c - Can't communicate with certain baud rates

Question

My application is able to communicate with baud rates like 4800, 9600 and 115200 but can't with 14400 or 38400. I have to include asm/termios because I need struct termios2 since I'm going to use c_ispeed and c_ospeed members for any buad rate.

Also the second problem I encounter is that read function doesn't return afterVTIME. Do you know why this happens? Any help is appreciated. Thanks.

#include <asm/termios.h>

int serialDevice = open("/dev/ttyUSB0", O_RDWR | O_NOCTTY | O_SYNC);

serialSettings.baudRate = 4800;
serialSettings.dataBits = 8;
serialSettings.hardwareFlowControl = 0;
serialSettings.parity = 0;
serialSettings.parityOdd = 0;
serialSettings.stopBits = 1;
serialSettings.xonxoff = 1;

setSerialSettings(serialDevice, &serialSettings);
//-------------------------------------------------------
int8_t setSerialSettings(int serialDevice, Serial_Params_t *settings)
{
    struct termios2 tty;
    memset(&tty, 0, sizeof tty);

    // get current serial settings
    if (ioctl(serialDevice, TCGETS2, &tty) == -1)
    {
        sendLog("Can't get serial attributes | setSerialSettings", LOG_TYPE_ERROR);
        return FALSE;
    }

    // baudrate
    tty.c_cflag &= ~CBAUD;
    tty.c_cflag |= BOTHER;
    tty.c_ispeed = MAX(110, MIN(settings->baudRate, MAX_BAUDRATE));
    tty.c_ospeed = MAX(110, MIN(settings->baudRate, MAX_BAUDRATE));

    // enable input parity check
    tty.c_iflag |= INPCK;

    // data bits: CS5, CS6, CS7, CS8
    tty.c_cflag &= ~CSIZE;
    switch (settings->dataBits)
    {
    case 5:
        tty.c_cflag |= CS5;
        break;
    case 6:
        tty.c_cflag |= CS6;
        break;
    case 7:
        tty.c_cflag |= CS7;
        break;
    case 8:
    default:
        tty.c_cflag |= CS8;
        break;
    }

    // stop bit
    switch (settings->stopBits)
    {
    case 1:
    default:
        tty.c_cflag &= ~CSTOPB;
        break;
    case 2:
        tty.c_cflag |= CSTOPB;
    }

    // parity
    if (settings->parity == 1)
        tty.c_cflag |= PARENB;
    else
        tty.c_cflag &= ~PARENB;

    // odd/even parity
    if (settings->parityOdd == 1)
        tty.c_cflag |= PARODD;
    else
        tty.c_cflag &= ~PARODD;

    // flow control
    // XON/XOFF
    if (settings->xonxoff == 1)
        tty.c_iflag |= (IXON | IXOFF | IXANY);
    else
        tty.c_iflag &= ~(IXON | IXOFF | IXANY);

    // enable RTS/CTS
    if (settings->hardwareFlowControl == 1)
        tty.c_cflag |= CRTSCTS;
    else
        tty.c_cflag &= ~CRTSCTS;

    tty.c_cc[VMIN] = 1;            // return read function when receive 1 byte
    tty.c_cc[VTIME] = 10;          // 1 seconds read timeout (deciseconds)
    tty.c_cflag |= CREAD | CLOCAL; // turn on READ & ignore ctrl lines

    // non-canonical mode
    tty.c_iflag &= ~(IGNBRK | BRKINT | PARMRK | ISTRIP | INLCR | IGNCR | ICRNL);
    tty.c_lflag &= ~(ECHO | ECHONL | ICANON | ISIG | IEXTEN);
    tty.c_oflag &= ~OPOST;

    // flush port & apply attributes
    tcflush(serialDevice, TCIFLUSH);
    if (ioctl(serialDevice, TCSETS2, &tty) == -1)
    {
        sendLog("Can't set serial attributes | setSerialSettings", LOG_TYPE_ERROR);
        return FALSE;
    }
    return TRUE;
}

Answer 1

My application is able to communicate with baud rates like 4800, 9600 and 115200 but can't with 14400 or 38400.

There is a pretty nice writeup for how custom serial speed setting works here: https://github.com/npat-efault/picocom/blob/master/termios2.txt.

In brief, given a struct termios2 identified by tty, to set both input and output speed to custom values, you must

• ensure that tty.c_cflag & CBAUD == BOTHER. You appear to do this correctly.
• set the desired output speed in tty.c_ospeed. You do this, too.

• either

  • ensure that (tty.c_cflag >> IBSHIFT) & CBAUD == B0, in which case the output speed will also be used as the input speed, or
  • ensure that (tty.c_cflag >> IBSHIFT) & CBAUD == BOTHER, in which case tty.c_ispeed will be used as the input speed.

  
  You do not do either of those. I'm uncertain why this would cause incorrect communication for some speeds and not others, but the driver is reputed to play some interesting games with speed settings, and maybe you've stumbled across one.

As for

read function doesn't return after VTIME

I think you have incorrect expectations. You are setting VMIN and VTIME both to nonzero values. In this case, VTIME is the maximum inter-character time, not an overall read timeout. With these settings, a blocking read will wait indefinitely for the first character, then will keep reading subsequent characters, up to the requested number, as long as each one arrives within VTIME deciseconds of the previous one.

If you want an overall timeout on every read() call, then set VMIN to 0, and be prepared for some read() calls to read 0 bytes. As always, read() may also read a positive number of bytes but fewer than requested. That may be more likely to happen in this configuration than in the one you're presently using, depending on your choice of VTIME and the behavior of the peer.