ATmega88PA Automotive Atmel Corporation, ATmega88PA Automotive Datasheet - Page 217

• Manufacturer Part Number: ATmega88PA Automotive
• Manufacturer: Atmel Corporation

22.4
9223B–AVR–09/11
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 mas-
ters 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
• Different masters may use different SCL frequencies. A scheme must be devised to
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.
Figure 22-7. SCL Synchronization Between Multiple Masters
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.
synchronize the serial clocks from all masters, in order to let the transmission proceed in a
lockstep fashion. This will facilitate the arbitration process.
Atmel ATmega48PA/88PA/168PA [Preliminary]
SCL from
SCL from
Master A
Master B
SCL Bus
Line
TA
Counting Low Period
low
Masters Start
TB
low
TA
Counting High Period
high
Masters Start
TB
high
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