How to properly set up serial communication on Linux
I prefer Serial Programming Guide for POSIX Operating Systems.
You should delete the fcntl(mainfd, F_SETFL)
statement, since it's not required and incorrectly implemented (F_GETFL not done prior and missing third argument).
Try using cfmakeraw to setup non-canonical mode, since your initialization code is incomplete:
options->c_iflag &= ~(IGNBRK | BRKINT | PARMRK | ISTRIP
| INLCR | IGNCR | ICRNL | IXON);
options->c_oflag &= ~OPOST;
For non-canonical mode, you also need to define
options.c_cc[VMIN] = 1;
options.c_cc[VTIME] = 2;
1 and 2 are just suggested values.
Add testing of return status after all system calls.
rc = tcgetattr(mainfd, &options);
if (rc < 0) {
printf("failed to get attr: %d, %s\n", mainfd, strerror(errno));
exit (-3);
}
Try testing with slower baudrates (e.g. 115200 or even 9600).
sj755
Updated on May 11, 2020Comments
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sj755 almost 4 years
I'm attempting to read and write data from and to an FPGA board. The board itself came with a driver that create a terminal device called ttyUSB0 whenever the board is plugged in. On the FPGA, an asynchronous receiver and transmitter were implemented, and they seem to work.
However, there seems to be an issue on C side of things. I've been using some test vectors to test if the FPGA is outputting the proper information. I've noticed a few things things:
- The device sometimes does not open correctly
- The terminal attributes sometimes fail to be retrieved or set.
- The read is sometimes non-blocking and doesn't retrieve the proper value.
Below is how I set up terminal and file descriptor options. Much of it was taken from here: http://slackware.osuosl.org/slackware-3.3/docs/mini/Serial-Port-Programming
Any advice or comments as to why the program may be failing would be greatly helpful.
#include <stdio.h> // Standard input/output definitions #include <string.h> // String function definitions #include <unistd.h> // UNIX standard function definitions #include <fcntl.h> // File control definitions #include <errno.h> // Error number definitions #include <termios.h> // POSIX terminal control definitions int open_port(void){ int fd; // File descriptor for the port fd = open("/dev/ttyUSB0", O_RDWR | O_NOCTTY); if (fd == -1){ fprintf(stderr, "open_port: Unable to open /dev/ttyUSB0 %s\n",strerror(errno)); exit(EXIT_FAILURE); } return (fd); } int main(void){ int fd = 0; // File descriptor struct termios options; // Terminal options fd = open_port(); // Open tty device for RD and WR fcntl(fd, F_SETFL); // Configure port reading tcgetattr(fd, &options); // Get the current options for the port cfsetispeed(&options, B230400); // Set the baud rates to 230400 cfsetospeed(&options, B230400); options.c_cflag |= (CLOCAL | CREAD); // Enable the receiver and set local mode options.c_cflag &= ~PARENB; // No parity bit options.c_cflag &= ~CSTOPB; // 1 stop bit options.c_cflag &= ~CSIZE; // Mask data size options.c_cflag |= CS8; // Select 8 data bits options.c_cflag &= ~CRTSCTS; // Disable hardware flow control // Enable data to be processed as raw input options.c_lflag &= ~(ICANON | ECHO | ISIG); // Set the new attributes tcsetattr(fd, TCSANOW, &options); //////////////////////////////////// // Simple read and write code here// //////////////////////////////////// // Close file descriptor & exit close(fd) return EXIT_SUCCESS }
UPDATE I've modified my code based on the first answer. This is what I have now:
#include <errno.h> // Error number definitions #include <stdint.h> // C99 fixed data types #include <stdio.h> // Standard input/output definitions #include <stdlib.h> // C standard library #include <string.h> // String function definitions #include <unistd.h> // UNIX standard function definitions #include <fcntl.h> // File control definitions #include <termios.h> // POSIX terminal control definitions // Open usb-serial port for reading & writing int open_port(void){ int fd; // File descriptor for the port fd = open("/dev/ttyUSB0", O_RDWR | O_NOCTTY); if (fd == -1){ fprintf(stderr, "open_port: Unable to open /dev/ttyUSB0 %s\n",strerror(errno)); exit(EXIT_FAILURE); } return fd; } int main(void){ int fd = 0; // File descriptor struct termios options; // Terminal options int rc; // Return value fd = open_port(); // Open tty device for RD and WR // Get the current options for the port if((rc = tcgetattr(fd, &options)) < 0){ fprintf(stderr, "failed to get attr: %d, %s\n", fd, strerror(errno)); exit(EXIT_FAILURE); } // Set the baud rates to 230400 cfsetispeed(&options, B230400); // Set the baud rates to 230400 cfsetospeed(&options, B230400); cfmakeraw(&options); options.c_cflag |= (CLOCAL | CREAD); // Enable the receiver and set local mode options.c_cflag &= ~CSTOPB; // 1 stop bit options.c_cflag &= ~CRTSCTS; // Disable hardware flow control options.c_cc[VMIN] = 1; options.c_cc[VTIME] = 2; // Set the new attributes if((rc = tcsetattr(fd, TCSANOW, &options)) < 0){ fprintf(stderr, "failed to set attr: %d, %s\n", fd, strerror(errno)); exit(EXIT_FAILURE); } //////////////////////////////// // Simple Read/Write Code Here// //////////////////////////////// // Close file descriptor & exit close(fd); return EXIT_SUCCESS; }
Just to clarify, the receiver and transmitter use 8 data bits, 1 stop bit, and no parity bit.