-DOWN SUPPLY CURRENT Vs. V \ POWER-SAVE SUPPLY CURRENT Vs. V Atmel
Figure 34-16 Power-Down Supply Current vs. VCC (Watchdog Timer Enabled)
POWER-DOWN S UP P LY CURRENT vs. V
CC
WATCHDOG TIMER ENABLED
85 °C
25 °C
-40 °C
5
9
13
17
21
25
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (uA)
34.4. Power-Save Supply Current
Figure 34-17 Power-Save Supply Current vs. VCC (Watchdog Timer Disabled)
POWER-S AVE SUP P LY CURRENT vs. VCC
WATCHDOG TIMER DISABLED
25 °C
5
6
7
8
9
10
11
12
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (uA)
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STANDBY SUPPLY CURRENT vs, V Atmel
34.5. Standby Supply Current
Figure 34-18 Standby Supply Current vs. VCC
STANDBY S UP P LY CURRENT vs . V
CC
6MHz Xtal
6MHz Res
4MHz Xtal
4MHz Re s
450kHz Res
2MHz Xtal
2MHz Re s
1MHz Re s
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
Figure 34-19 Standby Supply Current vs. VCC (CKOPT Programmed)
STANDBY S UP P LY CURRENT vs . V
CC
6MHz Xtal
4MHz Xtal
16MHz Xtal
12MHz Xtal
0
0.4
0.8
1.2
1.6
2
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
CKOP T P rogrammed
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I/O PIN PULL-UP RESISTOR CURRENT vsI INPUT VOLTAGE I/O PIN PULL-UP RESISTOR CURRENT vs, INPUT VOLTAGE Atmel
34.6. Pin Pull-up
Figure 34-20 I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 5V)
I/O PIN PULL-UP RESISTOR CURRENT vs . INPUT VOLTAGE
Vcc = 5V
0
20
40
60
80
100
120
140
0123456
VOP (V)
IOP (uA)
85 °C
25 °C
-40 °C
Figure 34-21 I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 2.7V)
85 °C
25 °C
-40 °C
I/O PIN PULL-UP RESISTOR CURRENT vs . INPUT VOLTAGE
Vcc = 2.7V
0
10
20
30
40
50
60
70
80
0 0.5 1 1.5 2 2.5 3
VOP (V)
IOP (uA)
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RESET PULL-UP RESISTOR CURRENT vs. RESET PIN VOLTAGE \\ RES ET PULL-UP RESISTOR CURRENT vs, RESET PIN VOLTAGE Atmel
Figure 34-22 Reset Pull-up Resistor Current vs. Reset Pin Voltage (VCC = 5V)
RESET P ULL-UP RESISTOR CURRENT vs . RESET P IN VOLTAGE
Vcc = 5V
0
20
40
60
80
100
120
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
VRES ET (V)
IRES ET (uA)
85 °C
25 °C
-40 °C
Figure 34-23 Reset Pull-up Resistor Current vs. Reset Pin Voltage (VCC = 2.7V)
85 °C
25 °C
-40 °C
RESET P ULL-UP RESISTOR CURRENT vs . RESET P IN VOLTAGE
Vcc = 2.7V
0
10
20
30
40
50
60
0 0.5 1 1.5 2 2.5 3
VRES ET (V)
IRES ET (uA)
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PEN PULL-UP RESISTOR CURRENT vs, PEN PIN VOLTAGE RES ET PULL-UP RESISTOR CURRENT vs, RESET PIN VOLTAGE Atmel
Figure 34-24 PEN Pull-up Resistor Current vs. PEN Pin Voltage (VCC = 5V)
PEN P ULL-UP RESIS TOR CURRENT vs . PEN PIN VOLTAGE
VCC = 5V
0
20
40
60
80
100
120
140
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
VPEN (V)
IPEN (uA)
85 °C
25 °C
-40 °C
Figure 34-25 PEN Pull-up Resistor Current vs. PEN Pin Voltage (VCC = 2.7V)
85 °C
25 °C
-40 °C
RESET P ULL-UP RESISTOR CURRENT vs . RESET P IN VOLTAGE
Vcc = 2.7V
0
10
20
30
40
50
60
0 0.5 1 1.5 2 2.5 3
VRES ET (V)
IRES ET (uA)
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I/O PIN SOURCE CURRENT vs, OUTPUT VOLTAGE I/O PIN SOURCE CURRENT VST OUTPUT VOLTAGE Atmel
34.7. Pin Driver Strength
Figure 34-26 I/O Pin Source Current vs. Output Voltage (VCC = 5V)
85 °C
25 °C
-40 °C
I/O PIN SOURCE CURRENT vs . OUTPUT VOLTAGE
Vcc = 5V
0
10
20
30
40
50
60
70
80
90
3 3.4 3.8 4.2 4.6 5
VOH (V)
IOH (mA)
Figure 34-27 I/O Pin Source Current vs. Output Voltage (VCC = 2.7V)
I/O PIN SOURCE CURRENT vs . OUTPUT VOLTAGE
Vcc = 2.7V
85 °C
25 °C
-40 °C
0
5
10
15
20
25
30
35
0.5 1 1.5 2 2.5 3
VOH (V)
IOH (mA)
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I/O PIN SINK CURRENT vs, OUTPUT VOLTAGE I/O PIN SINK CURRENT vs, OUTPUT VOLTAGE Atmel
Figure 34-28 I/O Pin Sink Current vs. Output Voltage (VCC = 5V)
I/O PIN SINK CURRENT vs . OUTP UT VOLTAGE
Vcc = 5V
85 °C
25 °C
-40 °C
0
10
20
30
40
50
60
70
80
90
0 0.4 0.8 1.2 1.6 2
VOL (V)
IOL (mA)
Figure 34-29 I/O Pin Sink Current vs. Output Voltage (VCC = 2.7V)
I/O PIN SINK CURRENT vs . OUTP UT VOLTAGE
Vcc = 2.7V
85 °C
25 °C
-40 °C
0
5
10
15
20
25
30
35
0 0.4 0.8 1.2 1.6 2
VOL (V)
IOL (mA)
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[/0 PIN INPUT THRESHOLD VOLTAGE vs. V \ / I/O PEN INPUT THRESHOLD VOLTAGE vs. V \ Atmel
34.8. Pin Thresholds and Hysteresis
Figure 34-30 I/O Pin Input Threshold Voltage vs. VCC (VIH, I/O Pin Read as '1')
I/O PIN INPUT THRESHOLD VOLTAGE vs . V
CC
VIH, IO P IN READ AS '1'
85 °C
25 °C
-40 °C
1
1.4
1.8
2.2
2.6
3
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
Thres hold (V)
Figure 34-31 I/O Pin Input Threshold Voltage vs. VCC (VIL, I/O Pin Read as '0')
I/O PIN INPUT THRESHOLD VOLTAGE vs . V
CC
VIL, IO P IN READ AS '0'
85 °C
25 °C
-40 °C
1
1.3
1.6
1.9
2.2
2.5
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
Thres hold (V)
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Atmel \\ \\ RESET INPUT THRESHOLD VOLTAGE vs, V
Figure 34-32 I/O Pin Input Hysteresis vs. VCC
I/O PIN INPUT HYSTERESIS vs. VCC
85 °C
25 °C
-40 °C
0
0.2
0.4
0.6
0.8
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
Input Hys tere s is (mV)
Figure 34-33 Reset Input Threshold Voltage vs. VCC (VIH, Reset Pin Read as '1')
RESET INP UT THRESHOLD VOLTAGE vs . V
CC
VIH, IO P IN READ AS '1'
85 °C
25 °C
-40 °C
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
Thres hold (V)
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RESET INPUT THRESHOLD VOLTAGE vs. V / / Atmel
Figure 34-34 Reset Input Threshold Voltage vs. VCC (VIL, Reset Pin Read as '0')
RESET INP UT THRESHOLD VOLTAGE vs . V
CC
VIL, IO P IN READ AS '0'
85 °C
25 °C
-40 °C
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
Thres hold (V)
Figure 34-35 Reset Input Pin Hysteresis vs. VCC
RESET INP UT PIN HYSTERES IS vs. VCC
85 °C
25 °C
-40 °C
0
0.1
0.2
0.3
0.4
0.5
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
Input Hys tere s is (mV)
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Atmel
34.9. BOD Thresholds and Analog Comparator Offset
Figure 34-36 BOD Thresholds vs. Temperature (BODLEVEL is 4.0V)
BOD THRESHOLDS vs. TEMP ERATURE
BOD LEVEL IS 4.0 V
3.95
4
4.05
4.1
4.15
4.2
-40 -25 -10 5 20 35 50 65 80 95
Tempera ture (
°C
)
Thres hold (V)
Falling Vcc
Rising Vcc
Figure 34-37 BOD Thresholds vs. Temperature (BODLEVEL is 2.7V)
BOD THRESHOLDS vs. TEMP ERATURE
BOD LEVEL IS 2.7 V
2.58
2.61
2.64
2.67
2.7
2.73
2.76
-40 -25 -10 5 20 35 50 65 80 95
Tempera ture (
°C
)
Thres hold (V)
Rising Vcc
Falling Vcc
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Atmel \\
Figure 34-38 Bandgap Voltage vs. VCC
BANDGAP VOLTAGE vs . V
CC
85 °C
25 °C
-40 °C
1.18
1.185
1.19
1.195
1.2
1.205
1.21
1.215
2.5 3 3.5 4 4.5 5 5.5
Vcc (V)
Bandgap Voltage (V)
34.10. Internal Oscillator Speed
Figure 34-39 Watchdog Oscillator Frequency vs. VCC
WATCHDOG OSCILLATOR FREQUENCY vs . Vcc
85 °C
25 °C
-40 °C
1040
1060
1080
1100
1120
1140
1160
1180
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
FRC (kHz)
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\\ Atmel
Figure 34-40 Calibrated 1 MHz RC Oscillator Frequency vs. Temperature
CALIBRATED 1MHz RC OSCILLATOR FREQUENCY vs . TEMPERATURE
5.5 V
5.0 V
4.5 V
4.0 V
3.6 V
3.3 V
2.7 V
