ATmega88PA Automotive Atmel Corporation, ATmega88PA Automotive Datasheet - Page 218

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

218
Atmel ATmega48PA/88PA/168PA [Preliminary]
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 con-
tinue into the data packet.
Figure 22-8. 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 compo-
sition 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.
Synchronized
SCL Line
SDA from
SDA from
Master A
Master B
SDA Line
START
Arbitration, SDA
Master A Loses
A
SDA
9223B–AVR–09/11