Datasheet

• and 16x sample rate using fractional baud rate generation (CTRLA.SAMPR = 1).
The USART uses a break detection threshold of greater than 11 nominal bit times at the configured baud
rate. At any time, if more than 11 consecutive dominant bits are detected on the bus, the USART detects
a Break Field. When a Break Field has been detected, the Receive Break interrupt flag
(INTFLAG.RXBRK) is set and the USART expects the Sync Field character to be 0x55. This field is used
to update the actual baud rate in order to stay synchronized. If the received Sync character is not 0x55,
then the Inconsistent Sync Field error flag (STATUS.ISF) is set along with the Error interrupt flag
(INTFLAG.ERROR), and the baud rate is unchanged.
After a break field is detected and the start bit of the Sync Field is detected, a counter is started. The
counter is then incremented for the next 8 bit times of the Sync Field. At the end of these 8 bit times, the
counter is stopped. At this moment, the 13 most significant bits of the counter (value divided by 8) give
the new clock divider (BAUD.BAUD), and the 3 least significant bits of this value (the remainder) give the
new Fractional Part (BAUD.FP).
When the Sync Field has been received, the clock divider (BAUD.BAUD) and the Fractional Part
(BAUD.FP) are updated after a synchronization delay. After the Break and Sync Fields are received,
multiple characters of data can be received.
26.6.3.5 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 26-12. 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 26-13. Collision Detected
Collision checked and ok
TXD
RXD
Collision detected
Tri-state
TXEN
When a collision is detected, the USART follows this sequence:
1. Abort the current transfer.
SAM D21 Family
SERCOM USART
© 2018 Microchip Technology Inc.
Datasheet Complete
DS40001882D-page 479