
- •Features
- •1. Pin Configurations
- •2. Overview
- •2.1 Block Diagram
- •2.2 Pin Descriptions
- •2.2.3 Port A (PA7:PA0)
- •2.2.4 Port B (PB7:PB0)
- •2.2.5 Port C (PC7:PC0)
- •2.2.6 Port D (PD7:PD0)
- •2.2.7 RESET
- •2.2.8 XTAL1
- •2.2.9 XTAL2
- •2.2.10 AVCC
- •2.2.11 AREF
- •3. Resources
- •4. Data Retention
- •5. About Code Examples
- •6. AVR CPU Core
- •6.1 Overview
- •6.3 Status Register
- •6.3.1 SREG – AVR Status Register
- •6.4 General Purpose Register File
- •6.5 Stack Pointer
- •6.5.1 SPH and SPL – Stack Pointer High and Low Register
- •6.6 Instruction Execution Timing
- •6.7 Reset and Interrupt Handling
- •6.7.1 Interrupt Response Time
- •7. AVR Memories
- •7.1 Overview
- •7.3 SRAM Data Memory
- •7.3.1 Data Memory Access Times
- •7.4 EEPROM Data Memory
- •7.4.1 EEPROM Read/Write Access
- •7.4.3 Preventing EEPROM Corruption
- •7.5 I/O Memory
- •7.6 Register Description
- •7.6.1 EEARH and EEARL – EEPROM Address Register
- •7.6.2 EEDR – EEPROM Data Register
- •7.6.3 EECR – EEPROM Control Register
- •8. System Clock and Clock Options
- •8.1 Clock Systems and their Distribution
- •8.2 Clock Sources
- •8.3 Default Clock Source
- •8.4 Crystal Oscillator
- •8.6 External RC Oscillator
- •8.7 Calibrated Internal RC Oscillator
- •8.8 External Clock
- •8.9 Timer/Counter Oscillator
- •8.10 Register Description
- •8.10.1 OSCCAL – Oscillator Calibration Register
- •9. Power Management and Sleep Modes
- •9.1 Sleep Modes
- •9.2 Idle Mode
- •9.3 ADC Noise Reduction Mode
- •9.6 Standby Mode
- •9.7 Extended Standby Mode
- •9.8 Minimizing Power Consumption
- •9.8.1 Analog to Digital Converter
- •9.8.2 Analog Comparator
- •9.8.4 Internal Voltage Reference
- •9.8.5 Watchdog Timer
- •9.8.6 Port Pins
- •9.9 Register Description
- •9.9.1 MCUCR – MCU Control Register
- •10. System Control and Reset
- •10.1 Resetting the AVR
- •10.2 Reset Sources
- •10.2.2 External Reset
- •10.2.4 Watchdog Reset
- •10.3 Internal Voltage Reference
- •10.4 Watchdog Timer
- •10.5 Register Description
- •10.5.1 MCUCSR – MCU Control and Status Register
- •10.5.2 WDTCR – Watchdog Timer Control Register
- •11. Interrupts
- •11.1 Interrupt Vectors in ATmega32A
- •11.1.1 Moving Interrupts Between Application and Boot Space
- •11.2 Register Description
- •11.2.1 GICR – General Interrupt Control Register
- •12. I/O Ports
- •12.1 Overview
- •12.2 Ports as General Digital I/O
- •12.2.1 Configuring the Pin
- •12.2.2 Reading the Pin Value
- •12.2.3 Digital Input Enable and Sleep Modes
- •12.2.4 Unconnected pins
- •12.3 Alternate Port Functions
- •12.3.1 Alternate Functions of Port A
- •12.3.2 Alternate Functions of Port B
- •12.3.3 Alternate Functions of Port C
- •12.3.4 Alternate Functions of Port D
- •12.4 Register Description
- •12.4.1 SFIOR – Special Function I/O Register
- •12.4.2 PORTA – Port A Data Register
- •12.4.3 DDRA – Port A Data Direction Register
- •12.4.4 PINA – Port A Input Pins Address
- •12.4.5 PORTB – Port B Data Register
- •12.4.6 DDRB – Port B Data Direction Register
- •12.4.7 PINB – Port B Input Pins Address
- •12.4.8 PORTC – Port C Data Register
- •12.4.9 DDRC – Port C Data Direction Register
- •12.4.10 PINC – Port C Input Pins Address
- •12.4.11 PORTD – Port D Data Register
- •12.4.12 DDRD – Port D Data Direction Register
- •12.4.13 PIND – Port D Input Pins Address
- •13. External Interrupts
- •13.1 Register Description
