Datasheet

ATmega164A/PA/324A/PA/644A/PA/1284/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 215
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, that is 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.
Figure 21-7. SCL synchronization between multiple masters
Arbitration is carried out by all masters continuously monitoring the SDA line after outputting data. If the value
read from the SDA line does not match the value the Master had output, it has lost the arbitration. Note that a
Master can only lose arbitration when it outputs a high SDA value while another Master outputs a low value. The
losing Master should immediately go to Slave mode, checking if it is being addressed by the winning Master.
The SDA line should be left high, but losing masters are allowed to generate a clock signal until the end of the
current data or address packet. Arbitration will continue until only one Master remains, and this may take many
bits. If several masters are trying to address the same Slave, arbitration will continue into the data packet.
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