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 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 Figure 1.
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 2. This error bit, NACK, for Non Acknowledge. In this case, the character lasts one additional bit time, as the guard time does not change and is added to the error bit time, which lasts one bit time.
When the USART is the receiver and it detects an error, it does not load the erroneous character in US_RHR. It sets US_SR.PARE so that the software can handle the error.
