GPS – Charles Ihler

WWVB signal generated from GPS

I sat at the bench this morning waiting for the inspiration to continue on with a project I’m working on with Francesco; I gave in to procrastination.

I wrote some assembler code to generate WWVB signal from a PIC a few months ago, remember? That stirred some interest from overseas folks looking for a way for their automatically update their “atomic” watches… apparently there isn’t some NTP device or anything else easily found to be purchased to do this. I was asked to do this NTP, but I’ll leave that for someone with a ‘Pi–I gave mine away to a kid. I did this project using GPS and a PIC micro.

It’s a pretty simple setup, gobble up a GPRMC sentence, pick off the data I need and spit out time. I generate 60KHz with the PWM module within the PIC (okay it’s 62KHz, but that should be close enough).. switch it through a 4066 to generate my time code.

GPRMC: there are other sentences you can use, but my cheapo module only spits out 4 sentences and GPRMC is the only one with time and date, so it’ll do. I re-used my GPGGA parsing code.. there is probably some left-overs in there if you look at the code below. I also dump the working string (first 63 bits of GPGGA) followed by the day number of the year which is calculated on the the UART, pick it up off the TX line @ 9,600 baud.. It’s spits it out about once a minute (in between time transmissions). I ended up using some DOY code online because I found some nice code that included determining if it was a leap year for day-of-year determination; I left the URL in the code.

The code isn’t finished nor is it accurate: it doesn’t recognize the valid data bit yet, and just spits out time whenever it feels like it so the time can be off by up to 59 seconds. I’ll work on this later on since I was just looking for proof of concept so I could get the hardware going and off a breadboard. I built up the circuit in eagle.. I’ll check it a few more times and have a couple PCBs made for round one of testing. The prototype was built on a Tautic 18F26K22 dev board, I considered just having the dev board plug-in but that’s probably a waste of $$.

You’ll note the SMA jack, perhaps this is over kill since I’ll never have a good enough antenna to match 60KHz… I don’t really know what to do with that yet, wire antenna for now I suppose. (version 2?) It’s likely illegal to sell these since it’s purposefully broadcasting, fat chance I’m doing the research beyond the FCC. I’ll look it up tomorrow but I suspect Part 15 doesn’t apply to 60KHz.

The code is a total hack job.. poorly documented, perhaps even incorrectly documented. I know I have “place-holder” functions.. but like I said.. it was a start to get my time code and it was close enough for testing; you get what you pay for! 🙂

/*
 * File:   main.c
 * Author: Charles M Douvier
 * Contact at: http://iradan.com / 0xEE.net / @chasxmd
 * Created on April 4th, 2014
 *
 * Target Device: 18F26K22
 *
 * Project: GPS --> WWVB simulator
 *
 * This version uses the GPRMC block
 * This is a limitation because GPRMC doesn't pass seconds
 * The time passed will always be up to 60 seconds off
 * I have to deterine DDMMYY --> Day of year, year
 * No simple leap year info in GPS :(
 *
 * TODO
 * Determine GPS lock and output to LED
 * Consider re-writing how I am writing my time code
 *
 * I'll re-write this sometime with a better (more expensive) GPS module.
 *
 * Version:
 * 0.1      First build I could prove I had GPS lock
 * 0.2      Output time/date on 232
 *
 */
#ifndef _XTAL_FREQ
#define _XTAL_FREQ 8000000 //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 8000000 //defined for delay

    char    ctxt[120], wstr[120];            //buff NMEA string, working string
    char    str1[20], str2[20], c, latstr, lonstr, setstr, doych[8];
    char    hourch[3], minch[3], secch[3], daych[3], monthch[3], yearch[3];
    char buffer[32] = "none";                        //temp dump
    volatile unsigned int     ping, isrcall, index, reading, new_rx;
    int     ready, gpgga, gprmc, mode;        //gps associated vars
    int     leap_year, dayint, monthint, yearint, year4int, secondint, minuteint, hourint;
    long    doy;
    int     min_40, min_20, min_10, min_8, min_4, min_2, min_1;
    int     hour_20, hour_10, hour_8, hour_4, hour_2, hour_1;
    int     doy_200, doy_100, doy_80, doy_40, doy_20, doy_10;
    int     doy_8, doy_4, doy_2, doy_1, leapint;
    int     year_80, year_40, year_20, year_10, year_8, year_4, year_2, year_1;

