Datasheet

Figure 23-20. An Arbitration Example
Device 1
MASTER
TRANSMITTER
Device 2
MASTER
TRANSMITTER
Device 3
SLA VE
RECEIVER
Device n
SD A
SCL
........
R1 R2
V
CC
Several different scenarios may arise during arbitration, as described below:
• Two or more masters are performing identical communication with the same Slave. In this case,
neither the Slave nor any of the masters will know about the bus contention.
• Two or more masters are accessing the same Slave with different data or direction bit. In this case,
arbitration will occur, either in the READ/WRITE bit or in the data bits. The masters trying to output
a '1' on SDA while another Master outputs a zero will lose the arbitration. Losing masters will switch
to not addressed Slave mode or wait until the bus is free and transmit a new START condition,
depending on application software action.
• Two or more masters are accessing different slaves. In this case, arbitration will occur in the SLA
bits. Masters trying to output a '1' on SDA while another Master outputs a zero will lose the
arbitration. Masters losing arbitration in SLA will switch to Slave mode to check if they are being
addressed by the winning Master. If addressed, they will switch to SR or ST mode, depending on
the value of the READ/WRITE bit. If they are not being addressed, they will switch to not addressed
Slave mode or wait until the bus is free and transmit a new START condition, depending on
application software action.
This is summarized in the next figure. Possible status values are given in circles.
Atmel ATmega644A [DATASHEET]
Atmel-42716C-ATmega644A_Datasheet_Complete-10/2016
288