GPS and PIC16F876A

Question

I am trying to configure a GPS receiver with PIC16F876A. In the code below i am receiving characters from UART, and checking whether received character is $. If condition is true, receive characters and store it in character array gps until , is seen.

If I type in characters through serial prompt in Linux, code works as it is supposed to. But if I connect it to the GPS receiver I only get first two characters and LCD will show OVERRUN. I am new to PIC. what should I do??

char a, gps[50];
int i, j, k, flag = 0;
void GPS_init()
{
        TXSTA = 0X24; //00100100
        RCSTA = 0X90; //10010000
        SPBRG = 25;
}

char uart_rx()
{
        while(!RCIF);
        //RCIF=0;
        a=RCREG;
        return(a);

}


void GPS()
{
    for(i=0; ;i++)
    {
        a = uart_rx();
        TXREG = a;   //view received data in serial terminal
        while(TRMT == 0);
        data(a); 
        if(OERR == 1)
        {
            command(0Xc0);
            display("OVERRUN"); //Display string to LCD
            CREN = 0;
        }

        if(a == '$')
        {
            gps[0] = '$';
            for(k=1; ;k++)
            {
                gps[k] = uart_rx();
                data(gps[k]);   //display character to LCD
                TXREG = gps[k]; //view received data in serial terminal
                while(TRMT == 0);

                if(gps[k] == ',')
                {
                    gps[k] = '\0';
                    flag = 1;
                    break;      
                }
            }
        }

        if(flag == 1)
        {
            flag = 0;
            break;
        }
    }
}

main()
{
    GPS_init();
    LCD_init();
    command(0x80);
    display("Initializing");
    command(0x01);
    GPS();


    while(1)
    {
        command(0XC0);
        display(gps);
    }

}

Answer 1

The overrun error means that you aren't taking the bytes out of the UART receiver as fast as they are coming in. My guess would be that the problem is with this code

while(TRMT == 0);

I assume that this causes execution to pause until the byte put into TXREG has actually been transmitted. This pause is what is killing your ability to keep up with the input.

Bottom line is that you shouldn't be trying to do both processes (receiving and transmitting) in lock-step. You need to use a data buffer (e.g., your gps array) to allow the two processes to run asynchronously.