35.11.1 Hardware Flow Control

The hardware flow control is selected by setting the FLO bits to ‘10’.

The hardware flow control consists of three lines. The RS-232 signal names for two of these are RTS and CTS. Both are low true. The third line is called TXDE for transmit drive enable which may be used to control an RS-485 transceiver. This output is high when the TX output is actively sending a character and low at all other times. The UART is configured as DTE (computer) equipment, which means RTS is an output and CTS is an input.

The RTS and CTS signals work as a pair to control the transmission flow. A DTE-to-DTE configuration connects the RTS output of the receiving UART to the CTS input of the sending UART. Refer to the following figure.

The UART receiving data asserts the RTS output low when the input FIFO is empty. When a character is received, the RTS output goes high until the UxRXB is read to free up both FIFO locations.

When the CTS input goes high after a byte has started to transmit, the transmission will complete normally. The receiver accommodates this by accepting the character in the second FIFO location even when the CTS input is high.