ATmega32A datasheet(181/352 Pages) ATMEL

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8155B–AVR–07/09

ATmega32A

mode, checking if it is being addressed by the winning master. The SDA line should be left high,

but losing masters are allowed to generate a clock signal until the end of the current data or

address packet. Arbitration will continue until only one master remains, and this may take many

bits. If several masters are trying to address the same slave, arbitration will continue into the

data packet.

Figure 20-8. Arbitration between Two Masters

Note that arbitration is not allowed between:

• A REPEATED START condition and a data bit

• A STOP condition and a data bit

• A REPEATED START and a STOP condition

It is the user software’s responsibility to ensure that these illegal arbitration conditions never

occur. This implies that in multi-master systems, all data transfers must use the same composi-

tion of SLA+R/W and data packets. In other words: All transmissions must contain the same

number of data packets, otherwise the result of the arbitration is undefined.

20.5

Overview of the TWI Module

The TWI module is comprised of several submodules, as shown in Figure 20-9. All registers

drawn in a thick line are accessible through the AVR data bus.

SDA from

Master A

SDA from

Master B

SDA Line

Synchronized

SCL Line

START

Master A Loses

Arbitration, SDA

SDA

ATmega32A Datasheet(PDF) 181 Page - ATMEL Corporation