0.92
0.94
0.96
0.98
1
1.02
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80
Tempera ture
(
°C
)
FRC (MHz)
Figure 34-41 Calibrated 1 MHz RC Oscillator Frequency vs. VCC
CALIBRATED 1MHz RC OSCILLATOR FREQUENCY vs . Vcc
85 °C
25 °C
-40 °C
0.92
0.94
0.96
0.98
1
1.02
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
FRC (MHz)
Atmel ATmega64A [DATASHEET]
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Figure 34-42 Calibrated 1 MHz RC Oscillator Frequency vs. Osccal Value
CALIBRATED 1MHz RC OSCILLATOR FREQUENCY vs . OS CCAL VALUE
25 °C
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240 256
OS CCAL VALUE
FRC (MHz)
Figure 34-43 Calibrated 2 MHz RC Oscillator Frequency vs. Temperature
CALIBRATED 2MHz RC OSCILLATOR FREQUENCY vs . TEMPERATURE
5.5 V
5.0 V
4.5 V
4.0 V
3.6 V
3.3 V
3.0 V
2.7 V
1.8
1.85
1.9
1.95
2
2.05
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80
FRC (MHz)
Tempera ture
(
°C
)
Atmel ATmega64A [DATASHEET]
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Atmel \\
Figure 34-44 Calibrated 2 MHz RC Oscillator Frequency vs. VCC
CALIBRATED 2MHz RC OSCILLATOR FREQUENCY vs . Vcc
85 °C
25 °C
-40 °C
1.8
1.85
1.9
1.95
2
2.05
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
FRC (MHz)
Figure 34-45 Calibrated 2 MHz RC Oscillator Frequency vs. Osccal Value
CALIBRATED 2MHz RC OSCILLATOR FREQUENCY vs . OS CCAL VALUE
25 °C
0.8
1.2
1.6
2
2.4
2.8
3.2
3.6
0 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240 256
OS CCAL VALUE
FRC (MHz)
Atmel ATmega64A [DATASHEET]
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:
Figure 34-46 Calibrated 4 MHz RC Oscillator Frequency vs. Temperature
CALIBRATED 4MHz RC OSCILLATOR FREQUENCY vs . TEMPERATURE
5.5 V
5.0 V
4.5 V
4.0 V
3.6 V
3.3 V
2.7 V
3.5
3.6
3.7
3.8
3.9
4
4.1
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80
FRC (MHz)
Tempera ture
(
°C
)
Figure 34-47 Calibrated 4 MHz RC Oscillator Frequency vs. VCC
CALIBRATED 4MHz RC OSCILLATOR FREQUENCY vs . Vcc
85 °C
25 °C
-40 °C
3.5
3.6
3.7
3.8
3.9
4
4.1
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
FRC (MHz)
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
459
Atmel
Figure 34-48 Calibrated 4 MHz RC Oscillator Frequency vs. Osccal Value
CALIBRATED 4MHz RC OSCILLATOR FREQUENCY vs . OS CCAL VALUE
25 °C
1
2
3
4
5
6
7
8
0 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240 256
OS CCAL VALUE
FRC (MHz)
Figure 34-49 Calibrated 8 MHz RC Oscillator Frequency vs. Temperature
CALIBRATED 8MHz RC OSCILLATOR FREQUENCY vs . TEMPERATURE
5.5 V
5.0 V
4.5 V
4.0 V
3.6 V
3.3 V
2.7 V
6.4
6.6
6.8
7
7.2
7.4
7.6
7.8
8
8.2
8.4
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80
FRC (MHz)
Tempera ture
(
°C
)
Atmel ATmega64A [DATASHEET]
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Atmel
Figure 34-50 Calibrated 8 MHz RC Oscillator Frequency vs. VCC
CALIBRATED 8MHz RC OSCILLATOR FREQUENCY vs . Vcc
85 °C
25 °C
-40 °C
6.6
6.8
7
7.2
7.4
7.6
7.8
8
8.2
8.4
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
FRC (MHz)
Figure 34-51 Calibrated 8 MHz RC Oscillator Frequency vs. Osccal Value
CALIBRATED 8MHz RC OSCILLATOR FREQUENCY vs . OS CCAL VALUE
25 °C
3
5
7
9
11
13
15
0 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240 256
OS CCAL VALUE
FRC (MHz)
Atmel ATmega64A [DATASHEET]
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Atmel \\ ADC CURRENT vs. V \\
34.11. Current Consumption of Peripheral Units
Figure 34-52 Brownout Detector Current vs. VCC
BROWNOUT DETECTOR CURRENT vs . VCC
85 °C
25 °C
-40 °C
8
10
12
14
16
18
20
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (uA)
Figure 34-53 ADC Current vs. VCC (ADC CLK = 50 kHz)
ADC CURRENT vs . V
CC
ADC CLK = 50 KHz
85 °C
25 °C
-40 °C
200
225
250
275
300
325
350
375
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (uA)
Atmel ATmega64A [DATASHEET]
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AREF CURRENT vs‘ V // / / / / :/ / Atmel
Figure 34-54 Aref Current vs. VCC
AREF CURRENT vs. V
CC
ADC CLK = 1 MHz
85 °C
25 °C
-40 °C
75
100
125
150
175
200
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (uA)
Figure 34-55 Analog Comparator Current vs. VCC
ANALOG COMPARATOR CURRENT vs . VCC
85 °C
25 °C
-40 °C
30
35
40
45
50
55
60
65
70
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (uA)
Atmel ATmega64A [DATASHEET]
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PROGRAMMING CURRENT vs‘ Vac v v/ / RESET SUPPLY CURRENT vs, V / / / / / / / //// // / // // /////é//,. / / / é;// / Atmel
Figure 34-56 Programming Current vs. VCC
PROGRAMMING CURRENT vs. Vcc
Ext Clk
85 °C
25 °C
-40 °C
1
2
3
4
5
6
7
8
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
ICC (mA)
34.12. Current Consumption in Reset and Reset Pulse width
Figure 34-57 Reset Supply Current vs. VCC (0.1 - 1.0 MHz, Excluding Current through the Reset Pull-up)
RESET S UPPLY CURRENT vs . VCC
0.1 - 1.0 MHz EXCLUDING CURRENT THROUGH THE RESET PULLUP
5.5 V
5.0 V
4.5 V
4.0 V
3.6 V
3.3 V
2.7 V
0
0.5
1
1.5
2
2.5
3
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Fre quency (MHz)
ICC (mA)
Atmel ATmega64A [DATASHEET]
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RES ET SUPPLY CURRENT vs. V / // ////,35v éé6/,2Jv Atmel
Figure 34-58 Reset Supply Current vs. VCC (1 - 16 MHz, Excluding Current through the Reset Pull-up)
5.5 V
5.0 V
4.5 V
4.0 V
3.6 V
3.3 V
2.7 V
RESET S UPPLY CURRENT vs . VCC
1 - 16 MHz EXCLUDING CURRENT THROUGH THE RES ET P ULLUP
0
4
8
12
16
0 2 4 6 8 10 12 14 16
Fre quency (MHz)
ICC (mA)
Figure 34-59 Minimum Reset Pulse Width vs. VCC
MINIMUM RESET P ULSE WIDTH vs . VCC
85 °C
25 °C
-40 °C
0
200
400
600
800
2.5 3 3.5 4 4.5 5 5.5
VCC (V)
Pulsewidth (ns )
Atmel ATmega64A [DATASHEET]
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Atmel \\\\ \\\\
35. Typical Characteristics – TA = -40°C to 105°C
The following charts show typical behavior. These figures are not tested during manufacturing. All current
consumption measurements are performed with all I/O pins configured as inputs and with internal pull-ups
enabled. A sine wave generator with rail-to-rail output is used as clock source.
All Active- and Idle current consumption measurements are done with all bits in the PRR register set and
thus, the corresponding I/O modules are turned off. Also the Analog Comparator is disabled during these
measurements.
The power consumption in Power-down mode is independent of clock selection.
The current consumption is a function of several factors such as: operating voltage, operating frequency,
loading of I/O pins, switching rate of I/O pins, code executed and ambient temperature. The dominating
factors are operating voltage and frequency.
The current drawn from capacitive loaded pins may be estimated (for one pin) as CL*VCC*f where CL =
load capacitance, VCC = operating voltage and f = average switching frequency of I/O pin.
The parts are characterized at frequencies higher than test limits. Parts are not guaranteed to function
properly at frequencies higher than the ordering code indicates.
The difference between current consumption in Power-down mode with Watchdog Timer enabled and
Power-down mode with Watchdog Timer disabled represents the differential current drawn by the
Watchdog Timer.
