RS-485 Mode

RS-485 is a communication standard supported by the USART peripheral. It is a physical interface that defines the setup of a communication circuit. Data are transmitted using differential signaling, making communication robust against noise. RS-485 is enabled by writing to the RS485 bit field in the USARTn.CTRLA register.

The RS-485 mode supports external line driver devices that convert a single USART transmission into corresponding differential pair signals. Writing RS485[0] to ‘1’ enables the automatic control of the XDIR pin that can be used to enable transmission or reception for the line driver device. The USART automatically drives the XDIR pin high while the USART is transmitting and pulls it low when the transmission is complete. An example of such a circuit is shown in the figure below.

Figure 1. RS-485 Bus Connection

The XDIR pin goes high one baud clock cycle in advance of data being shifted out to allow some guard time to enable the external line driver. The XDIR pin will remain high for the complete frame, including Stop bit(s).

Figure 2. XDIR Drive Timing

Writing RS485[1] to ‘1’ enables the RS-485 mode, which automatically sets the TXD pin to output one clock cycle before starting transmission and sets it back to input when the transmission is complete.

RS-485 mode is compatible with One-Wire mode. One-Wire mode enables 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. An example of such a circuit is shown in the figure below.

Figure 3. RS-485 with Loop-Back Mode Connection