SNIU028D February 2016 – September 2020 UCD3138 , UCD3138064 , UCD3138064A , UCD3138128 , UCD3138A , UCD3138A64
The UART uses a programmable frame format. All frames consist of the following:
The frame format for both the transmitter and receiver is programmable through the bits in the UARTCTRL0 register. Both receive and transmit data is in non-return to zero (NRZ) format, which means that the transmit and receive lines are at logic high when idle.
Each frame transmission begins with a start bit, in which the transmitter pulls the SCI line low (logic low). Following the start bit, the frame data is sent and received least significant bit first (LSB).
A parity bit is present in every frame when the PARITY ENA bit (UARTCTRL0.5) is set. The value of the parity bit depends on the number of one bits in the frame and whether odd or even parity has been selected via the PARITY bit (UARTCTRL0.6).
All frames include one stop bit, which is always a high level. This high level at the end of each frame is used to indicate the end of a frame to ensure synchronization between communicating devices. Two stop bits are transmitted if the STOP bit (UARTCTRL0.7) is set. The example shown in Figure below uses one stop bit per frame.