35.1. Active Supply Current
Figure 35-1 Active Supply Current vs. Frequency (0.1 - 1.0MHz)
ACTIVE S UP P LY CURRENT vs. LOW FREQUENCY
0.1 - 1.0 MHz
5.5V
5.0V
4.5V
4.0V
3.6V
3.3V
3.0V
2.7V
0.2
0.5
0.8
1.1
1.4
1.7
2.0
2.3
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Fre quency (MHz)
ICC (mA)
Atmel ATmega64A [DATASHEET]
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ACTIVE SUPPLY CURRENT Vs. V Atmel
Figure 35-2 Active Supply Current vs. Frequency (1 - 16MHz)
ACTIVE S UP P LY CURRENT vs. FREQUENCY
1 - 16 MHz
0
2
4
6
8
10
12
14
16
18
20
0 2 4 6 8 10 12 14 16
Fre quency (MHz)
ICC (mA)
5.5V
5.0V
4.5V
4.0V
3.6V
3.3V
2.7V
Figure 35-3 Active Supply Current vs. VCC (Internal RC Oscillator, 8MHz)
ACTIVE S UP P LY CURRENT vs. VCC
INTERNAL RC OS CILLATOR, 8 MHz
105°C
85°C
25°C
-40°C
4
5
6
7
8
9
10
11
12
2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5
VCC (V)
ICC (mA)
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
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ACTIVE SUPPLY CURRENT vs. V ACTIVE SUPPLY CURRENT vs, V \\ \ \ Atmel
Figure 35-4 Active Supply Current vs. VCC (Internal RC Oscillator, 4MHz)
ACTIVE S UP P LY CURRENT vs . VCC
INTERNAL RC OS CILLATOR, 4 MHz
105°C
85°C
25°C
-40°C
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5
VCC (V)
ICC (mA)
Figure 35-5 Active Supply Current vs. VCC (Internal RC Oscillator, 2MHz)
ACTIVE S UP P LY CURRENT vs. VCC
INTERNAL RC OS CILLATOR, 2 MHz
105°C
85°C
25°C
-40°C
1.5
2.0
2.5
3.0
3.5
4.0
4.5
2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5
VCC (V)
ICC (mA)
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
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ACTIVE SUPPLY CURRENT vs. V \\ IDLE SUPPLY CURRENT vs. LOW FREQUENCY Atmel
Figure 35-6 Active Supply Current vs. VCC (Internal RC Oscillator, 1MHz)
ACTIVE S UP P LY CURRENT vs . VCC
INTERNAL RC OS CILLATOR, 1 MHz
105°C
85°C
25°C
-40°C
1.1
1.3
1.5
1.7
1.9
2.1
2.3
2.5
2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5
VCC (V)
ICC (mA)
35.2. Idle Supply Current
Figure 35-7 Idle Supply Current vs. Frequency (0.1 - 1.0MHz)
IDLE S UP P LY CURRENT vs. LOW FREQUENCY
0.1 - 1.0 MHz
5.5V
5.0V
4.5V
4.0V
3.6V
3.3V
2.7V
0.05
0.11
0.17
0.23
0.29
0.35
0.41
0.47
0.53
0.59
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Fre quency (MHz)
ICC (mA)
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
469
IDLE SUPPLY CURRENT vs, FREQUENCY / 40v 311V / 33v / -2 7v IDLE SUPPLY CURRENT vs. V \\ Atmel
Figure 35-8 Idle Supply Current vs. Frequency (1 - 16MHz)
5.5V
5.0V
4.5V
4.0V
3.6V
3.3V
2.7V
IDLE S UP P LY CURRENT vs. FREQUENCY
1 - 16 MHZ
0
1
2
3
4
5
6
7
8
0 2 4 6 8 10 12 14 16
Fre quency (MHz)
ICC (mA)
Figure 35-9 Idle Supply Current vs. VCC (Internal RC Oscillator, 8MHz)
IDLE S UP P LY CURRENT vs. VCC
INTERNAL RC OS CILLATOR, 8 MHz
105°C
85°C
25°C
-40°C
1.6
2.1
2.6
3.1
3.6
4.1
4.6
5.1
2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5
VCC (V)
ICC (mA)
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
470
IDLE SUPPLY CURRENT vs, V \\ \ \\ Atmel
Figure 35-10 Idle Supply Current vs. VCC (Internal RC Oscillator, 4MHz)
IDLE S UP P LY CURRENT vs. VCC
INTERNAL RC OS CILLATOR, 4 MHz
105°C
85°C
25°C
-40°C
1.0
1.3
1.6
1.9
2.2
2.5
2.8
2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5
VCC (V)
ICC (mA)
Figure 35-11 Idle Supply Current vs. VCC (Internal RC Oscillator, 2MHz)
IDLE S UP P LY CURRENT vs. VCC
INTERNAL RC OS CILLATOR, 2 MHz
105°C
85°C
25°C
-40°C
0.43
0.53
0.63
0.73
0.83
0.93
1.03
1.13
1.23
2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5
VCC (V)
ICC (mA)
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
471
IDLE SUPPLYCURRENT vs, V \\ /, POWER-DOWN SUPPLY CURRENT vs. V Atmel
Figure 35-12 Idle Supply Current vs. VCC (Internal RC Oscillator, 1MHz)
IDLE S UP P LY CURRENT vs. VCC
INTERNAL RC OS CILLATOR, 1 MHz
105°C
85°C
25°C
-40°C
0.26
0.32
0.38
0.44
0.5
0.56
0.62
0.68
0.74
2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5
VCC (V)
ICC (mA)
35.3. Power-down Supply Current
Figure 35-13 Power-down Supply Current vs. VCC (Watchdog Timer Disabled)
POWER-DOWN S UP P LY CURRENT vs . VCC
WATCHDOG TIMER DISABLED
105°C
85°C
25°C
-40°C
0
1
2
3
4
5
6
2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5
VCC (V)
ICC (µA)
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
472
POWER-DOWN SUPPLY CURRENT vs. V I/O PIN PULL-UP RESISTOR CURRENT vs. INPUT VOLTAGE \ Atmel
Figure 35-14 Power-down Supply Current vs. VCC (Watchdog Timer Enabled)
POWER-DOWN S UP P LY CURRENT vs. VCC
WATCHDOG TIMER ENABLED
105°C
85°C
25°C
-40°C
7
9
11
13
15
17
19
21
23
25
27
2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5
VCC (V)
ICC (µA)
35.4. Pin Pull-up
Figure 35-15 I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 5V)
105°C
85°C
25°C
-40°C
I/O PIN PULL-UP RESISTOR CURRENT vs . INPUT VOLTAGE
VCC = 5V
0
20
40
60
80
100
120
140
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
VOP (V)
IOP (µA)
Atmel ATmega64A [DATASHEET]
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\ RESET PULL-UP RESISTOR CURRENT vs. RESET PIN VOLTAGE (MA) RESET Atmel
Figure 35-16 I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 2.7V)
I/O PIN PULL-UP RESISTOR CURRENT vs . INPUT VOLTAGE
VCC = 2.7V
0
10
20
30
40
50
60
70
80
0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7
VOP (V)
IOP (µA)
105°C
85°C
25°C
-40°C
Figure 35-17 Reset Pull-up Resistor Current vs. Reset Pin Voltage (VCC = 5V)
105°C
85°C
25°C
-40°C
RESET PULL-UP RESISTOR CURRENT vs. RESET PIN VOLTAGE
00.5 13.5
3
2.5
1.5 244.5 5
120
105
90
75
60
45
30
15
0
IRESET (μA)
VRESET (V)
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
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RESET PULL»UP RESISTOR CURRENT vs. RESET PIN VOLTAGE mm VRESET [/0 PIN OUTPUT VOLTAGE vs, SOURCE CURRENT \ \ Atmel
Figure 35-18 Reset Pull-up Resistor Current vs. Reset Pin Voltage (VCC = 2.7V)
RESET PULL-UP RESISTOR CURRENT vs. RESET PIN VOLTAGE
VRESET(V)
25°C
-40°C
85°C
105°C
60
50
40
30
20
10
0
00.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7
IRESET(μA)
35.5. Pin Driver Strength
Figure 35-19 I/O Pin Output Voltage vs. Source Current, Port B (VCC= 5V)
I/O PIN OUTP UT VOLTAGE vs . SOURCE CURRENT
VCC = 5V
105°C
85°C
25°C
-40°C
4.3
4.4
4.5
4.6
4.7
4.8
4.9
5
0 2 4 6 8 10 12 14 16 18 20
IOH (mA)
VOH (V)
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
