35.6.7.8 RS485 Configuration

RS485 is available with the following configuration:

  • USART frame format (CTRLA.FORM = 0x00 or 0x01)
  • RS485 Tx (CTRLA.TXPO = 0x3).
  • RS485 Rx (CTRLA.RXPO = 0x3).
  • Guard time CTRLC.GTIME = 0x1-3
  • Asynchronous mode (CTRLA.CMODE = 0)
  • USART with internal clock (CTRLA.MODE = 0x1)
  • CTRLA.SAMPR = 0x0,2,4

The RS485 feature enables control of an external line driver as shown in the following figure. While operating in RS485 mode, the transmit enable (TE) pin is driven high when the transmitter is active.

The TE pin will remain high for the complete frame including stop bits. If a Guard Time is programmed, CTRLC.GTIME, the Tx line will remain driven after the last character completion.

The following figure shows a transfer with one stop bit and CTRLC.GTIME = 3.

Figure 35-21. RS485 Example of TE Drive with Guard Time

The Transmit Complete interrupt flag (INTFLAG.TXC) will be raised after the guard time is complete and TE goes low.

Figure 35-22. Typical USART Half Duplex RS-485 Asynchronous Example with no Flow Control
Note:
  1. The recommended ground inductor must be a power line ferrite ≥ 680 Ω @ 100 MHz w/DCR ≤ 0.15 Ω.
  2. The recommended Transient Voltage Suppressor (TVS), must be a bi-directional Zener w/Reverse Standoff voltage rating > 10% of the maximum signal operating levels, and a Peak Pulse Current rating ≥ 10 amps.
Figure 35-23. Typical USART Full Duplex RS-485 Asynchronous Example with no Flow Control
Note:
  1. The recommended ground inductor should be a power line ferrite ≥ 680 Ω @ 100 MHz w/DCR ≤ 0.15 Ω.
  2. The recommended TVS must be a bi-directional Zener w/Reverse Standoff voltage rating > 10% of the max signal operating levels, and a Peak Pulse Current rating ≥ 10 amps.