Datasheet

210
7810C–AVR–10/12
Atmel 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
12 789
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
12 789
Data Byte
Data MSB Data LSB ACK
SDA
SCL
START
12 789
Addr MSB Addr LSB R/W
ACK
SLA+R/W STOP