35.11.3 XON/XOFF Flow Control

XON/XOFF flow control is selected by setting the FLO bits to ‘01’.

XON/XOFF is a data-based flow control method. The signals to suspend and resume transmission are special characters sent by the receiver to the transmitter. The advantage is that additional hardware lines are not needed.

XON/XOFF flow control requires full-duplex operation because the transmitter must be able to receive the signal to suspend transmitting while the transmission is in progress. Although XON and XOFF are not defined in the ASCII code, the generally accepted values are 13h for XOFF and 11h for XON. The UART uses those codes.

The transmitter defaults to XON, or transmitter enabled. This state is also indicated by the read-only XON bit.

When an XOFF character is received, the transmitter stops transmitting after completing the character actively being transmitted. The transmitter remains disabled until an XON character is received.

XON will be forced on when software toggles the TXEN bit.

When the RUNOVF bit is set, the XON and XOFF characters continue to be received and processed without the need to clear the input FIFO by reading UxRXB. However, if the RUNOVF bit is clear then UxRXB must be read to avoid a receive overflow which will suspend flow control when the receive buffer overflows.