AM335x based Custom board : Custom UART Baud Rate

Hi Richin,

Divisor value for the desired UART baud rate is calculated as:

Thank you for the quick reply.

1) Is it possible to set this baud rate for a particular UART port(eg. UART2)? If so will it affect other UART ports?

2) which files in the SDK should be modified for implementing the changes?

Thanks,

Richin Johns

1. Yes, this is individual per UART.

2. I will ask somebody from the SW team to answer this.

Hi,

The driver that needs to be modified is omap-serial.c, it is located in drivers/tty/serial

Your starting point should be serial_omap_get_divisor() function.

Have in mind that changing baud rate (divisor value) is not just a matter of assigning new values to DLL & DLH. The divisors are calculated per use case. Currently, as I see in SDK7, divisor is set to 16 if baud rate is MODE16x & 13 otherwise.

Hope this helps.

Best Regards,

Yordan

As the you suggestion I have gone through the omap-serial.c file and was able to understand that the divisor is calculated automatically based on the BAUD we set for the particular UART port.(either 13 or 16).  As suggested the serial_omap_get_divisor() function gets the BAUD rate from uart_get_baud_rate() function which is defined in serial_core.c. And in uart_get_baud_rate() function baud is returned from tty_termios_baud_rate() function defined in tty_iotcl.c.

here is that function.

speed_t tty_termios_baud_rate(struct ktermios *termios)
{
    unsigned int cbaud;

    cbaud = termios->c_cflag & CBAUD;

#ifdef BOTHER
    /* Magic token for arbitrary speed via c_ispeed/c_ospeed */
    if (cbaud == BOTHER)
        return termios->c_ospeed;
#endif
    if (cbaud & CBAUDEX) {
        cbaud &= ~CBAUDEX;

        if (cbaud < 1 || cbaud + 15 > n_baud_table)
            termios->c_cflag &= ~CBAUDEX;
        else
            cbaud += 15;
    }
    return baud_table[cbaud];
}

now the baud rate which is returned is found from the baud_table and cbaud points to the index. and cbaud is calculated based on : cbaud = termios->c_cflag & CBAUD.

1) Can u tell me in which file is this c_cflag value defined so that I can modify it for 10400baud.

What are the others things related to baud that I should modify to set 10400baud for UART2 and where these files are located?

2) It is also specified we can define BOTHER for custom baud rate but where should it be defined and which file should i set the value of termios->c_ospeed = 10400.

3) also I think i can hardcode 10400 as altbaud in uart_get_baud_rate() in serial_core.c in the driver file. But how to request standard baud rate of 38,4kbps via a termios call for UART2.

Thanks

Richin

Thank you Yordan.

So if i set altbaud in uart_get_baud_rate() in serial_core.c as 10400 in the kernal file and define BOTHER and whenever i request to set baud for particular UART port through system call as 38400 then it will set it to 10400. correct?

And no it is not for a debug console. it is for initialising a communication request with a hardware and then I have to change the baud to 115200.

Thanks ,

Richin

Hi Richin,

Yes, that is my understanding as well.

Best Regards,

Yordan