ATmega16 datasheet(194/315 Pages) ATMEL

194 / 315 page

194

ATmega16(L)

2466E–AVR–10/02

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 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.

This is summarized in Figure 96. Possible status values are given in circles.

Figure 96. Possible Status Codes Caused by Arbitration

Own

Address / General Call

received

Arbitration lost in SLA

TWI bus will be released and not addressed slave mode will be entered

A START condition will be transmitted when the bus becomes free

Arbitration lost in Data

Direction

Yes

Write

Data byte will be received and NOT ACK will be returned

Data byte will be received and ACK will be returned

Last data byte will be transmitted and NOT ACK should be received

Data byte will be transmitted and ACK should be received

Read

68/78

SLA

START

Data

STOP

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