47.7.4.2 Protocol T = 0

In T = 0 protocol, a character is made up of 1 start bit, 8 data bits, 1 parity bit and 1 guard time, which lasts two bit times. The transmitter shifts out the bits and does not drive the I/O line during the guard time.

If no parity error is detected, the I/O line remains at 1 during the guard time and the transmitter can continue with the transmission of the next character, as shown in the following figure.

If a parity error is detected by the receiver, it drives the I/O line to 0 during the guard time, as shown in figure "T = 0 Protocol with Parity Error" below. 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.

When the USART is the receiver and it detects an error, it does not load the erroneous character in the Receive Holding register (FLEX_US_RHR). It appropriately sets the PARE bit in the Status register (FLEX_US_CSR) so that the software can handle the error.