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[/0 PIN OUTPUT VOLTAGE vs, SOURCE CURRENT ,\ // / // / // / / // I/O PIN OUTPUT VOLTAGE vs. SINK CURRENT \\ Atmel
Figure 35-20 I/O Pin Output Voltage vs. Source Current, Port B (VCC = 3V)
I/O PIN OUTP UT VOLTAGE vs . SOURCE CURRENT
NORMAL P OWER P INS
105°C
85°C
25°C
-40°C
1.9
2.1
2.3
2.5
2.7
2.9
3.1
0 2 4 6 8 10 12 14 16 18 20
IOH (mA)
VOH (V)
Figure 35-21 I/O Pin Output Voltage vs. Sink Current, Port B (VCC = 5V)
I/O PIN OUTP UT VOLTAGE vs . SINK CURRENT
VCC = 5V
105°C
85°C
25°C
-40°C
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 2 4 6 8 10 12 14 16 18 20
IOL (mA)
VOL (V)
Atmel ATmega64A [DATASHEET]
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I/O PIN INPUT THRESHOLD VOLTAGE vs. V Atmel
Figure 35-22 I/O Pin Output Voltage vs. Sink Current, Port B (VCC = 3V)
I/O PIN OUTP UT VOLTAGE vs . SINK CURRENT
VCC = 3V
105°C
85°C
25°C
-40°C
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0 2 4 6 8 10 12 14 16 18 20
IOL (mA)
VOL (V)
35.6. Pin Thresholds and Hysteresis
Figure 35-23 I/O Pin Input Threshold Voltage vs. VCC (VIH, I/O Pin Read as “1”)
I/O PIN INPUT THRESHOLD VOLTAGE vs. VCC
VIH, IO P IN READ AS '1'
105°C
85°C
25°C
-40°C
1.3
1.5
1.7
1.9
2.1
2.3
2.5
2.7
2.9
3.1
2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5
VCC (V)
Thre s hold (V)
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
477
Atmel \\
Figure 35-24 I/O Pin Input Threshold Voltage vs. VCC (VIL, I/O Pin Read as “0”)
I/O PIN INPUT THRESHOLD VOLTAGE vs. VCC
VIL, IO P IN READ AS '0'
105°C
85°C
25°C
-40°C
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5
VCC (V)
Thre s hold (V)
Figure 35-25 I/O Pin Input Hysteresis vs. VCC
I/O PIN INPUT HYSTERESIS vs. VCC
105°C
85°C
25°C
-40°C
0.30
0.35
0.40
0.45
0.50
0.55
0.60
0.65
0.70
2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5
VCC (V)
Input Hyste re sis (mV)
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
478
Reset Input Threshold Voltage vs. Vol: (V , Reset Pin Read as "1") .40°C // 25°C /7 Threshold (V) \ Reset Input Threshold Voltage vs. Vcc (V , Reset Pin Read as ”0”) Threshold (V) Atmel
Figure 35-26 Reset Input Threshold Voltage vs. VCC (VIH,Reset Pin Read as “1”)
105°C
85°C
25°C
-40°C
Reset Input Threshold Voltage vs. Vcc (VIH, Reset Pin Read as "1")
2.4
2.2
2
1.8
1.6
1.4
1.2
2.5 2.8 3.1 3.4 3.7 44.3 4.6 4.9 5.2 5.5
Threshold (V)
VCC (V)
Figure 35-27 Reset Input Threshold Voltage vs. VCC (VIL,Reset Pin Read as “0”)
105°C
85°C
25°C
-40°C
Reset Input Threshold Voltage vs. VCC (VIL, Reset Pin Read as "0")
2.4
2.2
2
1.8
1.6
1.4
1.2
1
2.5 2.8 3.1 3.4 3.7 44.3 4.6 4.9 5.2 5.5
Threshold (V)
VCC (V)
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
479
Reset Input Pin Hysteresis vs. V Input Hysteresis (V) BOD THRESHOLDS vs. TEMPERATURE Thre shold (V) Atmel
Figure 35-28 Reset Input Pin Hysteresis vs. VCC
105°C
85°C
25°C
-40°C
Reset Input Pin Hysteresis vs. VCC
0.5
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
2.5 2.8 3.1 3.4 3.7 44.3 4.6 4.9 5.2 5.5
Input Hysteresis (V)
VCC (V)
35.7. BOD Thresholds and Analog Comparator Offset
Figure 35-29 BOD Thresholds vs. Temperature (BOD Level is 4.3V)
BOD THRESHOLDS vs. TEMP ERATURE
Rising Vcc
Fa lling Vcc
3.755
3.775
3.795
3.815
3.835
3.855
3.875
3.895
3.915
-45 -35 -25 -15 -5 5 15 25 35 45 55 65 75 85 95 105
Tempe ra ture (°C)
Thres hold (V)
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
480
BOD THRESHOLDS vs. TEMPERATURE Tmunou (V) BANDGAP VOLTAGE VS. TEMPERATURE ,3 v =1: Q 9 a E” g V m V V Atmel
Figure 35-30 BOD Thresholds vs. Temperature (BOD Level is 2.7V)
BOD THRESHOLDS vs. TEMP ERATURE
Rising Vcc
Fa lling Vcc
2.595
2.615
2.635
2.655
2.675
2.695
2.715
2.735
-45 -35 -25 -15 -5 5 15 25 35 45 55 65 75 85 95 105
Tempe rature ( °C)
Thre s hold (V)
Figure 35-31 Bandgap Voltage vs. Temperature
BANDGAP VOLTAGE vs . TEMPERATURE
5.5V
5.0V
4.5V
2.7V
1.161
1.166
1.171
1.176
1.181
1.186
1.191
1.196
1.201
-45 -35 -25 -15 -5 5 15 25 35 45 55 65 75 85 95 105
Tempe ra ture (°C)
Bandga p Voltage (V)
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
481
WATCHDOG OSCILLATOR FREQUENCY vs. OPERATING VOLTAGE (kHz) 1-‘ / 4/ / / \ \\ WATCHDOG OSCILLATOR FREQUENCY vs. TEMPERATURE (km) F < atmel="">
35.8. Internal Oscillator Speed
Figure 35-32 Watchdog Oscillator Frequency vs. VCC
WATCHDOG OSCILLATOR FREQUENCY vs . OP ERATING VOLTAGE
105°C
85°C
25°C
-40°C
1060
1080
1100
1120
1140
1160
1180
1200
1220
2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5
VCC (V)
FRC (kHz)
Figure 35-33 Watchdog Oscillator Frequency vs. Temperature
WATCHDOG OSCILLATOR FREQUENCY vs . TEMPERATURE
5.5V
5.0V
4.5V
4.0 V
3.6 V
3.3 V
2.7 V
1060
1080
1100
1120
1140
1160
1180
1200
1220
-45 -35 -25 -15 -5 5 15 25 35 45 55 65 75 85 95 105
Tempe rature (°C)
FRC (kHz)
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
482
Calibrated 1MHZ RC Oscillator Frequency vs. Temperature \\\\\\ ‘ \\ \\ \\\\\\\\\\\\ \ \ \ 1: \\\\\\\ \\\\\\\ g \\ \\ \\ \ \ V \\ \ \ \ \ \ \ \ u. \\\ \\\\\\\ \ \ \ \\\ \\\ \ \ \ \ \ 15 25 35 45 Temperature (‘0) Calibrated 1MHz RC Oscillator Frequency vs. V / / / / / E / / // E // / u. / / / / / // / Atmel
Figure 35-34 Calibrated 1MHz RC Oscillator Frequency vs. Temperature
Calibrated 1MHz RC Oscillator Frequency vs. Temperature
1.03
1.01
0.99
0.97
0.95
0.93
0.91
-45 -35 -25 -15 -5 515 25 35 45 55 65 75 85 95 105
5.5V
5.0V
4.5V
4.0V
3.6V
3.3V
2.7V
FRC (MHz)
Temperature (°C)
Figure 35-35 Calibrated 1MHz RC Oscillator Frequency vs. VCC
Calibrated 1MHz RC Oscillator Frequency vs. VCC
1.03
1.015
1
0.985
0.97
0.955
0.94
0.925
0.91
2.5 2.8 3.1 3.4 3.7 44.3 4.6 4.9 5.2 5.5
105°C
85°C
25°C
-40°C
VCC (V)
F
RC (MHz)
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
483
Calibrated 2MHZ RC Oscillator Frequency vs. Temperature I I I I I / I I I I / I I / I / I/I/ / / I/I// \ \ \ \ \ \ // / / / / //// 15 25 35 4 Temperature (°C) Calibrated 2MHz RC Oscillator Frequency vs. V \ \\ \ \\\ 1: / I E // / // u. // /// / 4 Vet: Atmel
Figure 35-36 Calibrated 2MHz RC Oscillator Frequency vs. Temperature
Calibrated 2MHz RC Oscillator Frequency vs. Temperature
2.09
2.04
1.99
1.94
1.89
1.84
1.79
-45 -35 -25 -15 -5 515 25 35 45 55 65 75 85 95 105
2.7V
3.3V
3.6V
4.0V
4.5V
5.0V
5.5V
Temperature (°C)
F
RC (MHz)
Figure 35-37 Calibrated 2MHz RC Oscillator Frequency vs. VCC
Calibrated 2MHz RC Oscillator Frequency vs. VCC
2.08
2.04
2
1.96
1.92
1.88
1.84
1.8
2.5 2.8 3.1 3.4 3.7 44.3 4.6 4.9 5.2 5.5
