133
8008H–AVR–04/11
ATtiny48/88
Figure 15-6. Typical Data Transmission
15.5
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-master 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.
Different masters may use different SCL frequencies. A scheme must be devised to
synchronize the serial clocks from all masters, in order to let the transmission proceed in a
lockstep fashion. This will facilitate the arbitration process.
Figure 15-7. SCL Synchronization Between Multiple Masters
12
7
8
9
Data Byte
Data MSB
Data LSB
ACK
SDA
SCL
START
12
789
Addr MSB
Addr LSB
R/W
ACK
SLA+R/W
STOP
TA
low
TA
high
SCL from
Master A
SCL from
Master B
SCL Bus
Line
TB
low
TB
high
Masters Start
Counting Low Period
Masters Start
Counting High Period
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