31.6.1 Principle of Operation

The USART uses the following lines for data transfer:

  • RxD for receiving
  • TxD for transmitting
  • XCK for the transmission clock in synchronous operation

USART data transfer is frame based. A serial frame consists of:

  • 1 start bit
  • From 5 to 9 data bits (MSB or LSB first)
  • No, even or odd parity bit
  • 1 or 2 stop bits

A frame starts with the Start bit followed by one character of Data bits. If enabled, the parity bit is inserted after the Data bits and before the first Stop bit. After the stop bit(s) of a frame, either the next frame can follow immediately, or the communication line can return to the Idle (high) state. The figure below illustrates the possible frame formats. Values inside brackets ([x]) denote optional bits.

Figure 31-2. Frame Formats
St
Start bit. Signal is always low.
n, [n]
Data bits. 0 to [5..9]
[P]
Parity bit. Either odd or even.
Sp, [Sp]
Stop bit. Signal is always high.
IDLE
No frame is transferred on the communication line. Signal is always high in this state.