This is what my function looks like to open the serial port (using Ubuntu 12.04):
int open_port(void)
{
int fd; /* File descriptor for the port */
fd = open("/dev/ttyUSB0", O_RDWR | O_NOCTTY | O_NDELAY);
if (fd == -1)
{
// Could not open the port.
perror("open_port: Unable to open /dev/ttyUSB0 - ");
}
else
fcntl(fd, F_SETFL, 0);
struct termios options;
tcgetattr(fd, &options);
//setting baud rates and stuff
cfsetispeed(&options, B19200);
cfsetospeed(&options, B19200);
options.c_cflag |= (CLOCAL | CREAD);
tcsetattr(fd, TCSANOW, &options);
tcsetattr(fd, TCSAFLUSH, &options);
options.c_cflag &= ~PARENB;//next 4 lines setting 8N1
options.c_cflag &= ~CSTOPB;
options.c_cflag &= ~CSIZE;
options.c_cflag |= CS8;
//options.c_cflag &= ~CNEW_RTSCTS;
options.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG); //raw input
options.c_iflag &= ~(IXON | IXOFF | IXANY); //disable software flow control
return (fd);
}
What the problem is, is that when I run this program and if my serial device was already plugged in, the buffer has content in it. I need a way to clear the buffer before I start reading from it. I thought using tcsetattr(fd, TCSAFLUSH, &options);
would fix this problem, by flushing the IO buffers before initializing the port, but no such luck. Any insight?
Flush Serial Port Device Inputs and Outputs Create a connection to a serial port device. Write some data to the device and view the number of bytes available to be read in the input buffer. Flush both the input and output buffers. flush(device);
The input buffer is computer memory allocated by the serial port object to store data that is to be read from the device. When reading data from your device, the data flow follows these two steps: The data read from the device is stored in the input buffer.
Description. Waits for the transmission of outgoing serial data to complete. (Prior to Arduino 1.0, this instead removed any buffered incoming serial data.)
The size of this serial port buffer was originally only one byte, but today it is usually 16 bytes (more in higher priced serial ports).
I think I figured it out. For some reason, I need to add a delay before flushing. These two lines added before returning fd
seem to have done the trick:
sleep(2); //required to make flush work, for some reason
tcflush(fd,TCIOFLUSH);
The cause of this problem lies in using a USB serial port. If you use a regular serial port, you will not have this problem.
Most USB serial port drivers don't support flushing properly, probably because there's no way of knowing if there's still data in the internal shift register, FIFO or in the USB subsystem.
See also Greg's reply to a similar problem reported earlier here.
Your sleep
may cure the problem, but it's only a work-around. Unfortunately there is no solution other than using a regular serial port.
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