25.3.2.3 Data Transmission - USART Transmitter

When the transmitter has been enabled, the normal port operation of the TxD pin is overridden by the USART and given the function as the transmitter's serial output. The direction of the pin n must be configured as output by writing the Direction register for the corresponding port (PORTx.DIR[n]). If the USART is configured for one-wire operation, the USART will automatically override the RxD/TxD pin to output, when the transmitter is enabled.