GPS on the PIC 18F26K22

I was asked to create a box that notified by LED if you were too far off a N/S or E/W course. I failed, I doubted the specified GPS accuracy/drift as my Garmin seems to stay pretty steady.. perhaps that’s some integration in the processing?

I started with one of the 18F26K22 TAUTIC development boards I bought from him off Tindie; They’re nice little breadboarding microcontrollers… I’ve recommended them before. I had purchased some GPS modules I got from eBay, so check .. they all have TTL NMEA output, some with 1PPS… and finally some tidbits I picked up out of my parts collection. Wrote some code.. tested it and found my drifting issue. After pondering the number 42 a while, I decided there was no sense in continuing because every idea I had meant something in the specification of the project was out.

I’m not going to go deep into explanation of the code since the project didn’t work but like all failures I learned something. I also expanded my knowledge of XC8 which was nice..  I’ve actively been coding in XC8 for a few months now; in some ways I miss ASM but XC8 does shorten the development time (if you don’t count how long it takes me to figure out atol() should have been used instead of atoi().

This project will be converted into a piece of test equipment of sorts. I’ve been meaning to hook a GPS receiver up to my WWVB transmitter and check it out. Of course WWVB is way more accurate … but if you can’t receive WWVB .. you’re up a creek.

 

GPS Receiver and Microcontroller outside locked.
GPS Receiver and Microcontroller outside locked.

Keep in mind this code is un-optimized, and essentially abandoned while I was building the framework. I leave it to you in case you want a jump start on grabbing some data out of a NMEA string.

/*
/*
 * File:   main.c
 * Author: Charles M Douvier
 * Contact at: http://iradan.com / 0xEE.net / @chasxmd
 * Created on April 4th, 2014
 *
 * Target Device: TAUTIC.com dev board / 18F26K22
 *
 * Project: AG GPS Indicator
 * Using a PIC to indicate if you're staying in lat/long "groove" by GPS.
 * *** This is not complete ***
 *
 *
 * Version:
 * 0.1      First build I could prove I had GPS lock
 * 0.2      Output GPS on 232 and position set, no debounce, etc.. abandonded
 */
#ifndef _XTAL_FREQ
#define _XTAL_FREQ 4000000 //4Mhz FRC internal osc
#define __delay_us(x) _delay((unsigned long)((x)*(_XTAL_FREQ/4000000.0)))
#define __delay_ms(x) _delay((unsigned long)((x)*(_XTAL_FREQ/4000.0)))
#endif

#include 
#include 
#include 
#include 


//config bits
#pragma config FOSC=INTIO67, WDTEN=OFF, PWRTEN=OFF, CP0=OFF, CP1=OFF, BOREN=ON
#pragma config STVREN=ON, LVP=OFF, HFOFST=OFF, IESO=OFF, FCMEN=OFF

//WRT=OFF, FOSC=INTOSC, MCLRE=ON

#define _XTAL_FREQ 4000000 //defined for delay

    char    ctxt[120], wstr[120];            //buff NMEA string, working string
    char    str[60], c, latstr, lonstr, setstr;
    char buffer[32] = "none";                        //temp dump
    volatile unsigned int     ping, isrcall, index, reading, new_rx;
    int     ready, gpgga, gprmc, mode;        //gps associated vars
    long long    position_set, position_now;
    //char    *rxdata;
    //volatile unsigned int uart_data;    // use 'volatile' qualifer as this is changed in ISR

/*
 *  Interrupt Service
 */
void interrupt ISR() {

   if (PIR1bits.RCIF){          // see if interrupt caused by incoming data

        isrcall = 0x01;
        char temp;
        temp = RCREG1;     // read the incoming data
        if(temp=='$' && new_rx==0)      //if first char of a GPS string..
        {
            index = 0;                  //reset index
            reading = 1;                //from now on go to else if
        }
        else if(reading == 1)           //in middle of GPS sentence
        {
            ctxt[index] = temp;         //load it up
            index++;                    //increment index
            ping = 1;                   //this is for debugging
            if(index > 50)              //thats more than enough data
                {
                index = 0;              //reset index
                reading = 0;            //no longer storing the string
                new_rx = 1;             //"ding"
                }
        }


   }
}


/*
 * Set up my ports
 */
void init_io(void) {

    INTCONbits.GIE = 0;         //no interruptions please

    LATA = 0x00;

    TRISAbits.TRISA0 = 0; //Onboard LED
    TRISAbits.TRISA1 = 0; //LED
    TRISAbits.TRISA2 = 0; //MCU (ON)
    TRISAbits.TRISA3 = 1; // input
    TRISAbits.TRISA4 = 1; // input
    TRISAbits.TRISA5 = 1; // input
    TRISAbits.TRISA6 = 1; // input
    TRISAbits.TRISA7 = 1; // input


    TRISBbits.TRISB0 = 0; // output
    TRISBbits.TRISB1 = 0; // output
    TRISBbits.TRISB2 = 0; // output
    TRISBbits.TRISB3 = 0; // output
    TRISBbits.TRISB4 = 0; // SCK
    TRISBbits.TRISB5 = 1; // input
    TRISBbits.TRISB6 = 0; // SCK
    TRISBbits.TRISB7 = 1; // input

    LATC = 0x00;

    TRISCbits.TRISC0 = 0; // N/W
    TRISCbits.TRISC1 = 0; // S/E
    TRISCbits.TRISC2 = 0; // GPGGA DETECT
    TRISCbits.TRISC3 = 1; // MODE SELECT (LAT/LONG)
    TRISCbits.TRISC4 = 1; // SET INPUT
    TRISCbits.TRISC5 = 1; // input
    TRISCbits.TRISC6 = 1; // input
    TRISCbits.TRISC7 = 1; // input
    
