AT90USB646 datasheet(222/434 Pages) ATMEL

Part #	AT90USB646

Description	Microcontroller with 64/128K Bytes of ISP Flash and USB Controller

PDF	434 Pages

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Manufacturer	ATMEL [ATMEL Corporation]

Direct Link	http://www.atmel.com

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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

AT90USB646 Datasheet(PDF) 222 Page - ATMEL Corporation