A TWIM read sequence is started by triggering the STARTRX task. After the STARTRX task has been triggered, TWIM generates a start condition on the TWI bus, followed by clocking out the address and the READ/WRITE bit set to 1 (WRITE = 0, READ = 1). The address must match the address of the slave device that the master wants to read from. The READ/WRITE bit is followed by an ACK/NACK bit (ACK=0 or NACK = 1) generated by the slave.
After sending the ACK bit, the TWI slave sends data to the master using the clock generated by TWIM.
Data received will be stored in RAM at the address specified in the RXD.PTR register. The TWI master will generate an ACK after all but the last byte have been received from the slave. TWIM generates a NACK after the last byte received to indicate that the read sequence shall stop.
A typical TWIM read sequence is illustrated in the following figure, including clock stretching performed by TWIM following a SUSPEND task.
A SUSPENDED event indicates that the SUSPEND task has taken effect. This event can be used to synchronize the software.
TWIM generates a LASTRX event when it is ready to receive the last byte. If RXD.MAXCNT > 1, the LASTRX event is generated after sending the ACK of the previously received byte. If RXD.MAXCNT = 1, the LASTRX event is generated after receiving the ACK following the address and READ bit.
TWIM is stopped by triggering the STOP task. This task must be triggered before the NACK bit is supposed to be transmitted. The STOP task can be triggered at any time during the reception of the last byte. It is recommended to use the shortcut between LASTRX and STOP to accomplish this.
TWIM does not stop by itself when the RAM buffer is full, or when an error occurs. The STOP task must be issued, through the use of a local or PPI shortcut, or in software as part of the error handler.
TWIM cannot be stopped while suspended, so the STOP task must be issued after TWIM has been resumed.