Datasheet

173
2503Q–AVR–02/11
ATmega32(L)
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 82. 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.
Figure 83. Arbitration between Two Masters
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
SDA from
Master A
SDA from
Master B
SDA Line
Synchronized
SCL Line
START
Master A Loses
Arbitration, SDA
A
SDA