    ADCON0 = 0b00000000;        //don't need any ADC
    ADCON1 = 0b00000000;        //speed Vref=AVdd, VssRef=AVss

    ANSELA = 0x00;
    ANSELB = 0x00;
    ANSELC = 0x00;
}

void uart_xmit(unsigned int mydata_byte) {  //send a character to the UART
    while(!TXSTA1bits.TRMT);    // make sure buffer full bit is high before transmitting
    TXREG1 = mydata_byte;       // transmit data
}

void uart_write(const char *txt)            //sent a multiple characters
{
    while(*txt != 0) uart_xmit(*txt++);     //this send a string to the TX buffer
                                            //one character at a time
}


void serial_init(void)
{
    //9600 8N1

    
    TXSTA1bits.BRGH=1;       // select low speed Baud Rate 
    TXSTA1bits.TX9=0;        // select 8 data bits
    TXSTA1bits.TXEN = 1;     // enable transmit


    RCSTA1bits.SPEN=1;       // serial port is enabled
    RCSTA1bits.RX9=0;        // select 8 data bits
    RCSTA1bits.CREN=1;       // receive enabled

    SPBRG1=25;  // here is calculated value of SPBRGH and SPBRGL
    SPBRGH1=0;

    PIR1bits.RCIF=0;        // make sure receive interrupt flag is clear

    __delay_ms(50);        // give time for voltage levels on board to settle
    uart_write("RESET");         // transmit some data for testing

}


/*
 *  Append a string with a character
 *  append(str, c);
 */


void append(char* s, char c)
{
        int len = strlen(s);
        s[len] = c;
        s[len+1] = '\0';
}



/*
 *
 * convert the decminal bits of lat or long to integer
 * send over RS-232 for review
 *
 *
 */



void lat(void){
    str[0] = wstr[17];
    str[1] = wstr[18];
    str[2] = wstr[19];
    str[3] = wstr[20];   // .21 is a decimal place
    str[4] = wstr[22];
    str[5] = wstr[23];
    str[6] = wstr[24];
    str[7] = wstr[25];

    position_now = atol(str);

    uart_write(str);
    uart_write(" ");

        //check to set position
    if (PORTCbits.RC4 && ready) {
        position_set = position_now;
    }

    if (position_set){
        if (position_now > (position_set + 3)) {
            LATCbits.LATC1 = 1;
        }   else {
            LATCbits.LATC1 = 0;
        }
        
        if (position_now < (position_set - 3)) {
            LATCbits.LATC0 = 1;
        }   else {
            LATCbits.LATC0 = 0;
        }
    }
        sprintf(buffer, "%lld", position_now);
        uart_write(buffer);
        uart_write(" ");
        sprintf(buffer, "%lld", position_set);
        uart_write(buffer);
    uart_write("\r");
}

void lon(void){
 //not started
}

void determine_mode(void){  //determine lat or long mode
 //not started   
}


int main(void) {

    // set up oscillator control register, using internal OSC at 4MHz.
    OSCCONbits.IRCF = 0x05; //set OSCCON IRCF bits to select OSC frequency 4MHz
    OSCCONbits.SCS = 0x02; //set the SCS bits to select internal oscillator block
    __delay_ms(50);         //lets think about life a bit before proceeding..


    ping = 0;
    new_rx = 0;
    isrcall = 0;
    
    init_io();
    serial_init();

    //RCONbits.IPEN = 0;
    PIE1bits.RC1IE = 1;         //Enable RX Interrupt
    INTCONbits.PEIE = 1;        // Enable peripheral interrupt
    INTCONbits.GIE = 1;         // enable global interrupt

    
    ready = 0;

    while (1) {
    isrcall = 0;
    ping = 0;
    if (ready){
        LATCbits.LATC2 = 1;
    }

    LATAbits.LATA2 = 1;         //startup heartbeat LED
    __delay_ms(1);
    LATAbits.LATA2 = 0;

        if (new_rx == 1)            //got our string...
        {
            if (strstr(ctxt, "GPGGA"))
            {
                gpgga = 1;
                strncpy((char*)wstr, (char*)ctxt, sizeof(ctxt));  
            }
            new_rx=0;              //finished with GPS string
        }

        if (gpgga) {
            LATAbits.LA1 = 0;
            if(ctxt[42] == '1')     //this is the 43rd bit but we didn't drop the $ into the buffer
            {                       //If "$GPGGA" NMEA message has '1' sign in the 43rd
                                    //position it means that tha GPS receiver has a position fix
                                    //
            ready = 1;              //This is my "locked" variable for future code
            LATAbits.LATA1 = 1;     //LOCK LED
            }
        }

    if (ready) {

        if (mode){

        }
        if (!mode) {
            lat();
        }
        
    }
    __delay_ms(149); //delay don't really even need to update this often
    }
}

If you noticed my posts/week is way down.. well it’s summer. This will happen every summer. My daughter is home and so my time gets happily gobbled up by her 🙂 I’ve got a huge backlog of projects.. thankfully the day job has eased off a bunch. I’m working on a 0xEE project which I probably should have been doing today.. but I needed a break. Also I recently found out our annual backpacking trip is going to be a bit more intense; I’ll have to train appropriately .. so less time on the bench. It’ll be this way until September I imagine.

Leave a Reply