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_player1537
Guru-in-Training (Posts: 2983)

Working with COM Ports in C
07 May 2011 07:32:31 pm
Last edited by _player1537 on 07 May 2011 09:25:45 pm; edited 1 time in total

I am wanting to write a C program to work with my eZ430 Chronos watch, which uses a COM port. I got some help the other night from Jonimus, Tari, and (sorry) my brain forgets who else. Anyways, I remember Kerm saying that he already had code that did most of this (working with them in general), so I figured I would post here so he would post it Smile

For what it's worth, here is the code that I was able to get, but it doesn't work right now.
Code:

#include <stdio.h>


#include <termios.h>


#include <unistd.h>





FILE* OpenPort( void );





int main(int argc, char* argv[]) {


   char startAccessPoint[] = { 'B', 0xFF, 0x07, 0x03 };


   char accDataRequest[] = { 'B', 0xFF, 0x08, 0x07, 0x00, 0x00, 0x00, 0x00 };


   char recvBuf[10];


   int length;


   struct termios options;


   FILE* ser = OpenPort();


   int fd = fileno(ser);


      printf("Port opened\n");


   fwrite(startAccessPoint, 1, 4, ser); // Start access point


      printf("Access point started\n");


   


   // Set options


   tcgetattr(fd, &options);


   cfsetispeed(&options, B115200);


   cfsetospeed(&options, B115200);


      printf("Settings... set\n");


   while( 1 ) {


         printf("in while(1) loop\n");


      fwrite(accDataRequest, 1, 8, ser); // Request data


      length = fread(recvBuf, 1, 7, ser);


         printf("x:%d y:%d z:%d\n",recvBuf[2], recvBuf[1], recvBuf[0]); // x:128 y:64 z:200


         printf("\tdatatype:%d\n",recvBuf[3]); // datatype:2


   }


   


   fclose(ser);


   return 0;


}





FILE* OpenPort( void ) {


   FILE* fp; // File descriptor


   


   fp = fopen("/dev/ttyACM0", "r+b");


   if( fp < 0 ) {


      perror("Could not open port");


   } else {


      //fcntl(fp, F_SETFL, 0);


   }


   


   return fp;


}

Most of the data was taken from various websites, I'll try to find them all:
http://chemicaloliver.net/arduino/graphing-ti-ez430-chronos-watch-data-in-linux/ (Copy paste)
http://pastebin.com/fde255fd (from 1)
http://www.easysw.com/~mike/serial/serial.html (Which I switched from, but most of the code was taken from examples there)

Edit: chemicaLOLiver got caught in the filter Razz

Tari
3
Systems Wrangler (Posts: 3496)

Re: Working with COM Ports in C
07 May 2011 09:08:54 pm

_player1537 wrote:

Code:

   // Set options


   tcgetattr(fd, &options);


   cfsetispeed(&options, B115200);


   cfsetospeed(&options, B115200);


      printf("Settings... set\n");

Wrong comments are worse than no comments. You never actually set the baud rate on the hardware, just in your termios struct.

(possibly) fixed:

Code:

    tcgetattr(fd, &options);


    cfsetispeed(&options, B115200);


    cfsetospeed(&options, B115200);


    tcsetattr(fd, TCSANOW, &options);

You should also fix your tabs. I can tell what was copy/pasted from the varying indentation levels and styles (some <tab> characters, some spaces). Not a huge issue, but it's annoying when trying to read the code.

_player1537
Guru-in-Training (Posts: 2983)

07 May 2011 09:35:27 pm

Unfortunately, that still doesn't seem to work for me, though I see why I should have set the options after editing them. Also, regarding the tabs, those were for my reading purposes because it was getting hard for me to read between the printf functions I was using for debugging. I'm probably still going to play with the styling for printf() functions because I'd probably just use them as debugging statements for now. And for the comments, is this any better?
Code:

   tcgetattr(fd, &options); // Get termios struct


   cfsetispeed(&options, B115200); // Set baud rate (input)


   cfsetospeed(&options, B115200); // Set baud rate (output)


   tcsetattr(fd, TCSANOW, &options); // Set termios struct for port

Tari
3
Systems Wrangler (Posts: 3496)

07 May 2011 10:02:58 pm

Better, but a bit verbose. With the new call to tcsetattr, the old one-line comment was fine. It was just bad because you claimed to set it but didn't actually.

KermMartian
Site Admin (Posts: 64056)

08 May 2011 05:33:08 pm

First, are you running this on Windows or Linux? Under Windows, there are some annoying things you need to do to properly configure the serial port. Assuming it's Linux, then we save a lot of trouble. I'm very concerned that you don't set parity and stop bits anywhere, only bitrate, and you don't bother with flow control toggling at all.

_player1537
Guru-in-Training (Posts: 2983)

08 May 2011 05:37:52 pm

Under Linux, that's why I am using /dev/ttyXXX. Would you mind explaining why I would need to set parity and stop bits, and how I would do so?

KermMartian
Site Admin (Posts: 64056)

08 May 2011 05:48:04 pm

_player1537 wrote:

Under Linux, that's why I am using /dev/ttyXXX. Would you mind explaining why I would need to set parity and stop bits, and how I would do so?

Good point about the /dev/ttyXXX stuff; I should have spotted that myself. Smile

As a random example, here's some code extracted from the gCnClient source code:

Code:

      tcgetattr(hSerial,&oldtio); /* save current serial port settings */


      bzero(&newtio, sizeof(newtio)); /* clear struct for new port settings */


      newtio.c_cflag = B115200 | CS8 | CLOCAL | CREAD ;//|CRTSCTS;// | 


      newtio.c_iflag = ~(IGNBRK | BRKINT | ICRNL | INLCR | ISTRIP | IXON);


      newtio.c_oflag = ~(OCRNL | ONLCR | ONLRET | ONOCR | OFILL | 


                     OPOST);


      newtio.c_lflag = ~(ECHO | ECHONL | ICANON | IEXTEN | ISIG);


      newtio.c_cc[VMIN] = 0;


      newtio.c_cc[VTIME] = 0;


      cfsetispeed(&newtio,B115200);


      cfsetospeed(&newtio, B115200);


      tcflush(hSerial, TCIFLUSH);


      tcsetattr(hSerial,TCSANOW,&newtio);

christop
Power User (Posts: 385)

08 May 2011 05:51:23 pm

This page will blow your mind: http://pubs.opengroup.org/onlinepubs/007908799/xbd/termios.html

(OK, it probably won't, but it goes into a LOT of detail about how the tty/termios stuff works)

The parity and stop bits stuff is all controlled by the c_cflag member of struct termios (so modify options.c_cflag to set or clear options). See the section titled "Control Modes" in that page above.

To set parity I would do something like this:

Code:




  options.c_cflag |= PARENB; // parity enable


  options.c_cflag |= PARODD; // odd parity


  options.c_cflag &= ~PARODD; // even parity (only this one or the previous one)

To set two stop bits, do something like this:

Code:




  options.c_cflag |= CSTOPB; // set two stop bits


  options.c_cflag &= ~CSTOPB; // set one stop bit

Of course, you'll have to figure out what set of serial control modes your device needs to communicate with the computer. The default control modes might work fine, which means you don't have to set or clear any of them.

EDIT: Kerm replied while I was writing this post. You can also follow his example code, which combines control modes into one line.

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