- •13.1.1 MCUCR – MCU Control Register
- •13.1.2 MCUCSR – MCU Control and Status Register
- •13.1.3 GICR – General Interrupt Control Register
- •13.1.4 GIFR – General Interrupt Flag Register
- •14. 8-bit Timer/Counter0 with PWM
- •14.1 Features
- •14.2 Overview
- •14.2.1 Registers
- •14.2.2 Definitions
- •14.3 Timer/Counter Clock Sources
- •14.4 Counter Unit
- •14.5 Output Compare Unit
- •14.5.1 Force Output Compare
- •14.5.2 Compare Match Blocking by TCNT0 Write
- •14.5.3 Using the Output Compare Unit
- •14.6 Compare Match Output Unit
- •14.6.1 Compare Output Mode and Waveform Generation
- •14.7 Modes of Operation
- •14.7.1 Normal Mode
- •14.7.2 Clear Timer on Compare Match (CTC) Mode
- •14.7.3 Fast PWM Mode
- •14.7.4 Phase Correct PWM Mode
- •14.8 Timer/Counter Timing Diagrams
- •14.9 Register Description
- •14.9.1 TCCR0 – Timer/Counter Control Register
- •14.9.2 TCNT0 – Timer/Counter Register
- •14.9.3 OCR0 – Output Compare Register
- •14.9.4 TIMSK – Timer/Counter Interrupt Mask Register
- •14.9.5 TIFR – Timer/Counter Interrupt Flag Register
- •15. Timer/Counter0 and Timer/Counter1 Prescalers
- •15.1 Overview
- •15.2 Internal Clock Source
- •15.3 Prescaler Reset
- •15.4 External Clock Source
- •15.5 Register Description
- •15.5.1 SFIOR – Special Function IO Register
- •16. 16-bit Timer/Counter1
- •16.1 Features
- •16.2 Overview
- •16.2.1 Registers
- •16.2.2 Definitions
- •16.2.3 Compatibility
- •16.3.1 Reusing the Temporary High Byte Register
- •16.4 Timer/Counter Clock Sources
- •16.5 Counter Unit
- •16.6 Input Capture Unit
- •16.6.1 Input Capture Pin Source
- •16.6.2 Noise Canceler
- •16.6.3 Using the Input Capture Unit
- •16.6.4 Output Compare Units
- •16.6.5 Force Output Compare
- •16.6.6 Compare Match Blocking by TCNT1 Write
- •16.6.7 Using the Output Compare Unit
- •16.7 Compare Match Output Unit
- •16.7.1 Compare Output Mode and Waveform Generation
- •16.8 Modes of Operation
- •16.8.1 Normal Mode
- •16.8.2 Clear Timer on Compare Match (CTC) Mode
- •16.8.3 Fast PWM Mode
- •16.8.4 Phase Correct PWM Mode
- •16.8.5 Phase and Frequency Correct PWM Mode
- •16.9 Timer/Counter Timing Diagrams
- •16.10 Register Description
- •16.10.1 TCCR1A – Timer/Counter1 Control Register A
- •16.10.2 TCCR1B – Timer/Counter1 Control Register B
- •16.10.3 TCNT1H and TCNT1L – Timer/Counter1
- •16.10.4 OCR1AH and OCR1AL – Output Compare Register 1 A
- •16.10.5 OCR1BH and OCR1BL – Output Compare Register 1 B
- •16.10.6 ICR1H and ICR1L – Input Capture Register 1
- •16.10.8 TIFR – Timer/Counter Interrupt Flag Register
- •17. 8-bit Timer/Counter2 with PWM and Asynchronous Operation
- •17.1 Features
- •17.2 Overview
- •17.2.1 Registers
- •17.2.2 Definitions
- •17.3 Timer/Counter Clock Sources
- •17.4 Counter Unit
- •17.5 Output Compare Unit
- •17.5.1 Force Output Compare
- •17.5.2 Compare Match Blocking by TCNT2 Write
- •17.5.3 Using the Output Compare Unit
- •17.6 Compare Match Output Unit
- •17.6.1 Compare Output Mode and Waveform Generation
- •17.7 Modes of Operation
- •17.7.1 Normal Mode
- •17.7.2 Clear Timer on Compare Match (CTC) Mode
- •17.7.3 Fast PWM Mode
- •17.7.4 Phase Correct PWM Mode
- •17.8 Timer/Counter Timing Diagrams
- •17.9 Asynchronous Operation of the Timer/Counter
- •17.10 Timer/Counter Prescaler
- •17.11 Register Description
- •17.11.1 TCCR2 – Timer/Counter Control Register
- •17.11.2 TCNT2 – Timer/Counter Register
