Question

I’m writing a RS485 driver for an ARM AT91SAM9260 board on Linux.

When I initialise the UART, the RTS signal line gets high (1). I guess this would and should be the standard behaviour in RS232 operation mode. In RS485 mode however this is not wanted.

I’m using the standard functions provided by the arm-arch section to initialise the UART. Therefore the significant steps are:

at91_register_uart(AT91SAM9260_ID_US2, 3, ATMEL_UART_CTS | ATMEL_UART_RTS);
//consisting of:

// >> configure/mux the pins
    at91_set_A_periph(AT91_PIN_PB10, 1);        /* TXD */
    at91_set_A_periph(AT91_PIN_PB11, 0);        /* RXD */

    if (pins & ATMEL_UART_RTS)
        at91_set_B_periph(AT91_PIN_PC8, 0);     /* RTS */
    if (pins & ATMEL_UART_CTS)
        at91_set_B_periph(AT91_PIN_PC10, 0);    /* CTS */

// >> associate the clock
axm_clock_associate("usart3_clk", &pdev->dev, "usart");

// >> et voilà

As you can see with

at91_set_B_periph(AT91_PIN_PC8, 0);

the pull-up on the RTS pin isn’t activated.

• Why does the UART set the RTS high?
  Just because this would be the standard
  behaviour in RS232 mode?

• Wouldn’t it be a better standard for
  the UART to keep silent till the
  operation mode is explicitly set?

Answer 1

A high RTS signal after initialisation seems to be the standard behaviour on many platforms. It manly depends which serial operation mode the start-up routines anticipates for the interface.

To prevent RTS-high on a ATMEL AT91SAM9260 board running Linux, you have to put the UART into the right mode BEFORE muxing at91_set_X_periph() the Pins and register the device.

Since Linux Kernel Version 2.6.35, the ATMEL serial driver supports the RS485 mode. In this driver the UART is properly configured before setting the Pins (GPIO) to there role.

For my embedded device which is running an older Linux version, I solved the problem with the following line of codes:

/* write control flags */

control |= ATMEL_US_RTSEN;
mode |= ATMEL_US_USMODE_RS485;  

  UART_PUT(uartbaseaddr, ATMEL_US_CR, control);
  UART_PUT(uartbaseaddr, ATMEL_US_MR,mode);

Now the Pins can be muxed the their role

at91_set_X_periph(RTS_PIN, 0);