Datasheet

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

data packet.

Figure 21-8. 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 softw are’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.

21.5 Overview of the TWI module

The TWI module is comprised of several submodules, as shown in Figure 21-9 on page 217. 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