105°C
85°C
25°C
-40°C
VCC (V)
F
RC(MHz)
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
484
Calibrated 4MHZ RC Oscillator Frequency vs. Temperature \\ \\\\\ \ \ \ \\ \ \\\\ \\\\\\\\\\\ A \ \ \ \ N \ \ \ \ \ \ I \ \ \ \ \ \ E, \\\\\\ \ \ u- \\ \\:\:\\\ \\ \ \ \ \ \ \ \ \ \\ \ \ \\ 15 25 35 4 Temperature (‘0) Calibrated 4MHZ RC Oscillator Frequency vs. V / // / / / / // 1: / / I / e / /// / / u" / /// // Atmel
Figure 35-38 Calibrated 4MHz RC Oscillator Frequency vs. Temperature
Calibrated 4MHz RC Oscillator Frequency vs. Temperature
4.15
4.05
3.95
3.85
3.75
3.65
3.55
-45 -35 -25 -15 -5 515 25 35 45 55 65 75 85 95 105
2.7V
3.3V
3.6V
4.0V
4.5V
5.0V
5.5V
Temperature (°C)
F
RC (MHz)
Figure 35-39 Calibrated 4MHz RC Oscillator Frequency vs. VCC
Calibrated 4MHz RC Oscillator Frequency vs. VCC
4.15
4.05
3.95
3.85
3.75
3.65
3.55 2.5 2.8 3.1 3.4 3.7 44.3 4.6 4.9 5.2 5.5
105°C
85°C
25°C
-40°C
V
CC(V)
F
RC (MHz)
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
485
CALIBRATED SMIHZ RC OSCILLATOR FREQUENCYVS. TEMPERATURE (MHz) F <>< calibrated="" xmhz="" rc="" oscillator="" frequency="" vs.="" operating="" (ml-i7)="" f="" \\\="" atmel="">
Figure 35-40 Calibrated 8MHz RC Oscillator Frequency vs. Temperature
CALIBRATED 8MHz RC OSCILLATOR FREQUENCY vs . TEMPERATURE
5.5 V
5.0 V
4.5 V
4.0 V
3.6 V
3.3 V
3.0 V
2.7 V
6.6
6.8
7.0
7.2
7.4
7.6
7.8
8.0
8.2
8.4
-45 -35 -25 -15 -5 5 15 25 35 45 55 65 75 85 95 105
Tempe rature (°C)
FRC (MHz)
Figure 35-41 Calibrated 8MHz RC Oscillator Frequency vs. VCC
CALIBRATED 8MHz RC OSCILLATOR FREQUENCY vs . OP ERATING
VOLTAGE
105°C
85°C
25°C
-40°C
6.4
6.7
7.0
7.3
7.6
7.9
8.2
8.5
2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5
VCC (V)
FRC (MHz)
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
486
Calibrated 8MHz RC Oscillator Frequency vs. Osmal Value 74‘ I E / u. /69/ //,;=s°‘ f—figgé OSCCAL (x1) Brownoul Detector Current vs. V 2‘ & 0 _° Atmel
Figure 35-42 Calibrated 8MHz RC Oscillator Frequency vs. Osccal Value
Calibrated 8MHz RC Oscillator Frequency vs. Osccal Value
16
14
12
10
8
6
4
2
016 32 48 64 80 96 112 128 144 160 176 192 208 224 240 256
105°C
85°C
25°C
-40°C
OSCCAL (X1)
F
RC (MHz)
35.9. Current Consumption of Peripheral Units
Figure 35-43 Brownout Detector Current vs. VCC
Brownout Detector Current vs. VCC
19
18
17
16
15
14
13
12
11
10
9
2.5 2.8 3.1 3.4 3.7 44.3 4.6 4.9 5.2 5.5
105°C
85°C
25°C
-40°C
VCC (V)
ICC (µA)
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
487
ADC Current vs. V c (AREF = AV ) 420 400 380 360 340 320 300 'chA) Vcc AREF External Reference Current vs. V 190 180 170 160 150 140 130 'cc (MA) Atmel
Figure 35-44 ADC Current vs. VCC (AREF = AVCC)
105°C
85°C
25°C
-40°C
ADC Current vs. VCC (AREF = AVCC)
420
400
380
360
340
320
300
280
260
240
220
2.5 2.8 3.1 3.4 3.7 44.3 4.6 4.9 5.2 5.5
VCC (V)
ICC (µA)
Figure 35-45 AREF External Reference Current vs. VCC
105°C
85°C
25°C
-40°C
AREF External Reference Current vs. VCC
190
180
170
160
150
140
130
120
110
100
90
2.5 2.8 3.1 3.4 3.7 44.3 4.6 4.9 5.2 5.5
VCC (V)
ICC (µA)
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
488
Watchdog Timer Current vs. V 21 19 17 15 13 / 11 / '00 (VA) Vcc Analog Comparator Current vs. V Icc (1AA) Atmel
Figure 35-46 Watchdog Timer Current vs. VCC
105°C
85°C
25°C
-40°C
Watchdog Timer Current vs. VCC
21
19
17
15
13
11
9
7
5
2.5 2.8 3.1 3.4 3.7 44.3 4.6 4.9 5.2 5.5
VCC(V)
ICC (µA)
Figure 35-47 Analog Comparator Current vs. VCC
105°C
85°C
25°C
-40°C
Analog Comparator Current vs. VCC
85
80
75
70
65
60
55
50
45
40
352.5 2.8 3.1 3.4 3.7 44.3 4.6 4.9 5.2 5.5
VCC (V)
ICC (µA)
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
489
Programming Current vs. V '90 (mA) Reset Supply Current vs. Veg (0.1 -1.0MH2, Excludi enl Through The R 0.5 0.6 Frequency (MHZ) Atmel
Figure 35-48 Programming Current vs. VCC
105°C
85°C
25°C
-40°C
Programming Current vs. VCC
6
5.5
5
4.5
4
3.5
3
2.5
2
1.5
1
2.5 2.8 3.1 3.4 3.7 44.3 4.6 4.9 5.2 5.5
VCC (V)
ICC (mA)
35.10. Current Consumption in Reset and Reset Pulsewidth
Figure 35-49 Reset Supply Current vs. VCC (0.1 - 1.0
Reset Supply Current vs. VCC (0.1 - 1.0MHz,
Excluding Current Through The Reset Pull-up)
3
2.7
2.4
2.1
1.8
1.5
1.2
0.9
0.6
0.3
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
2.7V
3.3V
3.6V
4.0V
4.5V
5.0V
5.5V
Frequency (MHz)
ICC (mA)
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
490
Reset Supply Current vs. Vcc (1 >16MHz, Excluding Current Through The Reset Pull-up) 16 v 14 v 12 v 10 / V8 6 //, 3 6V _ // 3,3v 4 2 //%/‘ 2 7V / D D 2 4 6 8 10 12 14 16 Minimum Reset Pulse Width vs. V E 5 \ E a \\ a: fi \ \\ \\ \ \\ \§ \ \ \\\\ 4 Vet: Atmel
Figure 35-50 Reset Supply Current vs. VCC (1 - 16MHz, Excluding Current Through The Reset Pull-up)
5.5V
5.0V
4.5V
4.0V
3.6V
3.3V
2.7V
Reset Supply Current vs. VCC
(1 - 16MHz, Excluding Current Through The Reset Pull-up)
16
14
12
10
8
6
4
2
016
14
12
10
8
6
4
2
0
ICC(mA)
Figure 35-51 Minimum Reset Pulse Width vs. VCC
Minimum Reset Pulse Width vs. VCC
800
700
600
500
400
300
200
2.5 2.8 3.1 3.4 3.7 44.3 4.6 4.9 5.2 5.5
-40°C
25°C
85°C
105°C
VCC (V)
Pulsewidth (ns)
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
491
Atmel
36. Register Summary
Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
(0xFF) Reserved – – – – – – – –
: Reserved – – – – – – – –
(0x9E) Reserved – – – – – – – –
(0x9D) UCSR1C – UMSEL1 UPM11 UPM10 USBS1 UCSZ11 UCSZ10 UCPOL1
(0x9C) UDR1 USART1 I/O Data Register
(0x9B) UCSR1A RXC1 TXC1 UDRE1 FE1 DOR1 UPE1 U2X1 MPCM1
(0x9A) UCSR1B RXCIE1 TXCIE1 UDRIE1 RXEN1 TXEN1 UCSZ12 RXB81 TXB81
(0x99) UBRR1L USART1 Baud Rate Register Low
(0x98) UBRR1H – – – – USART1 Baud Rate Register High
(0x97) Reserved – – – – – – – –
(0x96) Reserved – – – – – – – –
(0x95) UCSR0C – UMSEL0 UPM01 UPM00 USBS0 UCSZ01 UCSZ00 UCPOL0
(0x94) Reserved – – – – – – – –
(0x93) Reserved – – – – – – – –
(0x92) Reserved – – – – – – – –
(0x91) Reserved – – – – – – – –
(0x90) UBRR0H – – – – USART0 Baud Rate Register High
(0x8F) Reserved – – – – – – – –
(0x8E) ADCSRB – – – – – ADTS2 ADTS1 ADTS0
(0x8D) Reserved – – – – – – – –
(0x8C) TCCR3C FOC3A FOC3B FOC3C – – – – –
(0x8B) TCCR3A COM3A1 COM3A0 COM3B1 COM3B0 COM3C1 COM3C0 WGM31 WGM30
(0x8A) TCCR3B ICNC3 ICES3 – WGM33 WGM32 CS32 CS31 CS30
(0x89) TCNT3H Timer/Counter3 – Counter Register High Byte
(0x88) TCNT3L Timer/Counter3 – Counter Register Low Byte
(0x87) OCR3AH Timer/Counter3 – Output Compare Register A High Byte
