ATmega644PA datasheet(212/468 Pages) ATMEL

212 / 468 page

212

8152G–AVR–11/09

ATmega164PA/324PA/644PA/1284P

20.3.4

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 20-5. Data Packet Format

20.3.5

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 condi-

tion, 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.

Figure 20-6 on page 212 shows 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 20-6. Typical Data Transmission

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

Data MSB

Data LSB

ACK

Addr MSB

Addr LSB

R/W

ACK

SLA+R/W

ATmega644PA Datasheet(PDF) 212 Page - ATMEL Corporation

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