ATmega128 datasheet(226/386 Pages) ATMEL

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2467S–AVR–07/09

ATmega128

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 105. 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.

•

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 106. Possible status values are given in circles.

Figure 106. Possible Status Codes Caused by Arbitration

Device 1

MASTER

TRANSMITTER

Device 2

MASTER

TRANSMITTER

Device 3

SLAVE

RECEIVER

Device n

SDA

SCL

........

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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