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 80. Data Packet Format
Data MSB
Data LSB
ACK
Aggregate
SDA
SDA from
Transmitter
SDA from
receiverR
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 81 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 81. 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
Multi-master Bus
Systems, Arbitration and
Synchronization
The TWI protocol allows bus systems with several masters. Special concerns have
been taken in order to ensure that transmissions will proceed as normal, even if two or
more masters initiate a transmission at the same time. Two problems arise in multi-mas-
ter systems:
•
An algorithm must be implemented allowing only one of the masters to complete the
transmission. All other masters should cease transmission when they discover that
they have lost the selection process. This selection process is called arbitration.
When a contending master discovers that it has lost the arbitration process, it
should immediately switch to slave mode to check whether it is being addressed by
the winning master. The fact that multiple masters have started transmission at the
same time should not be detectable to the slaves, i.e., the data being transferred on
the bus must not be corrupted.
172
ATmega32(L)
2503J–AVR–10/06
ATMEL [ ATMEL ]
