Atmel SAM4L Xplained Pro Starter Kit Atmel ATSAM4L-XSTK ATSAM4L-XSTK Hoja De Datos

42023E–SAM–07/2013

More than one master may access the bus at the same time without data corruption by using
arbitration. 

Arbitration starts as soon as two or more masters place information on the bus at the same time,
and stops (arbitration is lost) for the master that intends to send a logical one while the other
master sends a logical zero.

As soon as arbitration is lost by a master, it stops sending data and listens to the bus in order to
detect a STOP. The SR.ARBLST flag will be set. When the STOP is detected, the master who
lost arbitration may reinitiate the data transfer.

Arbitration is illustrated in 

If the user starts a transfer and if the bus is busy, the TWIM automatically waits for a STOP con-
dition on the bus before initiating the transfer (see 

).

Note:

The state of the bus (busy or free) is not indicated in the user interface.

Figure 27-10. User Sends Data While the Bus is Busy

TWCK

TWD

DATA sent by a master

STOP sent by the master

START sent by the TWI

DATA sent by the TWI

Bus is busy

Bus is free

A transfer is programmed

(DADR + W + START + Write THR)

Transfer is initiated

TWI DATA transfer

Transfer is kept

Bus is considered as free