(0x86) OCR3AL Timer/Counter3 – Output Compare Register A Low Byte
(0x85) OCR3BH Timer/Counter3 – Output Compare Register B High Byte
(0x84) OCR3BL Timer/Counter3 – Output Compare Register B Low Byte
(0x83) OCR3CH Timer/Counter3 – Output Compare Register C High Byte
(0x82) OCR3CL Timer/Counter3 – Output Compare Register C Low Byte
(0x81) ICR3H Timer/Counter3 – Input Capture Register High Byte
(0x80) ICR3L Timer/Counter3 – Input Capture Register Low Byte
(0x7F) Reserved – – – – – – – –
(0x7E) Reserved – – – – – – – –
(0x7D) ETIMSK – – TICIE3 OCIE3A OCIE3B TOIE3 OCIE3C OCIE1C
(0x7C) ETIFR – – ICF3 OCF3A OCF3B TOV3 OCF3C OCF1C
(0x7B) Reserved – – – – – – – –
(0x7A) TCCR1C FOC1A FOC1B FOC1C – – – – –
(0x79) OCR1CH Timer/Counter1 – Output Compare Register C High Byte
(0x78) OCR1CL Timer/Counter1 – Output Compare Register C Low Byte
(0x77) Reserved – – – – – – – –
(0x76) Reserved – – – – – – – –
(0x75) Reserved – – – – – – – –
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
492
Atmel
Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
(0x74) TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE
(0x73) TWDR Two-wire Serial Interface Data Register
(0x72) TWAR TWA6 TWA5 TWA4 TWA3 TWA2 TWA1 TWA0 TWGCE
(0x71) TWSR TWS7 TWS6 TWS5 TWS4 TWS3 – TWPS1 TWPS0
(0x70) TWBR Two-wire Serial Interface Bit Rate Register
(0x6F) OSCCAL Oscillator Calibration Register
(0x6E) Reserved – – – – – – – –
(0x6D) XMCRA – SRL2 SRL1 SRL0 SRW01 SRW00 SRW11 –
(0x6C) XMCRB XMBK – – – – XMM2 XMM1 XMM0
(0x6B) Reserved – – – – – – – –
(0x6A) EICRA ISC31 ISC30 ISC21 ISC20 ISC11 ISC10 ISC01 ISC00
(0x69) Reserved – – – – – – – –
(0x68) SPMCSR SPMIE RWWSB – RWWSRE BLBSET PGWRT PGERS SPMEN
(0x67) Reserved – – – – – – – –
(0x66) Reserved – – – – – – – –
(0x65) PORTG – – – PORTG4 PORTG3 PORTG2 PORTG1 PORTG0
(0x64) DDRG – – – DDG4 DDG3 DDG2 DDG1 DDG0
(0x63) PING – – – PING4 PING3 PING2 PING1 PING0
(0x62) PORTF PORTF7 PORTF6 PORTF5 PORTF4 PORTF3 PORTF2 PORTF1 PORTF0
(0x61) DDRF DDF7 DDF6 DDF5 DDF4 DDF3 DDF2 DDF1 DDF0
(0x60) Reserved – – – – – – – –
0x3F (0x5F) SREG I T H S V N Z C
0x3E (0x5E) SPH SP15 SP14 SP13 SP12 SP11 SP10 SP9 SP8
0x3D (0x5D) SPL SP7 SP6 SP5 SP4 SP3 SP2 SP1 SP0
0x3C (0x5C) XDIV XDIVEN XDIV6 XDIV5 XDIV4 XDIV3 XDIV2 XDIV1 XDIV0
0x3B (0x5B) Reserved – – – – – – – –
0x3A (0x5A) EICRB ISC71 ISC70 ISC61 ISC60 ISC51 ISC50 ISC41 ISC40
0x39 (0x59) EIMSK INT7 INT6 INT5 INT4 INT3 INT2 INT0 INT0
0x38 (0x58) EIFR INTF7 INTF6 INTF5 INTF4 INTF3 INTF2 INTF1 INTF0
0x37 (0x57) TIMSK OCIE2 TOIE2 TICIE1 OCIE1A OCIE1B TOIE1 OCIE0 TOIE0
0x36 (0x56) TIFR OCF2 TOV2 ICF1 OCF1A OCF1B TOV1 OCF0 TOV0
0x35 (0x55) MCUCR SRE SRW10 SE SM1 SM0 SM2 IVSEL IVCE
0x34 (0x54) MCUCSR JTD – – JTRF WDRF BORF EXTRF PORF
0x33 (0x53) TCCR0 FOC0 WGM00 COM01 COM00 WGM01 CS02 CS01 CS00
0x32 (0x52) TCNT0 Timer/Counter0 (8 Bit)
0x31 (0x51) OCR0 Timer/Counter0 Output Compare Register
0x30 (0x50) ASSR – – – – AS0 TCN0UB OCR0UB TCR0UB
0x2F (0x4F) TCCR1A COM1A1 COM1A0 COM1B1 COM1B0 COM1C1 COM1C0 WGM11 WGM10
0x2E (0x4E) TCCR1B ICNC1 ICES1 – WGM13 WGM12 CS12 CS11 CS10
0x2D (0x4D) TCNT1H Timer/Counter1 – Counter Register High Byte
0x2C (0x4C) TCNT1L Timer/Counter1 – Counter Register Low Byte
0x2B (0x4B) OCR1AH Timer/Counter1 – Output Compare Register A High Byte
0x2A (0x4A) OCR1AL Timer/Counter1 – Output Compare Register A Low Byte
0x29 (0x49) OCR1BH Timer/Counter1 – Output Compare Register B High Byte
0x28 (0x48) OCR1BL Timer/Counter1 – Output Compare Register B Low Byte
0x27 (0x47) ICR1H Timer/Counter1 – Input Capture Register High Byte
0x26 (0x46) ICR1L Timer/Counter1 – Input Capture Register Low Byte
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
493
Atmel
Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
0x25 (0x45) TCCR2 FOC2 WGM20 COM21 COM20 WGM21 CS22 CS21 CS20
0x24 (0x44) TCNT2 Timer/Counter2 (8 Bit)
0x23 (0x43) OCR2 Timer/Counter2 Output Compare Register
0x22 (0x42) OCDR IDRD/
OCDR7 OCDR6 OCDR5 OCDR4 OCDR3 OCDR2 OCDR1 OCDR0
0x21 (0x41) WDTCR – – – WDCE WDE WDP2 WDP1 WDP0
0x20 (0x40) SFIOR TSM – – – ACME PUD PSR0 PSR321
0x1F (0x3F) EEARH – – – – EEPROM Address Register High
0x1E (0x3E) EEARL EEPROM Address Register Low Byte
0x1D (0x3D) EEDR EEPROM Data Register
0x1C (0x3C) EECR – – – – EERIE EEMWE EEWE EERE
0x1B (0x3B) PORTA PORTA7 PORTA6 PORTA5 PORTA4 PORTA3 PORTA2 PORTA1 PORTA0
0x1A (0x3A) DDRA DDA7 DDA6 DDA5 DDA4 DDA3 DDA2 DDA1 DDA0
0x19 (0x39) PINA PINA7 PINA6 PINA5 PINA4 PINA3 PINA2 PINA1 PINA0
0x18 (0x38) PORTB PORTB7 PORTB6 PORTB5 PORTB4 PORTB3 PORTB2 PORTB1 PORTB0
0x17 (0x37) DDRB DDB7 DDB6 DDB5 DDB4 DDB3 DDB2 DDB1 DDB0
0x16 (0x36) PINB PINB7 PINB6 PINB5 PINB4 PINB3 PINB2 PINB1 PINB0
0x15 (0x35) PORTC PORTC7 PORTC6 PORTC5 PORTC4 PORTC3 PORTC2 PORTC1 PORTC0
0x14 (0x34) DDRC DDC7 DDC6 DDC5 DDC4 DDC3 DDC2 DDC1 DDC0
0x13 (0x33) PINC PINC7 PINC6 PINC5 PINC4 PINC3 PINC2 PINC1 PINC0
0x12 (0x32) PORTD PORTD7 PORTD6 PORTD5 PORTD4 PORTD3 PORTD2 PORTD1 PORTD0
0x11 (0x31) DDRD DDd7 DDD6 DDD5 DDD4 DDD3 DDD2 DDD1 DDD0
0x10 (0x30) PIND PIND7 PIND6 PIND5 PIND4 PIND3 PIND2 PIND1 PIND0
0x0F (0x2F) SPDR SPI Data Register
0x0E (0x2E) SPSR SPIF WCOL – – – – – SPI2X
0x0D (0x2D) SPCR SPIE SPE DORD MSTR CPOL CPHA SPR1 SPR0
0x0C (0x2C) UDR0 USART0 I/O Data Register
0x0B (0x2B) UCSR0A RXC0 TXC0 UDRE0 FE0 DOR0 UPE0 U2X0 MPCM0
0x0A (0x2A) UCSR0B RXCIE0 TXCIE0 UDRIE0 RXEN0 TXEN0 UCSZ02 RXB80 TXB80
0x09 (0x29) UBRR0L USART0 Baud Rate Register Low
0x08 (0x28) ACSR ACD ACBG ACO ACI ACIE ACIC ACIS1 ACIS0
0x07 (0x27) ADMUX REFS1 REFS0 ADLAR MUX4 MUX3 MUX2 MUX1 MUX0
0x06 (0x26) ADCSRA ADEN ADSC ADATE ADIF ADIE ADPS2 ADPS1 ADPS0
0x05 (0x25) ADCH ADC Data Register High Byte
0x04 (0x24) ADCL ADC Data Register Low byte
0x03 (0x23) PORTE PORTE7 PORTE6 PORTE5 PORTE4 PORTE3 PORTE2 PORTE1 PORTE0
0x02 (0x22) DDRE DDE7 DDE6 DDE5 DDE4 DDE3 DDE2 DDE1 DDE0
0x01 (0x21) PINE PINE7 PINE6 PINE5 PINE4 PINE3 PINE2 PINE1 PINE0
0x00 (0x20) PINF PINF7 PINF6 PINF5 PINF4 PINF3 PINF2 PINF1 PINF0
Note: 
1. For compatibility with future devices, reserved bits should be written to zero if accessed. Reserved
I/O memory addresses should never be written.
2. Some of the status flags are cleared by writing a logical one to them. Note that the CBI and SBI
instructions will operate on all bits in the I/O register, writing a one back into any flag read as set,
thus clearing the flag. The CBI and SBI instructions work with registers 0x00 to 0x1F only.
