Datasheet

When the USART repetition number reaches the programmed value in CTRLC.MAXITER, the
STATUS.ITER bit is set and the internal iteration counter is reset. If the repetition of the character is
acknowledged by the receiver before the maximum iteration is reached, the repetitions are stopped and
the iteration counter is cleared.
Disable Successive Receive NACK
The receiver can limit the number of successive NACKs sent back to the remote transmitter. This is
programmed by setting the Disable Successive NACK bit (CTRLC.DSNACK). The maximum number of
NACKs transmitted is programmed in the CTRLC.MAXITER field. As soon as the maximum is reached,
the character is considered as correct, an acknowledge is sent on the line, the STATUS.ITER bit is set
and the internal iteration counter is reset.
Protocol T=1
When operating in ISO7816 protocol T=1, the transmission is asynchronous (CTRL1.CMODE=0) with
one or two stop bits. After the stop bits are sent, the transmitter does not drive the I/O line.
Parity is generated when transmitting and checked when receiving. Parity error detection sets the
STATUS.PERR bit, and the erroneous character is written to the receive FIFO. When using T=1 protocol,
the receiver does not signal errors on the I/O line and the transmitter does not retransmit.
34.6.3.8 Collision Detection
When the receiver and transmitter are connected either through pin configuration or externally, transmit
collision can be detected after selecting the Collision Detection Enable bit in the CTRLB register
(CTRLB.COLDEN=1). To detect collision, the receiver and transmitter must be enabled (CTRLB.RXEN=1
and CTRLB.TXEN=1).
Collision detection is performed for each bit transmitted by comparing the received value with the transmit
value, as shown in the figure below. While the transmitter is idle (no transmission in progress), characters
can be received on RxD without triggering a collision.
Figure 34-20. Collision Checking
8-bit character, single stop bit
Collision checked
TXD
RXD
The next figure shows the conditions for a collision detection. In this case, the Start bit and the first Data
bit are received with the same value as transmitted. The second received Data bit is found to be different
than the transmitted bit at the detection point, which indicates a collision.
Figure 34-21. Collision Detected
Collision checked and ok
TXD
RXD
Collision detected
Tri-state
TXEN
When a collision is detected, the USART follows this sequence:
SAM D5x/E5x Family Data Sheet
SERCOM USART - SERCOM Synchronous and Asyn...
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Datasheet
DS60001507E-page 937