ATmega324P datasheet(214/377 Pages) ATMEL

214 / 377 page

214

7674F–AVR–09/09

ATmega164P/324P/644P

Figure 19-7. 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.

Figure 19-8. Arbitration Between Two Masters

low

high

SCL from

Master A

SCL from

Master B

SCL Bus

Line

low

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

SDA

ATmega324P Datasheet(PDF) 214 Page - ATMEL Corporation