SAM7X256_14 Datasheet, PDF (315/662 Pages) ATMEL Corporation – ARM-based Flash MCU

30.6.6 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 a RS485 bus is shown in Figure 30-26.
Figure 30-26. Typical Connection to a RS485 Bus
USART
RXD
TXD
RTS
Differential
Bus
The USART is set in RS485 mode by programming the USART_MODE field in the Mode Register (US_MR) to the value
0x1.
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 30-27 gives an example of the
RTS waveform during a character transmission when the timeguard is enabled.
Figure 30-27. Example of RTS Drive with Timeguard
Baud Rate
Clock
TG = 4
TXD
Write
US_THR
Start
Bit
D0
D1
D2
D3
D4
D5
D6
D7
Parity Stop
Bit Bit
TXRDY
TXEMPTY
RTS
SAM7X Series [DATASHEET]
6120K–ATARM–11-Feb-14
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