220
8025I–AVR–02/09
ATmega48P/88P/168P/328P
Figure 19-8. Arbitration Between Two Masters
Note that arbitration is not allowed between:
• A REPEATED START condition and a data bit.
• A STOP condition and a data bit.
• A REPEATED START and a STOP condition.
It is the user software’s responsibility to ensure that these illegal arbitration conditions never
occur. This implies that in multi-master systems, all data transfers must use the same composi-
tion of SLA+R/W and data packets. In other words: All transmissions must contain the same
number of data packets, otherwise the result of the arbitration is undefined.
SDA from
Master A
SDA from
Master B
SDA Line
Synchronized
SCL Line
START
Master A Loses
Arbitration, SDA
A
SDA
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