Atmel ATmega328P Xplained Mini MEGA328P-XMINI MEGA328P-XMINI Data Sheet
Product codes
MEGA328P-XMINI
230
ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
Atmel-8271H-AVR- ATmega-Datasheet_08/2014
22.8
Multi-master Systems and Arbitration
If multiple masters are connected to the same bus, transmissions may be initiated simultaneously by one or
more of them. The TWI standard ensures that such situations are handled in such a way that one of the masters
will be allowed to proceed with the transfer, and that no data will be lost in the process. An example of an
arbitration situation is depicted below, where two masters are trying to transmit data to a Slave Receiver.
more of them. The TWI standard ensures that such situations are handled in such a way that one of the masters
will be allowed to proceed with the transfer, and that no data will be lost in the process. An example of an
arbitration situation is depicted below, where two masters are trying to transmit data to a Slave Receiver.
Figure 22-20. An Arbitration Example
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.
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 one
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.
arbitration will occur, either in the READ/WRITE bit or in the data bits. The masters trying to output a one
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 one 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.
Masters trying to output a one 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
. Possible status values are given in circles.
Device 1
MASTER
TRANSMITTER
Device 2
MASTER
TRANSMITTER
Device 3
SLAVE
RECEIVER
Device n
SDA
SCL
........
R1
R2
V
CC