- •17.11.3 OCR2 – Output Compare Register
- •17.11.5 TIMSK – Timer/Counter Interrupt Mask Register
- •17.11.6 TIFR – Timer/Counter Interrupt Flag Register
- •17.11.7 SFIOR – Special Function IO Register
- •18. SPI – Serial Peripheral Interface
- •18.1 Features
- •18.2 Overview
- •18.3 SS Pin Functionality
- •18.3.1 Slave Mode
- •18.3.2 Master Mode
- •18.3.3 SPCR – SPI Control Register
- •18.3.4 SPSR – SPI Status Register
- •18.3.5 SPDR – SPI Data Register
- •18.4 Data Modes
- •19. USART
- •19.1 Features
- •19.2 Overview
- •19.2.1 AVR USART vs. AVR UART – Compatibility
- •19.3 Clock Generation
- •19.3.1 Internal Clock Generation – The Baud Rate Generator
- •19.3.2 Double Speed Operation (U2X)
- •19.3.3 External Clock
- •19.3.4 Synchronous Clock Operation
- •19.4 Frame Formats
- •19.4.1 Parity Bit Calculation
- •19.5 USART Initialization
- •19.6 Data Transmission – The USART Transmitter
- •19.6.1 Sending Frames with 5 to 8 Data Bit
- •19.6.2 Sending Frames with 9 Data Bit
- •19.6.3 Transmitter Flags and Interrupts
- •19.6.4 Parity Generator
- •19.6.5 Disabling the Transmitter
- •19.7 Data Reception – The USART Receiver
- •19.7.1 Receiving Frames with 5 to 8 Data Bits
- •19.7.2 Receiving Frames with 9 Databits
- •19.7.3 Receive Compete Flag and Interrupt
- •19.7.4 Receiver Error Flags
- •19.7.5 Parity Checker
- •19.7.6 Disabling the Receiver
- •19.7.7 Flushing the Receive Buffer
- •19.8 Asynchronous Data Reception
- •19.8.1 Asynchronous Clock Recovery
- •19.8.2 Asynchronous Data Recovery
- •19.8.3 Asynchronous Operational Range
- •19.9.1 Using MPCM
- •19.10 Accessing UBRRH/ UCSRC Registers
- •19.10.1 Write Access
- •19.10.2 Read Access
- •19.11 Register Description
- •19.11.1 UDR – USART I/O Data Register
- •19.11.2 UCSRA – USART Control and Status Register A
- •19.11.3 UCSRB – USART Control and Status Register B
- •19.11.4 UCSRC – USART Control and Status Register C
- •19.11.5 UBRRL and UBRRH – USART Baud Rate Registers
- •19.12 Examples of Baud Rate Setting
- •20. Two-wire Serial Interface
- •20.1 Features
- •20.2.1 TWI Terminology
- •20.2.2 Electrical Interconnection
- •20.3 Data Transfer and Frame Format
- •20.3.1 Transferring Bits
- •20.3.2 START and STOP Conditions
- •20.3.3 Address Packet Format
- •20.3.4 Data Packet Format
- •20.3.5 Combining Address and Data Packets into a Transmission
- •20.5 Overview of the TWI Module
- •20.5.1 SCL and SDA Pins
- •20.5.2 Bit Rate Generator Unit
- •20.5.3 Bus Interface Unit
- •20.5.4 Address Match Unit
- •20.5.5 Control Unit
- •20.6 Using the TWI
- •20.7 Transmission Modes
- •20.7.1 Master Transmitter Mode
- •20.7.2 Master Receiver Mode
- •20.7.3 Slave Receiver Mode
- •20.7.4 Slave Transmitter Mode
- •20.7.5 Miscellaneous States
- •20.7.6 Combining Several TWI Modes
- •20.9 Register Description
- •20.9.1 TWBR – TWI Bit Rate Register
- •20.9.2 TWCR – TWI Control Register
- •20.9.3 TWSR – TWI Status Register
- •20.9.4 TWDR – TWI Data Register
- •20.9.5 TWAR – TWI (Slave) Address Register
- •21. Analog Comparator
- •21.1 Overview
- •21.2 Analog Comparator Multiplexed Input
- •21.3 Register Description
- •21.3.1 SFIOR – Special Function IO Register
- •21.3.2 ACSR – Analog Comparator Control and Status Register
- •22. Analog to Digital Converter
- •22.1 Features
- •22.2 Overview
- •22.3 Operation
- •22.4 Starting a Conversion
- •22.5 Prescaling and Conversion Timing
