ATmega324P datasheet(212/377 Pages) ATMEL

212 / 377 page

212

7674F–AVR–09/09

ATmega164P/324P/644P

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 19-4. Address Packet Format

19.3.4

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 19-5. Data Packet Format

SDA

SCL

START

Addr MSB

Addr LSB

R/W

ACK

Data MSB

Data LSB

ACK

Aggregate

SDA

SDA from

Transmitter

SDA from

Receiver

SCL from

Master

SLA+R/W

Data Byte

STOP, REPEATED

START or Next

Data Byte

ATmega324P Datasheet(PDF) 212 Page - ATMEL Corporation

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