Figure 93. Arbitration Between two Masters
START
SDA from
Master A
SDA from
Master B
SDA Line
Synchronized
SCL Line
Master A loses
Arbitration, SDAA SDA
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 composition 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.
204 ATmega128
2467O–AVR–10/06