    //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 > 63)              //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) {
    // This code before the TRIS setup is for switching the RX2/TX2 to proper pins for the dev board
    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; // PWM1B
    TRISBbits.TRISB3 = 0; // output
    TRISBbits.TRISB4 = 0; // SCK
    TRISBbits.TRISB5 = 0; // PWM1C
    TRISBbits.TRISB6 = 0; // SCK
    TRISBbits.TRISB7 = 1; // input

    LATC = 0x00;

    TRISCbits.TRISC0 = 0; // N/W
    TRISCbits.TRISC1 = 0; // S/E
    TRISCbits.TRISC2 = 0; // PWM1A (output to 4066 control)
    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=51;  // 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

}


void pwm_init(){
//
//Take care if setting up the PWM pins (DISBALE A/D, etc)
//
//Select the 8-bit TimerX resource, (Timer2,Timer4 or Timer6) to be used for PWM generation
//by setting the CxTSEL bits in the CCPTMRSx register.(1)
//
//Load the PRx register for the selected TimerX with the PWM period value.
//
//Configure the CCP module for the PWM mode by loading the CCPxCON register with the
//appropriate values.
//
//Load the CCPRxL register and the DCxB bits of the CCPxCON register, with the PWM
//duty cycle value.
//

//    CCPR1L = 0x120;
    CCPR1Lbits.CCPR1L = 0x01;       //PWM duty cycle
    //CCPR2Lbits.CCPR2L = 0xCE;       //PWM duty cycle
    //CCPR3Lbits.CCPR3L = 0xCE;       //PWM duty cycle
    PR2 = 0x01;                     //Timer 2 Prescale
    //PR4 = 0xFF;                     //Timer 4 Prescale
    //PR6 = 0xFF;                     //Timer 6 Prescale
    CCPTMRS0bits.C1TSEL = 0x00;     //PWM1 TMR2 Selection
    //CCPTMRS0bits.C2TSEL = 0x01;     //PWM2 TMR4 Selection
    //CCPTMRS0bits.C3TSEL = 0x02;     //PWM3 TMR6 Selection
    CCP1CONbits.P1M = 0x00;         //single output mode
    CCP1CONbits.DC1B = 0x00;
    PWM1CONbits.P1RSEN = 0;
    PWM1CONbits.P1DC = 0x00;    //dead band delay
    ECCP1ASbits.CCP1AS = 0x00;
    ECCP1ASbits.CCP1ASE = 0;    //Auto-shutdown off
    CCP1CONbits.CCP1M = 0x0C;   //PWM Mode
    //CCP2CONbits.CCP2M = 0x0C;   //PWM Mode
    //CCP3CONbits.CCP3M = 0x0C;   //PWM Mode
    PSTR1CONbits.STR1A = 1;
    PSTR1CONbits.STR1B = 1;


    //T2CONbits.T2OUTPS = 0x0F;      //post scaler
    T2CONbits.T2CKPS = 2;       //16x prescaler
    //T4CONbits.T4CKPS = 2;
    //T6CONbits.T6CKPS = 2;
    T2CONbits.TMR2ON = 1;       //Turn the Timers On...
    //T4CONbits.TMR4ON = 1;
    //T6CONbits.TMR6ON = 1;



}


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


//taken from http://stackoverflow.com/questions/19377396/c-get-day-of-year-from-date

int yisleap(int year)
{
    return (year % 4 == 0 && year % 100 != 0);
}

int get_yday(int mon, int day, int year)
//use: int day = get_yday(1, 31, 2013);
{
    static const int days[2][13] = {
        {0, 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334},
        {0, 0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335}
    };
    int leap = yisleap(year);
    leapint = leap;

    return days[leap][mon] + day;
}

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

void marker(void){
    //send a marker frame - 800ms off, 200 ms on
    LATBbits.LATB0 = 0;
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);

