User Manual

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ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
8271E–AVR–07/2012
Figure 22-6. Typical Data Transmission
22.4 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.
The wired-ANDing of the bus lines is used to solve both these problems. The serial clocks from all masters will be
wired-ANDed, yielding a combined clock with a high period equal to the one from the Master with the shortest high
period. The low period of the combined clock is equal to the low period of the Master with the longest low period.
Note that all masters listen to the SCL line, effectively starting to count their SCL high and low time-out periods
when the combined SCL line goes high or low, respectively.
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