dsPIC30F Family Reference Manual
DS70074C-page 26-8 © 2004 Microchip Technology Inc.
A.4 Multi-master
The I
2
C protocol allows a system to have more than one master. This is called a multi-master
system. When two or more masters try to transfer data at the same time, arbitration and
synchronization occur.
A.4.1 Arbitration
Arbitration takes place on the SDA line, while the SCL line is high. The master which transmits a
high when the other master transmits a low, loses arbitration (Figure A-9) and turns off its data
output stage. A master which lost arbitration can generate clock pulses until the end of the data
byte where it lost arbitration. When the master devices are addressing the same device,
arbitration continues into the data.
Figure A-9: Multi-Master Arbitration (Two Masters)
Masters that also incorporate the slave function, and have lost arbitration must immediately
switch over to Slave-receiver mode. This is because the winning master-transmitter may be
addressing it.
Arbitration is not allowed between:
• A repeated Start condition
• A Stop condition and a data bit
• A repeated Start condition and a Stop condition
Care needs to be taken to ensure that these conditions do not occur.
Transmitter 1 Loses Arbitration
DATA 1 SDA
DATA 1
DATA 2
SDA
SCL
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