    LATBbits.LATB0 = 1;
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    LATBbits.LATB0 = 0;
}

void one(void){
    //send a one - 500 ms off, 500 ms on
    LATBbits.LATB0 = 0;
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    LATBbits.LATB0 = 1;
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
}

void zero(void){
    //send a zero - 200ms off, 800ms on
    LATBbits.LATB0 = 0;
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    LATBbits.LATB0 = 1;
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
    __delay_ms(50);
}



void sendtime(void){

    /*
    *;0
    xCALL    MARKER                      ;MARKER FRAME REFERENCE BIT
    xCALL    ONE                         ;40min
    xCALL    ZERO                        ;20min
    xCALL    ZERO                        ;10min
    xCALL    ZERO                        ;Reserved
    xCALL    ZERO                        ;8mins
    xCALL    ZERO                        ;4mins
    xCALL    ONE                         ;2mins
    xCALL    ZERO                        ;1mins
    xCALL    MARKER                      ;MARKER 1
     */

    marker();                       //marker, frame reference

    if (minuteint >= 40){           //40min
        minuteint = minuteint - 40;
        one();
    }else {
        zero();
    }
    if (minuteint >= 20){           //20min
        minuteint = minuteint - 20;
        one();
    }else {
        zero();
    }
    if (minuteint >= 10){           //10min
        minuteint = minuteint - 10;
        one();
    }else {
        zero();
    }    
    zero();                        //reserved (zero)
    if (minuteint >= 8){           //8min
        minuteint = minuteint - 8;
        one();
    }else {
        zero();
    }    
    if (minuteint >= 4){           //4min
        minuteint = minuteint - 4;
        one();
    }else {
        zero();
    }    
    if (minuteint >= 2){           //2min
        minuteint = minuteint - 2;
        one();
    }else {
        zero();
    }    
    if (minuteint >= 1){           //1min
        minuteint = minuteint - 1;
        one();
    }else {
        zero();
    }    
    marker();                       //marker 1
    
    /*
    ;10
    xCALL    ZERO                        ;RESERVED
    xCALL    ZERO                        ;RESERVED
    xCALL    ZERO                        ;20hours
    CALL    ZERO                        ;10hours
    CALL    ZERO                        ;RESERVED
    CALL    ZERO                        ;8hours
    CALL    ONE                         ;4hours
    CALL    ONE                         ;2hours
    CALL    ZERO                        ;1hour
    CALL    MARKER                      ;MARKER 2     
     */
    zero();                         //reserved
    zero();                         //reserved
    if (hourint >= 20){           //20 hours
        hourint = hourint - 20;
        one();
    }else {
        zero();
    }       
    if (hourint >= 10){           //10 hours
        hourint = hourint - 10;
        one();
    }else {
        zero();
    }  
    zero();                         //reserved    
    if (hourint >= 8){           //8 hours
        hourint = hourint - 8;
        one();
    }else {
        zero();
    }  
    if (hourint >= 4){           //4 hours
        hourint = hourint - 4;
        one();
    }else {
        zero();
    }  
    if (hourint >= 2){           //2 hours
        hourint = hourint - 2;
        one();
    }else {
        zero();
    }  
    if (hourint >= 1){           //1 hours
        hourint = hourint - 1;
        one();
    }else {
        zero();
    }  
    marker();                       //marker 2    
    /*
     * 
;20
    CALL    ZERO                        ;RESERVED
    CALL    ZERO                        ;RESERVED
    CALL    ZERO                        ;200 day of year
    CALL    ONE                         ;100 day of year
    CALL    ZERO                        ;RESERVED
    CALL    ZERO                        ;80 day of year
    CALL    ONE                         ;40 day of year
    CALL    ZERO                        ;20 day of year
    CALL    ONE                         ;10 day of year
    CALL    MARKER                      ;MARKER 3
     */
    zero();                         //reserved
    zero();                         //reserved
    if (doy >= 200){           //200th day
        doy = doy - 200;
        one();
    }else {
        zero();
    }           
    if (doy >= 100){           //100th day
        doy = doy - 100;
        one();
    }else {
        zero();
    }   
    zero();                         //reserved
    if (doy >= 80){           //80th day
        doy = doy - 80;
        one();
    }else {
        zero();
    }      
    if (doy >= 40){           //40th day
        doy = doy - 40;
        one();
    }else {
        zero();
    }   
    if (doy >= 20){           //20th day
        doy = doy - 20;
        one();
    }else {
        zero();
    } 
    if (doy >= 10){           //10th day
        doy = doy - 10;
        one();
    }else {
        zero();
    }     
    marker();                       //marker 3

