25.4 Frame Formats

A serial frame is defined to be one character of data bits with synchronization bits (start and stop bits), and optionally a parity bit for error checking. The USART accepts all 30 combinations of the following as valid frame formats:

  • 1 start bit
  • 5, 6, 7, 8, or 9 data bits
  • no, even or odd parity bit
  • 1 or 2 stop bits

A frame starts with the start bit, followed by the data bits (from five up to nine data bits in total): first the least significant data bit, then the next data bits ending with the most significant bit. If enabled, the parity bit is inserted after the data bits, before the one or two stop bits. When a complete frame is transmitted, it can be directly followed by a new frame, or the communication line can be set to an idle (high) state. the figure below illustrates the possible combinations of the frame formats. Bits inside brackets are optional.

Figure 25-3. Frame Formats
StStart bit, always low.
(n)Data bits (0 to 8).
PParity bit. Can be odd or even.
SpStop bit, always high.
IDLENo transfers on the communication line (RxDn or TxDn). An IDLE line must be high.
The frame format used by the USART is set by:
  • Character Size bits (UCSRnC.UCSZn[2:0]) select the number of data bits in the frame.
  • Parity Mode bits (UCSRnC.UPMn[1:0]) enable and set the type of parity bit.
  • Stop Bit Select bit (UCSRnC.USBSn) select the number of stop bits. The Receiver ignores the second stop bit.

The Receiver and Transmitter use the same setting. Note that changing the setting of any of these bits will corrupt all ongoing communication for both the Receiver and Transmitter. An FE (Frame Error) will only be detected in cases where the first stop bit is zero.