Atmel ATmega64A [DATASHEET]
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Atmel
37. Instruction Set Summary
ARITHMETIC AND LOGIC INSTRUCTIONS
Mnemonics Operands Description Operation Flags #Clocks
ADD Rd, Rr Add two Registers Rd ← Rd + Rr Z,C,N,V,H 1
ADC Rd, Rr Add with Carry two Registers Rd ← Rd + Rr + C Z,C,N,V,H 1
ADIW Rdl,K Add Immediate to Word Rdh:Rdl ← Rdh:Rdl + K Z,C,N,V,S 2
SUB Rd, Rr Subtract two Registers Rd ← Rd - Rr Z,C,N,V,H 1
SUBI Rd, K Subtract Constant from Register Rd ← Rd - K Z,C,N,V,H 1
SBC Rd, Rr Subtract with Carry two Registers Rd ← Rd - Rr - C Z,C,N,V,H 1
SBCI Rd, K Subtract with Carry Constant from Reg. Rd ← Rd - K - C Z,C,N,V,H 1
SBIW Rdl,K Subtract Immediate from Word Rdh:Rdl ← Rdh:Rdl - K Z,C,N,V,S 2
AND Rd, Rr Logical AND Registers Rd ← Rd · Rr Z,N,V 1
ANDI Rd, K Logical AND Register and Constant Rd ← Rd · K Z,N,V 1
OR Rd, Rr Logical OR Registers Rd ← Rd v Rr Z,N,V 1
ORI Rd, K Logical OR Register and Constant Rd ← Rd v K Z,N,V 1
EOR Rd, Rr Exclusive OR Registers Rd ← Rd ⊕ Rr Z,N,V 1
COM Rd One’s Complement Rd ← 0xFF - Rd Z,C,N,V 1
NEG Rd Two’s Complement Rd ← 0x00 - Rd Z,C,N,V,H 1
SBR Rd,K Set Bit(s) in Register Rd ← Rd v K Z,N,V 1
CBR Rd,K Clear Bit(s) in Register Rd ← Rd · (0xFF - K) Z,N,V 1
INC Rd Increment Rd ← Rd + 1 Z,N,V 1
DEC Rd Decrement Rd ← Rd - 1 Z,N,V 1
TST Rd Test for Zero or Minus Rd ← Rd · Rd Z,N,V 1
CLR Rd Clear Register Rd ← Rd ⊕ Rd Z,N,V 1
SER Rd Set Register Rd ← 0xFF None 1
MUL Rd, Rr Multiply Unsigned R1:R0 ← Rd x Rr Z,C 2
MULS Rd, Rr Multiply Signed R1:R0 ← Rd x Rr Z,C 2
MULSU Rd, Rr Multiply Signed with Unsigned R1:R0 ← Rd x Rr Z,C 2
FMUL Rd, Rr Fractional Multiply Unsigned R1:R0 ← (Rd x Rr) << 1 Z,C 2
FMULS Rd, Rr Fractional Multiply Signed R1:R0 ← (Rd x Rr) << 1 Z,C 2
FMULSU Rd, Rr Fractional Multiply Signed with Unsigned R1:R0 ← (Rd x Rr) << 1 Z,C 2
BRANCH INSTRUCTIONS
Mnemonics Operands Description Operation Flags #Clocks
RJMP k Relative Jump PC ← PC + k + 1 None 2
IJMP Indirect Jump to (Z) PC ← Z None 2
JMP(1) k Direct Jump PC ← k None 3
RCALL k Relative Subroutine Call PC ← PC + k + 1 None 3
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BRANCH INSTRUCTIONS
Mnemonics Operands Description Operation Flags #Clocks
ICALL Indirect Call to (Z) PC ← Z None 3
CALL(1) k Direct Subroutine Call PC ← k None 4
RET Subroutine Return PC ← STACK None 4
RETI Interrupt Return PC ← STACK I 4
CPSE Rd,Rr Compare, Skip if Equal if (Rd = Rr) PC ← PC + 2 or 3 None 1 / 2 / 3
CP Rd,Rr Compare Rd - Rr Z, N,V,C,H 1
CPC Rd,Rr Compare with Carry Rd - Rr - C Z, N,V,C,H 1
CPI Rd,K Compare Register with Immediate Rd - K Z, N,V,C,H 1
SBRC Rr, b Skip if Bit in Register Cleared if (Rr(b)=0) PC ← PC + 2 or 3 None 1 / 2 / 3
SBRS Rr, b Skip if Bit in Register is Set if (Rr(b)=1) PC ← PC + 2 or 3 None 1 / 2 / 3
SBIC P, b Skip if Bit in I/O Register Cleared if (P(b)=0) PC ← PC + 2 or 3 None 1 / 2 / 3
SBIS P, b Skip if Bit in I/O Register is Set if (P(b)=1) PC ← PC + 2 or 3 None 1 / 2 / 3
BRBS s, k Branch if Status Flag Set if (SREG(s) = 1) then PC←PC+k + 1 None 1 / 2
BRBC s, k Branch if Status Flag Cleared if (SREG(s) = 0) then PC←PC+k + 1 None 1 / 2
BREQ k Branch if Equal if (Z = 1) then PC ← PC + k + 1 None 1 / 2
BRNE k Branch if Not Equal if (Z = 0) then PC ← PC + k + 1 None 1 / 2
BRCS k Branch if Carry Set if (C = 1) then PC ← PC + k + 1 None 1 / 2
BRCC k Branch if Carry Cleared if (C = 0) then PC ← PC + k + 1 None 1 / 2
BRSH k Branch if Same or Higher if (C = 0) then PC ← PC + k + 1 None 1 / 2
BRLO k Branch if Lower if (C = 1) then PC ← PC + k + 1 None 1 / 2
BRMI k Branch if Minus if (N = 1) then PC ← PC + k + 1 None 1 / 2
BRPL k Branch if Plus if (N = 0) then PC ← PC + k + 1 None 1 / 2
BRGE k Branch if Greater or Equal, Signed if (N Å V= 0) then PC ← PC + k + 1 None 1 / 2
BRLT k Branch if Less Than Zero, Signed if (N Å V= 1) then PC ← PC + k + 1 None 1 / 2
BRHS k Branch if Half Carry Flag Set if (H = 1) then PC ← PC + k + 1 None 1 / 2
BRHC k Branch if Half Carry Flag Cleared if (H = 0) then PC ← PC + k + 1 None 1 / 2
BRTS k Branch if T Flag Set if (T = 1) then PC ← PC + k + 1 None 1 / 2
BRTC k Branch if T Flag Cleared if (T = 0) then PC ← PC + k + 1 None 1 / 2
BRVS k Branch if Overflow Flag is Set if (V = 1) then PC ← PC + k + 1 None 1 / 2
BRVC k Branch if Overflow Flag is Cleared if (V = 0) then PC ← PC + k + 1 None 1 / 2
BRIE k Branch if Interrupt Enabled if ( I = 1) then PC ← PC + k + 1 None 1 / 2
BRID k Branch if Interrupt Disabled if ( I = 0) then PC ← PC + k + 1 None 1 / 2
BIT AND BIT-TEST INSTRUCTIONS
Mnemonics Operands Description Operation Flags #Clocks
SBI P,b Set Bit in I/O Register I/O(P,b) ← 1 None 2
CBI P,b Clear Bit in I/O Register I/O(P,b) ← 0 None 2
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BIT AND BIT-TEST INSTRUCTIONS
Mnemonics Operands Description Operation Flags #Clocks
LSL Rd Logical Shift Left Rd(n+1) ← Rd(n), Rd(0) ← 0 Z,C,N,V 1
LSR Rd Logical Shift Right Rd(n) ← Rd(n+1), Rd(7) ← 0 Z,C,N,V 1
ROL Rd Rotate Left Through Carry Rd(0)←C,Rd(n+1)← Rd(n),C¬Rd(7) Z,C,N,V 1
ROR Rd Rotate Right Through Carry Rd(7)←C,Rd(n)← Rd(n+1),C←Rd(0) Z,C,N,V 1
ASR Rd Arithmetic Shift Right Rd(n) ← Rd(n+1), n=0:6 Z,C,N,V 1
SWAP Rd Swap Nibbles Rd(3:0)←Rd(7:4),Rd(7:4)¬Rd(3:0) None 1
BSET s Flag Set SREG(s) ← 1 SREG(s) 1
BCLR s Flag Clear SREG(s) ← 0 SREG(s) 1
BST Rr, b Bit Store from Register to T T ← Rr(b) T 1
BLD Rd, b Bit load from T to Register Rd(b) ← T None 1
SEC Set Carry C ← 1 C 1
CLC Clear Carry C ← 0 C 1
SEN Set Negative Flag N ← 1 N 1
CLN Clear Negative Flag N ← 0 N 1
SEZ Set Zero Flag Z ← 1 Z 1
CLZ Clear Zero Flag Z ← 0 Z 1
SEI Global Interrupt Enable I ← 1 I 1
CLI Global Interrupt Disable I ← 0 I 1
SES Set Signed Test Flag S ← 1 S 1
CLS Clear Signed Test Flag S ← 0 S 1
SEV Set Twos Complement Overflow. V ← 1 V 1
CLV Clear Twos Complement Overflow V ← 0 V 1
SET Set T in SREG T ← 1 T 1
CLT Clear T in SREG T ← 0 T 1
SEH Set Half Carry Flag in SREG H ← 1 H 1
CLH Clear Half Carry Flag in SREG H ← 0 H 1
DATA TRANSFER INSTRUCTIONS
Mnemonics Operands Description Operation Flags #Clocks
MOV Rd, Rr Move Between Registers Rd ← Rr None 1
MOVW Rd, Rr Copy Register Word Rd+1:Rd ← Rr+1:Rr None 1
LDI Rd, K Load Immediate Rd ← K None 1
LD Rd, X Load Indirect Rd ← (X) None 2
LD Rd, X+ Load Indirect and Post-Inc. Rd ← (X), X ← X + 1 None 2
LD Rd, - X Load Indirect and Pre-Dec. X ← X - 1, Rd ← (X) None 2
LD Rd, Y Load Indirect Rd ← (Y) None 2
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DATA TRANSFER INSTRUCTIONS
Mnemonics Operands Description Operation Flags #Clocks
LD Rd, Y+ Load Indirect and Post-Inc. Rd ← (Y), Y ← Y + 1 None 2
LD Rd, - Y Load Indirect and Pre-Dec. Y ← Y - 1, Rd ← (Y) None 2
LDD Rd,Y+q Load Indirect with Displacement Rd ← (Y + q) None 2
LD Rd, Z Load Indirect Rd ← (Z) None 2
LD Rd, Z+ Load Indirect and Post-Inc. Rd ← (Z), Z ← Z+1 None 2
LD Rd, -Z Load Indirect and Pre-Dec. Z ← Z - 1, Rd ← (Z) None 2
LDD Rd, Z+q Load Indirect with Displacement Rd ← (Z + q) None 2
LDS Rd, k Load Direct from SRAM Rd ← (k) None 2
ST X, Rr Store Indirect (X) ← Rr None 2
ST X+, Rr Store Indirect and Post-Inc. (X) ← Rr, X ← X + 1 None 2
ST - X, Rr Store Indirect and Pre-Dec. X ← X - 1, (X) ← Rr None 2
ST Y, Rr Store Indirect (Y) ¬ Rr None 2
ST Y+, Rr Store Indirect and Post-Inc. (Y) ← Rr, Y ← Y + 1 None 2
ST - Y, Rr Store Indirect and Pre-Dec. Y ← Y - 1, (Y) ← Rr None 2
STD Y+q,Rr Store Indirect with Displacement (Y + q) ← Rr None 2
ST Z, Rr Store Indirect (Z) ← Rr None 2
ST Z+, Rr Store Indirect and Post-Inc. (Z) ← Rr, Z ← Z + 1 None 2
ST - Z, Rr Store Indirect and Pre-Dec. Z ← Z - 1, (Z) ← Rr None 2
STD Z+q,Rr Store Indirect with Displacement (Z + q) ← Rr None 2
STS k, Rr Store Direct to SRAM (k) ← Rr None 2
LPM Load Program Memory R0 ← (Z) None 3
LPM Rd, Z Load Program Memory Rd ← (Z) None 3
LPM Rd, Z+ Load Program Memory and Post-Inc Rd ← (Z), Z ← Z+1 None 3
SPM Store Program Memory (Z) ← R1:R0 None -
IN Rd, P In Port Rd ← P None 1
OUT P, Rr Out Port P ← Rr None 1
PUSH Rr Push Register on Stack STACK ← Rr None 2
POP Rd Pop Register from Stack Rd ← STACK None 2
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MCU CONTROL INSTRUCTIONS
Mnemonics Operands Description Operation Flags #Clocks
NOP No Operation None 1
SLEEP Sleep (see specific descr. for Sleep function) None 1
WDR Watchdog Reset (see specific descr. for WDR/timer) None 1
BREAK Break For On-chip Debug Only None N/A
Note:  1. Instruction not available in all devices.
