XON/XOFF

XON/XOFF uses messages implemented as special byte values and does not require additional HW lines. An XON command is implemented by sending a byte value of 0x11 and an XOFF is implemented by a value of 0x13. There are two states of the control mechanism as indicated by the XON bit (UxSTAT[18]). By default, the UART is in the XON = 1 state and will transmit data as they become available in the TX buffer. If the device receives an XOFF command, the XON status bit is cleared and transmission stops until another XON command is received. XON/OFF commands are transmitted in the same manner as regular data.

The receiver keeps track of the UART buffer; if the RX buffer is left with two empty slots, an XOFF command is sent by the module automatically. After that, the module sends an XON command if more than two empty slots become available in the RX buffer.