RS485 Mode of Operation

The RS485 feature enables the support of external components to comply with the RS-485 standard.

Either an external line driver is supported as shown in the figure below (RS485=0x1 in USART.CTRLA), or control of the transmitter driving the TxD pin is provided (RS485=0x2).

While operating in RS485 mode, the Transmit Enable pin (TE) is driven high when the transmitter is active.

Figure 1. RS485 Bus Connection

The TE 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 TE pin will remain high for the complete frame including stop bit(s).

Figure 2. TE Drive Timing