- •22.5.1 Differential Gain Channels
- •22.6 Changing Channel or Reference Selection
- •22.6.1 ADC Input Channels
- •22.6.2 ADC Voltage Reference
- •22.7 ADC Noise Canceler
- •22.7.1 Analog Input Circuitry
- •22.7.2 Analog Noise Canceling Techniques
- •22.7.3 Offset Compensation Schemes
- •22.7.4 ADC Accuracy Definitions
- •22.8 ADC Conversion Result
- •22.9 Register Description
- •22.9.1 ADMUX – ADC Multiplexer Selection Register
- •22.9.2 ADCSRA – ADC Control and Status Register A
- •22.9.3 ADCL and ADCH – The ADC Data Register
- •22.9.4 SFIOR – Special FunctionIO Register
- •23. JTAG Interface and On-chip Debug System
- •23.1 Features
- •23.2 Overview
- •23.3 TAP – Test Access Port
- •23.4 TAP Controller
- •23.7.1 PRIVATE0; $8
- •23.7.2 PRIVATE1; $9
- •23.7.3 PRIVATE2; $A
- •23.7.4 PRIVATE3; $B
- •23.8 Using the JTAG Programming Capabilities
- •23.9 Register Description
- •23.10 Bibliography
- •24. IEEE 1149.1 (JTAG) Boundary-scan
- •24.1 Features
- •24.2 Overview
- •24.3 Data Registers
- •24.3.1 Bypass Register
- •24.3.2 Device Identification Register
- •24.3.3 Reset Register
- •24.4.1 EXTEST; $0
- •24.4.2 IDCODE; $1
- •24.4.3 SAMPLE_PRELOAD; $2
- •24.4.4 AVR_RESET; $C
- •24.4.5 BYPASS; $F
- •24.5.1 Scanning the Digital Port Pins
- •24.5.3 Scanning the RESET Pin
- •24.5.4 Scanning the Clock Pins
- •24.5.5 Scanning the Analog Comparator
- •24.5.6 Scanning the ADC
- •24.6 ATmega32A Boundary-scan Order
- •24.8 Register Description
- •24.8.1 MCU Control and Status Register – MCUCSR
- •25. Boot Loader Support – Read-While-Write Self-Programming
- •25.1 Features
- •25.2 Overview
- •25.3 Application and Boot Loader Flash Sections
- •25.3.1 Application Section
- •25.3.2 BLS – Boot Loader Section
- •25.5 Boot Loader Lock Bits
- •25.6 Entering the Boot Loader Program
- •25.8.1 Performing Page Erase by SPM
- •25.8.2 Filling the Temporary Buffer (Page Loading)
- •25.8.3 Performing a Page Write
- •25.8.4 Using the SPM Interrupt
- •25.8.5 Consideration while Updating BLS
- •25.8.7 Setting the Boot Loader Lock Bits by SPM
- •25.8.8 EEPROM Write Prevents Writing to SPMCR
- •25.8.9 Reading the Fuse and Lock Bits from Software
- •25.8.10 Preventing Flash Corruption
- •25.8.11 Programming Time for Flash when using SPM
- •25.8.12 Simple Assembly Code Example for a Boot Loader
- •25.8.13 Boot Loader Parameters
- •25.9 Register Description
- •25.9.1 SPMCR – Store Program Memory Control Register
- •26. Memory Programming
- •26.1 Program And Data Memory Lock Bits
- •26.2 Fuse Bits
- •26.2.1 Latching of Fuses
- •26.3 Signature Bytes
- •26.4 Calibration Byte
- •26.5 Page Size
- •26.6 Parallel Programming Parameters, Pin Mapping, and Commands
- •26.6.1 Signal Names
- •26.7 Parallel Programming
- •26.7.1 Enter Programming Mode
- •26.7.2 Considerations for Efficient Programming
- •26.7.3 Chip Erase
- •26.7.4 Programming the Flash
- •26.7.5 Programming the EEPROM
- •26.7.6 Reading the Flash
- •26.7.7 Reading the EEPROM
- •26.7.8 Programming the Fuse Low Bits
- •26.7.9 Programming the Fuse High Bits
- •26.7.10 Programming the Lock Bits
- •26.7.11 Reading the Fuse and Lock Bits
- •26.7.12 Reading the Signature Bytes
- •26.7.13 Reading the Calibration Byte
- •26.7.14 Parallel Programming Characteristics
- •26.8 SPI Serial Downloading
- •26.9 SPI Serial Programming Pin Mapping
- •26.9.1 SPI Serial Programming Algorithm
- •26.9.2 Data Polling Flash
