9.3.7.6 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 the following figure.

Figure 9-44. Typical Connection to an RS485 Bus

The USART is set to RS485 mode by writing the value 0x1 to the FLEX_US_MR.USART_MODE field.

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. The following figure gives an example of the RTS waveform during a character transmission when the timeguard is enabled.

Figure 9-45. Example of RTS Drive with Timeguard

Note: In case the minimum period between RTS rising edge and START bit falling edge is not suitable for the application, it is possible to drive the RTS signal by software when FLEX_US_MR.USART_MODE= 0 or 2. FLEX_US_CR.RTSEN/RTSDIS can be configured to manage RTS.