Datasheet

Protocol T=0
In T=0 protocol, a character is made up of:
• one start bit,
• eight data bits,
• one parity bit
• and one guard time, which lasts two bit times.
The transfer is synchronous (CTRLA.CMODE=1). The transmitter shifts out the bits and does not drive
the I/O line during the guard time. Additional guard time can be added by programming the Guard Time
(CTRLC.GTIME).
If no parity error is detected, the I/O line remains during the guard time and the transmitter can continue
with the transmission of the next character, as shown in the figure below.
Figure 34-18. T=0 Protocol without Parity Error
SCK
I/O
Start
Bit
D0 D1 D2 D3 D4 D5 D6 D7 P
Guard
Time1
Guard
Time2
Next
Start
Bit
If a parity error is detected by the receiver, it drives the I/O line to 0 during the guard time, as shown in the
next figure. This error bit is also named NACK, for Non Acknowledge. In this case, the character lasts 1
bit time more, as the guard time length is the same and is added to the error bit time, which lasts 1 bit
time.
Figure 34-19. T=0 Protocol with Parity Error
SCK
I/O
Start
Bit
D0 D1 D2 D3 D4 D5 D6 D7 P
Guard
Time1
Guard
Time2
Start
Bit
D0 D1
Repetition
Error
When the USART is the receiver and it detects a parity error, the parity error bit in the Status Register
(STATUS.PERR) is set and the character is not written to the receive FIFO.
Receive Error Counter
The receiver also records the total number of errors (receiver parity errors and NACKs from the remote
transmitter) up to a maximum of 255. This can be read in the Receive Error Count (RXERRCNT) register.
RXERRCNT is automatically cleared on read.
Receive NACK Inhibit
The receiver can also be configured to inhibit error generation. This can be achieved by setting the Inhibit
Not Acknowledge (CTRLC.INACK) bit. If CTRLC.INACK is 1, no error signal is driven on the I/O line even
if a parity error is detected. Moreover, if CTRLC.INACK is set, the erroneous received character is stored
in the receive FIFO, and the STATUS.PERR bit is set. Inhibit not acknowledge (CTRLC.INACK) takes
priority over disable successive receive NACK (CTRLC.DSNACK).
Transmit Character Repetition
When the USART is transmitting a character and gets a NACK, it can automatically repeat the character
before moving on to the next character. Repetition is enabled by writing the Maximum Iterations register
(CTRLC.MAXITER) to a non-zero value. The USART repeats the character the number of times specified
in CTRLC.MAXITER.
SAM D5x/E5x Family Data Sheet
SERCOM USART - SERCOM Synchronous and Asyn...
© 2019 Microchip Technology Inc.
Datasheet
DS60001507E-page 936