- •26.9.3 Data Polling EEPROM
- •26.9.4 SPI Serial Programming Characteristics
- •26.10 Programming via the JTAG Interface
- •26.10.1 Programming Specific JTAG Instructions
- •26.10.2 AVR_RESET ($C)
- •26.10.3 PROG_ENABLE ($4)
- •26.10.4 PROG_COMMANDS ($5)
- •26.10.5 PROG_PAGELOAD ($6)
- •26.10.6 PROG_PAGEREAD ($7)
- •26.10.7 Data Registers
- •26.10.8 Reset Register
- •26.10.9 Programming Enable Register
- •26.10.10 Programming Command Register
- •26.10.11 Virtual Flash Page Load Register
- •26.10.12 Virtual Flash Page Read Register
- •26.10.13 Programming Algorithm
- •26.10.14 Entering Programming Mode
- •26.10.15 Leaving Programming Mode
- •26.10.16 Performing Chip Erase
- •26.10.17 Programming the Flash
- •26.10.18 Reading the Flash
- •26.10.19 Programming the EEPROM
- •26.10.20 Reading the EEPROM
- •26.10.21 Programming the Fuses
- •26.10.22 Programming the Lock Bits
- •26.10.23 Reading the Fuses and Lock Bits
- •26.10.24 Reading the Signature Bytes
- •26.10.25 Reading the Calibration Byte
- •27. Electrical Characteristics
- •27.1 Absolute Maximum Ratings*
- •27.2 DC Characteristics
- •27.3 Speed Grades
- •27.4 Clock Characteristics
- •27.4.1 External Clock Drive Waveforms
- •27.4.2 External Clock Drive
- •27.5 System and Reset Characteristics
- •27.7 SPI Timing Characteristics
- •27.8 ADC Characteristics
- •28. Typical Characteristics
- •28.1 Active Supply Current
- •28.2 Idle Supply Current
- •28.5 Standby Supply Current
- •28.7 Pin Driver Strength
- •28.8 Pin Thresholds and Hysteresis
- •28.9 BOD Thresholds and Analog Comparator Offset
- •28.10 Internal Oscillator Speed
- •28.11 Current Consumption of Peripheral Units
- •28.12 Current Consumption in Reset and Reset Pulsewidth
- •29. Register Summary
- •30. Instruction Set Summary
- •31. Ordering Information
- •32. Packaging Information
- •33. Errata
- •33.1 ATmega32A, rev. G to rev. I
- •34. Datasheet Revision History
- •Table of Contents

Figure 20-16. Formats and States in the Slave Receiver Mode
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P or S |
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not acknowledged |
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A |
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$98 |
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Arbitration lost as master and |
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A |
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addressed as slave by general call |
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$78 |
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Any number of data bytes |
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From master to slave |
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and their associated acknowledge bits |
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From slave to master |
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n |
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to a defined state of the Two-wire Serial Bus. The |
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prescaler bits are zero or masked to zero
20.7.4Slave Transmitter Mode
In the Slave Transmitter mode, a number of data bytes are transmitted to a master receiver (see Figure 20-17). All the status codes mentioned in this section assume that the prescaler bits are zero or are masked to zero.
