Reading from UART port....

Hey,

A few days ago I connected a small PIC Microcontroller to my brandnew OMAP3550EVM and set up a serial connection which works quite well [:)].
The first thing to note is that the layer representing the UART3 port can be found under "/dev/ttyS2"

For first initial tests, I used a small c program that reads incomming bytes from UART3 port and then prints them to the console. The source-code might help you to get a small overview about serial programming.

#include <stdio.h>
#include <stdlib.h>
#include <termios.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/signal.h>
#include <sys/types.h>


#define BAUDRATE B9600
#define MODEMDEVICE "/dev/ttyS2"
#define _POSIX_SOURCE 1
#define FALSE 0
#define TRUE 1

volatile int STOP=FALSE;

int main(int argc, char *argv[])
{
    int fd,c,res;
    struct termios oldtio,newtio;
    char buf[255];
   
    printf("serial test start configuring...\n");

    fd = open(MODEMDEVICE, O_RDWR | O_NOCTTY | O_NDELAY);
    /*
        open(...); is a system call

        O_NCTTY program is not controlling terminal for that port
        O_NDELAY DCD line is not considered
        O_RDWR --> read write mode       
    */   

    if(fd<0){
        perror(MODEMDEVICE); // prints standard error
        exit(-1);
    }
   
    tcgetattr(fd,&oldtio);
    /*
        get current serial configuration and store
        it ins struct oldtio (function from termios.h)
    */
   
    bzero(&newtio,sizeof(newtio));
    /*
        reset configuration of newtio
    */

    // control options
    newtio.c_cflag = BAUDRATE | CRTSCTS | CS8 | CLOCAL | CREAD;
   
    // input options
    newtio.c_iflag = IGNPAR;
   
    // output options
    newtio.c_oflag = 0;
   
    // control characters
    newtio.c_cc[VTIME] = 0;
    newtio.c_cc[VMIN] = 5;
    /*
        configure new settings
    */
   

    tcflush(fd,TCIFLUSH);

    tcsetattr(fd,TCSANOW,&newtio);
    /*
        set new configuration
    */


    printf("serial device configured...\n");

    printf("start listening...");
    while(STOP==FALSE){
        res = read(fd,buf,255);
        buf[res] = 0;
        printf(":%s:%d\n",buf,res);
        if(buf[0] =='z')
            STOP = TRUE;
    }
    /*
        read from fd (which is configured to read at least 5 chars)
        print result
        if first char is 'z' exit
    */

    printf("finish");

    tcsetattr(fd,TCSANOW,&oldtio);
    /*
        restore serial device settings
    */
  return EXIT_SUCCESS;
}

As you can see the <termios> library that comes with Linux handles the whole serial connection stuff. For a far more detailed insight you should have a look at

http://tldp.org/HOWTO/Serial-HOWTO.html

http://tldp.org/HOWTO/Serial-Programming-HOWTO/

These papers really helped me to understand whats going on and how to handle the whole connection.Hope that helps...

Cheers,
Maik

Just 2 cents to add:

I was a bit disappointed when I noticed that only the Receive/Tranceive Pins are connected to the board ?! What about the other pins for flow control and so on?
Serial Connections seem to sneak away step by step [:D]

Sorry to post this on OMAP forum, but I think this may be related to Davinci more closely than we could imagine! Well I can write from serial port and read from serial port using Ubuntu Linux. I  was successful. It sends and reads prints valid results (printf statement with DGB and ERR for debugging). I changed "gcc" in the make file to "arm_v5t_le-gcc" to port the same code on dm6446. Now I expected to see same behaviour. whenever I send data it does what it did on Ubuntu and when it read/ recieves data I do get "0" on being successful read and "-1" when no character recieved so I am sure dm6446 is reading but I fail to understand why would it not echo any read character onto the screen which it does on ubuntu. In my boot parameters "bootarg" I have set console to serial port but my program overrides it and I have verified the same. Also since I have a static IP hence I am using SSH as I don't need to reenter bootargs each time I start evm. Any aspect I override you think could be affecting this behaviour.

Hi Maik,

I have the same requirements where I have to configure UART3 so that I can send data from a terminal emulator like teraterm and display it on the omap3evm lcd display. Basically, UART3 should act like a keyboard port. Should I have to configure the kernel to accomplish this? I am using OpenEmbedded with kernel version 2.6.32. Any help would be appreciated.

Thanks,

Krishnan.