34.6.4.1 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 the following:

  • 1 start bit
  • From 5 to 9 data bits (MSB or LSB first)
  • None, an even or an 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 following figure illustrates the possible frame formats. Brackets denote optional bits.

Figure 34-8. 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.