Figure 20-17. Data Transfer in Slave Transmitter Mode
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VCC |
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Device 1 |
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Device 2 |
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Device 3 |
........ |
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Device n |
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R1 |
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R2 |
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SLAVE |
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MASTER |
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TRANSMITTER |
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RECEIVER |
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SDA
SCL
196 ATmega32A
8155B–AVR–07/09

ATmega32A
To initiate the Slave Transmitter mode, TWAR and TWCR must be initialized as follows:
TWAR |
TWA6 |
TWA5 |
TWA4 |
TWA3 |
TWA2 |
TWA1 |
TWA0 |
TWGCE |
Value |
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Device’s Own Slave Address |
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The upper seven bits are the address to which the Two-wire Serial Interface will respond when addressed by a master. If the LSB is set, the TWI will respond to the general call address ($00), otherwise it will ignore the general call address.
TWCR |
TWINT |
TWEA |
TWSTA |
TWSTO |
TWWC |
TWEN |
– |
TWIE |
Value |
0 |
1 |
0 |
0 |
0 |
1 |
0 |
X |
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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 Table 20-5. The slave transmitter mode may also be entered if arbitration is lost while the TWI is in the Master mode (see state $B0).
If the TWEA bit is written to zero during a transfer, the TWI will transmit the last byte of the transfer. State $C0 or state $C8 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 $C8 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 TWEA is zero, the TWI does not respond to its own slave address. However, the Two-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 Two-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 Two-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 it to one). 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 that the Two-wire Serial Interface Data Register – TWDR does not reflect the last byte present on the bus when waking up from these sleep modes.
197
8155B–AVR–07/09

Table 20-5. |
Status Codes for Slave Transmitter Mode |
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Status Code |
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Application Software Response |
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(TWSR) |
Status of the Two-wire Serial Bus |
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To TWCR |
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Prescaler Bits |
and Two-wire Serial Interface |
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To/from TWDR |
STA |
STO |
TWINT |
TWEA |
Next Action Taken by TWI Hardware |
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are 0 |
Hardware |
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$A8 |
Own SLA+R has been received; |
Load data byte or |
X |
0 |
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1 |
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0 |
Last data byte will be transmitted and NOT ACK should |
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ACK has been returned |
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0 |
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1 |
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1 |
be received |
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Load data byte |
X |
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Data byte will be transmitted and ACK should be re- |
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ceived |
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$B0 |
Arbitration lost in SLA+R/W as |
Load data byte or |
X |
0 |
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1 |
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0 |
Last data byte will be transmitted and NOT ACK should |
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master; own SLA+R has been |
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0 |
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1 |
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1 |
be received |
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received; ACK has been returned |
Load data byte |
X |
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Data byte will be transmitted and ACK should be re- |
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ceived |
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$B8 |
Data byte in TWDR has been |
Load data byte or |
X |
0 |
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1 |
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0 |
Last data byte will be transmitted and NOT ACK should |
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transmitted; ACK has been |
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0 |
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1 |
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1 |
be received |
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received |
Load data byte |
X |
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Data byte will be transmitted and ACK should be re- |
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ceived |
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$C0 |
Data byte in TWDR has been |
No TWDR action or |
0 |
0 |
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1 |
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0 |
Switched to the not addressed Slave mode; |
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transmitted; NOT ACK has been |
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0 |
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1 |
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1 |
no recognition of own SLA or GCA |
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received |
No TWDR action or |
0 |
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Switched to the not addressed Slave mode; |
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own SLA will be recognized; |
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0 |
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1 |
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0 |
GCA will be recognized if TWGCE = “1” |
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No TWDR action or |
1 |
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Switched to the not addressed Slave mode; |
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no recognition of own SLA or GCA; |
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a START condition will be transmitted when the bus |
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0 |
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1 |
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becomes free |
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No TWDR action |
1 |
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Switched to the not addressed Slave mode; |
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own SLA will be recognized; |
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GCA will be recognized if TWGCE = “1”; |
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a START condition will be transmitted when the bus |
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becomes free |