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38. Packaging Information
38.1. 64A
2325 Orchard Parkway
San Jose, CA 95131
TITLE DRAWING NO. REV.
64A, 64-lead, 14 x 14mm Body Size, 1.0mm Body Thickness,
0.8mm Lead Pitch, Thin Prof le Plastic Quad Flat Package (TQFP) C
64A
2010-10-20
PIN 1 IDENTIFIER
0°~7°
PIN 1
L
C
A1 A2 A
D1
D
e
E1 E
B
COMMON DIMENSIONS
(Unit of measure = mm)
SYMBOL MIN NOM MAX NOTE
Notes:
1.This package conforms to JEDEC reference MS-026, Variation AEB.
2. Dimensions D1 and E1 do not include mold protrusion. Allowable
protrusion is 0.25mm per side. Dimensions D1 and E1 are maximum
plastic body size dimensions including mold mismatch.
3. Lead coplanarity is 0.10mm maximum.
A – – 1.20
A1 0.05 – 0.15
A2 0.95 1.00 1.05
D 15.75 16.00 16.25
D1 13.90 14.00 14.10 Note 2
E 15.75 16.00 16.25
E1 13.90 14.00 14.10 Note 2
B 0.30 – 0.45
C 0.09 – 0.20
L 0.45 – 0.75
e 0.80 TYP
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123 U’UUJU \ :ij UUUUUUUU 2232:3232: ccccctcccccccccc D 2 4mm nnnnnnnnnnnnnnnn _|_ E. * f! DRAWINGN O. Atmet
38.2. 64M1
2325 Orchard Parkway
San Jos e, CA 9513 1
TITLE DRAWING N O. RE V.
64M1 , 64-pad, 9 x 9 x 1.0 mm Bod y, Lead Pitch 0.50 mm , H
64M1
2010-10-19
COMMON DIMENSIONS
(Unit of Measure = mm)
SYMBOL MIN NOM MAX NOTE
A 0.80 0.90 1.00
A1 – 0.02 0.05
b 0.18 0.25 0.30
D
D2 5.20 5.40 5.60
8.90 9.00 9.10
8.90 9.00 9.10
E
E2 5.20 5.40 5.60
e 0.50 BSC
L 0.35 0.40 0.45
Notes:
1 . JEDEC Standard MO-220, (S AW Singulation) Fig . 1, VMM D.
2 . Dimension and tole rance con form to ASMEY14.5M-1994 .
TOP VIE W
SIDE VIEW
BOTTOM VIE W
D
E
Ma rked Pin# 1 I D
SE ATING PLAN E
A1
C
A
C
0.08
1
2
3
K 1.25 1.40 1.55
E2
D2
be
Pin #1 Co rner
L
Pin #1
Triangle
Pin #1
Cham fer
(C 0.30)
Option A
Option B
Pin #1
Notch
(0.20 R)
Option C
K
K
5.40 mm Exposed Pad, Micro Lead Frame Pa ckage (MLF)
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39. Errata
The revision letter in this section refers to the revision of the ATmega64A device.
39.1. ATmega64A Rev. D
•First Analog Comparator conversion may be delayed
•Interrupts may be lost when writing the timer registers in the asynchronous timer
•Stabilizing time needed when changing XDIV Register
•Stabilizing time needed when changing OSCCAL Register
•IDCODE masks data from TDI input
•Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt request
1. First Analog Comparator conversion may be delayed
If the device is powered by a slow rising VCC, the first Analog Comparator conversion will take
longer than expected on some devices.
Problem Fix/Workaround
When the device has been powered or reset, disable then enable the Analog Comparator before
the first conversion.
2. Interrupts may be lost when writing the timer registers in the asynchronous timer
The interrupt will be lost if a timer register that is synchronous timer clock is written when the
asynchronous Timer/Counter register (TCNTx) is 0x00.
Problem Fix/Workaround
Always check that the asynchronous Timer/Counter register neither have the value 0xFF nor 0x00
before writing to the asynchronous Timer Control Register (TCCRx), asynchronous Timer Counter
Register (TCNTx), or asynchronous Output Compare Register (OCRx).
3. Stabilizing time needed when changing XDIV Register
After increasing the source clock frequency more than 2% with settings in the XDIV register, the
device may execute some of the subsequent instructions incorrectly.
Problem Fix/Workaround
The NOP instruction will always be executed correctly also right after a frequency change. Thus,
the next 8 instructions after the change should be NOP instructions. To ensure this, follow this
procedure:
3.1. Clear the I bit in the SREG Register.
3.2. Set the new pre-scaling factor in XDIV register.
3.3. Execute 8 NOP instructions
3.4. Set the I bit in SREG
This will ensure that all subsequent instructions will execute correctly.
Assembly Code Example:
CLI ; clear global interrupt enable
OUT XDIV, temp ; set new prescale value
NOP ; no operation
NOP ; no operation
NOP ; no operation
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NOP ; no operation
NOP ; no operation
NOP ; no operation
NOP ; no operation
NOP ; no operation
SEI ; set global interrupt enable
4. Stabilizing time needed when changing OSCCAL Register
After increasing the source clock frequency more than 2% with settings in the OSCCAL register, the
device may execute some of the subsequent instructions incorrectly.
Problem Fix/Workaround
The behavior follows errata number 3., and the same Fix / Workaround is applicable on this errata.
5. IDCODE masks data from TDI input
The JTAG instruction IDCODE is not working correctly. Data to succeeding devices are replaced by
all-ones during Update-DR.
Problem Fix/Workaround
–If ATmega64A is the only device in the scan chain, the problem is not visible.
–Select the Device ID Register of the ATmega64A by issuing the IDCODE instruction or by
entering the Test-Logic-Reset state of the TAP controller to read out the contents of its Device
ID Register and possibly data from succeeding devices of the scan chain. Issue the BYPASS
instruction to the ATmega64A while reading the Device ID Registers of preceding devices of
the boundary scan chain.
–If the Device IDs of all devices in the boundary scan chain must be captured simultaneously,
the ATmega64A must be the first device in the chain.
6. Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt request.
Reading EEPROM by using the ST or STS command to set the EERE bit in the EECR register
triggers an unexpected EEPROM interrupt request.
Problem Fix/Workaround
Always use OUT or SBI to set EERE in EECR.
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40. Datasheet Revision History
Please note that the referring page numbers in this section are referred to this document. The referring
revision in this section refers to the document revision.
40.1. 8160E - 07/2015
1. New workflow used for the publication.
40.2. 8160D - 02/2013
1. Applied the new template that includes new logo and new last page.
2. Added Capacitive Touch Sensing on page 21.
3. Note is added Performing Page Erase by SPM on page 373.
4. Note 6 and Note 7 below Table 32-6 Two-wire Serial Bus Requirements on page 419 have been
removed.
5. Formulas in Table 32-6 Two-wire Serial Bus Requirements on page 419 have been updated.
6. Added Electrical Characteristics – TA = -40°C to 105°C on page 433.
7. Added Typical Characteristics – TA = -40°C to 105°C on page 466.
8. Updated Ordering Information on page 11 and added Ordering Information for 105°C devices.
40.3. 8160C - 07/2009
1. Updated Errata on page 502.
40.4. 8160B - 03/2009
1. Updated Typical Characteristics – TA = -40°C to 85°C on page 436 view.
2. Updated Figure 34-36 BOD Thresholds vs. Temperature (BODLEVEL is 4.0V) on page 454 and
Figure 34-37 BOD Thresholds vs. Temperature (BODLEVEL is 2.7V) on page 454 (BOD
Thresholds Characteristics).
3. Updated the last page.
40.5. 8160A - 08/2008
1. Initial revision (Based on the ATmega64/L datasheet 2490N-AVR-06/08).
2. Changes done compared to ATmega64/L datasheet 2490N-AVR-06/08:
–All Electrical Characteristics are moved to Electrical Characteristics – TA = -40°C to 85°C on
page 415.
–Register descriptions are moved to sub section at the end of each chapter.
–Updated DC Characteristics on page 415 with new VOL Max (0.9V and 0.6V) and typical
values for ICC.
–Added Speed Grades on page 417.
–Added System and Reset Characteristics on page 418.
–New graphics in Electrical Characteristics – TA = -40°C to 85°C on page 415.
–New Ordering Information on page 11.
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Atmet ‘Enabllng Unlimited Possnbllltles‘ “um-ID
Atmel Corporation 1600 Technology Drive, San Jose, CA 95110 USA T: (+1)(408) 441.0311 F: (+1)(408) 436.4200 | www.atmel.com
© 2015 Atmel Corporation. / Rev.: Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
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