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I
2
C Module Operation
15-11
USART Peripheral Interface, I
2
C Mode
Arbitration
If two or more master transmitters simultaneously start a transmission on the
bus, an arbitration procedure is invoked. Figure 15−10 illustrates the
arbitration procedure between two devices. The arbitration procedure uses
the data presented on SDA by the competing transmitters. The first master
transmitter that generates a logic high is overruled by the opposing master
generating a logic low. The arbitration procedure gives priority to the device
that transmits the serial data stream with the lowest binary value. The master
transmitter that lost arbitration switches to the slave receiver mode, and sets
the arbitration lost flag ALIFG. If two or more devices send identical first bytes,
arbitration continues on the subsequent bytes.
Figure 15−10. Arbitration Procedure Between Two Master Transmitters
1
00 0
1
00 0
11
111
n
Device #1 Lost Arbitration
and Switches Off
Bus Line
SCL
Data From
Device #1
Data From
Device #2
Bus Line
SDA
If the arbitration procedure is in progress when a repeated START condition
or STOP condition is transmitted on SDA, the master transmitters involved in
arbitration must send the repeated START condition or STOP condition at the
same position in the format frame. Arbitration is not allowed between:
- A repeated START condition and a data bit
- A STOP condition and a data bit
- A repeated START condition and a STOP condition