MEGA128CAN Datasheet, PDF (202/413 Pages) ATMEL Corporation – Microcontroller WITH 128K BYTES OF ISP FLASH AND CAN CONTROLLER

Figure 94. Data Packet Format
Aggregate
SDA
Data MSB
SDA from
Transmitter
SDA from
Receiver
SCL from
Master
SLA+R/W
1
2
7
Data Byte
Data LSB ACK
8
9
STOP, REPEATED
START or Next
Data Byte
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 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 95 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 95. Typical Data Transmission
SDA
Addr MSB
Addr LSB R/W ACK
SCL
START
1
2
7
8
9
SLA+R/W
Data MSB
Data LSB ACK
1
2
7
8
9
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
202 AT90CAN128
4250E–CAN–12/04