6.2 One-Wire Mode

Using only one wire effectively reduces the number of pins used for USART communication to one. RX and TX are internally connected, and only TX is used, which means that both incoming and outgoing data will share the same wire, so transmission and reception cannot happen simultaneously. This is called half-duplex communication.

Use the LBME bit in the CTRLA register to enable an internal loopback connection between RX and TX. An internal connection between RX and TX can be created by writing to USARTn.CTRLA.

USART1.CTRLA |= USART_LBME_bm;

This will internally connect the RX and TX pins, but only the TX pin is used. As the TX pin is used for both transmit and receive, the pin direction needs to be configured as an output before each transmission and switched back to input when the transmission ends.

Since RX is connected internally to TX during transmission, it will receive the data sent, which can be used as a collision detection mechanism. If there is another transmission occurring, the received data will not match the transmitted data. An advanced one-wire driver could take advantage of this strategy.

An MPLAB MCC generated code example for AVR128DA48 with the same functionality as the one described in this section can be found here: