25.3.3.2.6.1 One-Wire Mode

One-Wire mode is enabled by setting the Loop-Back Mode Enable (LBME) bit in the USARTn.CTRLA register. This will enable an internal connection between the TXD pin and the USART receiver, making the TXD pin a combined TxD/RxD line. The RXD pin will be disconnected from the USART receiver and may be controlled by a different peripheral.

In One-Wire mode, multiple devices are able to manipulate the TxD/RxD line at the same time. In the case where one device drives the pin to a logical high level (V_CC), and another device pulls the line low (GND), a short will occur. To accommodate this, the USART features an Open-Drain mode (the ODME bit in the USARTn.CTRLB register) which prevents the transmitter from driving a pin to a logical high level, thereby constraining it to only be able to pull it low. Combining this function with the internal pull-up feature (the PULLUPEN bit in the PORTx.PINnCTRL register) will let the line be held high through a pull-up resistor, allowing any device to pull it low. When the line is pulled low the current from V_CC to GND will be limited by the pull-up resistor. The TXD pin is automatically set to output by hardware when the Open-Drain mode is enabled.

When the USART is transmitting to the TxD/RxD line, it will also receive its own transmission. This can be used to check for overlapping transmissions by checking if the received data are the same as the transmitted data as it should be.