ATmega8(L)
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 sig-
nalled. 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 72. Data Packet Format
Data MSB
Data LSB
ACK
Aggregate
SDA
SDA from
Transmitter
SDA from
Receiver
SCL from
Master
1
2
7
8
9
STOP, REPEATED
START or Next
Data Byte
SLA+R/W
Data Byte
Combining Address and Data A transmission basically consists of a START condition, a SLA+R/W, one or more data
Packets into a Transmission
packets and a STOP condition. An empty message, consisting of a START followed by
a STOP condition, 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 73 shows a typical data transmission. Note that several data bytes can be trans-
mitted between the SLA+R/W and the STOP condition, depending on the software
protocol implemented by the application software.
Figure 73. Typical Data Transmission
Addr MSB
Addr LSB R/W
ACK
Data MSB
Data LSB ACK
SDA
SCL
1
2
7
8
9
1
2
7
8
9
START
SLA+R/W
Data Byte
STOP
163
2486M–AVR–12/03
ATMEL [ ATMEL ]
