45.6.6 RS485 Mode

The USART features the RS485 mode to enable line driver control. While operating in RS485 mode, the USART behaves as though in Asynchronous or Synchronous mode and configuration of all the parameters is possible. The difference is that the RTS pin is driven high when the transmitter is operating. The behavior of the RTS pin is controlled by the TXEMPTY bit. A typical connection of the USART to an RS485 bus is shown in Figure 45-35.

Figure 45-35. Typical Connection to a RS485 Bus

RS485 mode is enabled by writing the value 0x1 to the US_MR.USART_MODE.

The RTS pin is at a level inverse to the TXEMPTY bit. Significantly, the RTS pin remains high when a timeguard is programmed so that the line can remain driven after the last character completion. Figure 45-36 gives an example of the RTS waveform during a character transmission when the timeguard is enabled.

Figure 45-36. Example of RTS Drive with Timeguard