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$C8 |
Last data byte in TWDR has been |
No TWDR action or |
0 |
0 |
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1 |
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0 |
Switched to the not addressed Slave mode; |
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transmitted (TWEA = “0”); ACK |
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0 |
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1 |
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1 |
no recognition of own SLA or GCA |
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has been received |
No TWDR action or |
0 |
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Switched to the not addressed Slave mode; |
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own SLA will be recognized; |
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0 |
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1 |
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0 |
GCA will be recognized if TWGCE = “1” |
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No TWDR action or |
1 |
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Switched to the not addressed Slave mode; |
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no recognition of own SLA or GCA; |
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a START condition will be transmitted when the bus |
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0 |
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1 |
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becomes free |
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No TWDR action |
1 |
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Switched to the not addressed Slave mode; |
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own SLA will be recognized; |
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GCA will be recognized if TWGCE = “1”; |
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a START condition will be transmitted when the bus |
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becomes free |
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198 ATmega32A
8155B–AVR–07/09

ATmega32A
Figure 20-18. Formats and States in the Slave Transmitter Mode
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Reception of the own |
S |
SLA |
R |
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A |
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DATA |
A |
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DATA |
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P or S |
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slave address and one or |
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A |
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more data bytes |
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$A8 |
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$B8 |
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$C0 |
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Arbitration lost as master |
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A |
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and addressed as slave |
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$B0 |
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Last data byte transmitted. |
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A |
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All 1's |
P or S |
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Switched to not addressed |
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slave (TWEA = '0') |
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$C8 |
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Any number of data bytes |
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From master to slave |
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DATA |
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A |
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and their associated acknowledge bits |
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From slave to master |
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n |
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This number (contained in TWSR) corresponds |
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to a defined state of the Two-wire Serial Bus. The |
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prescaler bits are zero or masked to zero
20.7.5Miscellaneous States
There are two status codes that do not correspond to a defined TWI state, see Table 20-6.
Status $F8 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 $00 indicates that a bus error has occurred during a Two-wire Serial Bus transfer. A bus error occurs when a START or STOP condition occurs at an illegal position in the format frame. Examples of such illegal positions are during the serial transfer of an address byte, a data byte, or an acknowledge bit. When a bus error occurs, TWINT is set. To recover from a bus error, the TWSTO Flag must set and TWINT must be cleared by writing a logic one to it. This causes the TWI to enter the not addressed slave mode and to clear the TWSTO Flag (no other bits in TWCR are affected). The SDA and SCL lines are released, and no STOP condition is transmitted.
Table 20-6. |
Miscellaneous States |
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Status Code |
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Application Software Response |
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||||
(TWSR) |
Status of the Two-wire Serial |
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To TWCR |
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||
Prescaler Bits |
Bus and Two-wire Serial Inter- |
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To/from TWDR |
STA |
STO |
TWINT |
TWEA |
Next Action Taken by TWI Hardware |
|||
are 0 |
face Hardware |
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$F8 |
No relevant state information |
No TWDR action |
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No TWCR action |
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Wait or proceed current transfer |
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available; TWINT = “0” |
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$00 |
Bus error due to an illegal |
No TWDR action |
0 |
1 |
1 |
X |
Only the internal hardware is affected, no STOP condi- |
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START or STOP condition |
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tion is sent on the bus. In all cases, the bus is released |
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and TWSTO is cleared. |
20.7.6Combining 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:
1.The transfer must be initiated
2.The EEPROM must be instructed what location should be read
3.The reading must be performed
199
8155B–AVR–07/09

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 20-19. Combining Several TWI Modes to Access a Serial EEPROM
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Master Transmitter |
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Master Receiver |
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S |
SLA+W |
A |
ADDRESS |
A Rs |
SLA+R |
A |
DATA |
A P |
S = START |
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Rs = REPEATED START |
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20.8Multi-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 20-20. An Arbitration Example
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SDA
SCL
200 ATmega32A
8155B–AVR–07/09