35.2 UART Asynchronous Modes

The UART has five Asynchronous modes:

  • 7-bit
  • 8-bit
  • 8-bit with even parity in the 9th bit
  • 8-bit with odd parity in the 9th bit
  • 8-bit with address indicator in the 9th bit

The UART transmits and receives data using the standard Non-Return-to-Zero (NRZ) format. NRZ is implemented with two levels: A VOH Mark state, which represents a ‘1’ data bit, and a VOL Space state, which represents a ‘0’ data bit. NRZ implies that consecutively transmitted data bits of the same value stay at the output level of that bit without returning to a neutral level between each bit transmission. An NRZ transmission port idles in the Mark state. Each character transmission consists of one Start bit followed by seven or eight data bits, one optional parity or address bit, and is always terminated by one or more Stop bits. The Start bit is always a space and the Stop bits are always marks. The most common data format is eight bits with no parity. Each transmitted bit persists for a period of 1/ (Baud Rate). An on-chip dedicated 16-bit Baud Rate Generator is used to derive standard baud rate frequencies from the system oscillator. See the UART Baud Rate Generator section for more information.

In all Asynchronous modes, the UART transmits and receives the LSb first. The UART’s transmitter and receiver are functionally independent but share the same data format and baud rate. Parity is supported by the hardware with even and odd parity modes.