Writing this bit to '1' enables the USART Receiver. The Receiver will override normal port operation for the RxD pin when enabled. Disabling the Receiver will flush the receive buffer invalidating the FERR, BUFOVF, and PERR flags. In GENAUTO and LINAUTO mode, disabling the receiver will reset the auto-baud detection logic.

Writing this bit to '1' enables the USART Transmitter. The Transmitter will override normal port operation for the TxD pin when enabled. Disabling the Transmitter (writing TXEN to zero) will not become effective until ongoing and pending transmissions are completed, i.e., when the Transmit Shift Register and Transmit Buffer Register do not contain data to be transmitted. When disabled, the Transmitter will no longer override the TxD port.

Writing this bit to '1' enables the USART Start Frame Detection mode. The start frame detector is able to wake up the system from idle or standby sleep modes when a high (IDLE) to low (START) transition is detected on the RxD line.

Writing this bit to '1' will make the TxD pin to have open-drain functionality. A pull-up resistor is needed to prevent the line from floating when a logic one is output to TxD pin.

In CLK2X mode, the divisor of the baud rate divider will be reduced from 16 to 8 effectively doubling the transfer rate for asynchronous communication modes. For synchronous operation the CLK2X mode has no effect and RXMODE should always be written to zero. RXMODE must be zero when the USART Communication Mode is configured to IRCOM. Setting RXMODE to GENAUTO enables generic auto-baud where the SYNC character is valid when eight low and high bits has been registered. In this mode any SYNC character that gives a valid BAUD rate will be accepted. In LINAUTO mode the SYNC character is constrained and found valid if each bits falls within 16 +/- 3 baud samples of the internal baud rate and match data value 0x55. Both GENAUTO and LINAUTO mode is only supported for USART operated in asynchronous slave mode.

Writing a '1' to this bit enables the Multi-Processor Communication mode: the USART Receiver ignores all the incoming frames that do not contain address information. The Transmitter is unaffected by the MPCM setting. For more detailed information see Multiprocessor Communication Mode.