ATmega328P datasheet(210/345 Pages) ATMEL

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210

7810A–AVR–11/09

ATmega328P [Preliminary]

Figure 21-5. Data Packet Format

21.3.5

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 21-6 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 imple-

mented by the application software.

Figure 21-6. 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

SDA

SCL

START

Addr MSB

Addr LSB

R/W

ACK

SLA+R/W

STOP

ATmega328P Datasheet(PDF) 210 Page - ATMEL Corporation

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