/*
 ;30
    CALL    ONE                         ;8 day of year
    CALL    ZERO                        ;4 day of year
    CALL    ZERO                        ;2 day of year
    CALL    ZERO                        ;1 day of year
    CALL    ZERO                        ;RESERVED
    CALL    ZERO                        ;RESERVED
    CALL    ZERO                        ;UTI Sign +
    CALL    ZERO                        ;UTI Sign -
    CALL    ZERO                        ;UTI Sign +
    CALL    MARKER                      ;MARKER 4
 */

     if (doy >= 8){           //8th day
        doy = doy - 8;
        one();
    }else {
        zero();
    }
    if (doy >= 4){           //4th day
        doy = doy - 4;
        one();
    }else {
        zero();
    }
    if (doy >= 2){           //2nd day
        doy = doy - 2;
        one();
    }else {
        zero();
    }
    if (doy >= 1){           //1st day
        doy = doy - 1;
        one();
    }else {
        zero();
    }
    zero();                         //reserved
    zero();                         //reserved
    zero();                         //reserved
    zero();                         //reserved
    zero();                         //reserved
    marker();                       //marker 4

    /*
   ;40
    CALL    ZERO                        ;UTI Corr 0.8s
    CALL    ZERO                        ;UTI Corr 0.4s
    CALL    ZERO                        ;UTI Corr 0.2s
    CALL    ZERO                        ;UTI Corr 0.1s
    CALL    ZERO                        ;RESERVED
    CALL    ZERO                        ;80 year yearint
    CALL    ZERO                        ;40 year
    CALL    ZERO                        ;20 year
    CALL    ONE                         ;10 year
    CALL    MARKER                      ;MARKER 5
     */

    zero();                         //reserved
    zero();                         //reserved
    zero();                         //reserved
    zero();                         //reserved
    zero();                         //reserved

    if (yearint >= 80){           //80th year
        yearint = yearint - 80;
        one();
    }else {
        zero();
    }
    if (yearint >= 40){           //40th year
        yearint = yearint - 40;
        one();
    }else {
        zero();
    }
    if (yearint >= 20){           //20th year
        yearint = yearint - 20;
        one();
    }else {
        zero();
    }
    if (yearint >= 10){           //10th year
        yearint = yearint - 10;
        one();
    }else {
        zero();
    }
    marker();                       //marker 5

/*
     *
;50
    CALL    ZERO                        ;8 year
    CALL    ONE                         ;4 year
    CALL    ONE                         ;2 year
    CALL    ZERO                        ;1 year
    CALL    ZERO                        ;RESERVED
    CALL    ZERO                        ;LEAP YEAR TRUE
    CALL    ZERO                        ;LEAP SEC WARN
    CALL    ONE                        ;DST
    CALL    ONE                        ;DST
    CALL    MARKER                        ;FRAME BIT P0
    *
*/

    if (yearint >= 8){           //8th year
        yearint = yearint - 8;
        one();
    }else {
        zero();
    }
    if (yearint >= 4){           //4th day
        yearint = yearint - 4;
        one();
    }else {
        zero();
    }
    if (yearint >= 2){           //2nd day
        yearint = yearint - 2;
        one();
    }else {
        zero();
    }
    if (yearint >= 1){           //1st day
        yearint = yearint - 1;
        one();
    }else {
        zero();
    }
    zero();                         //reserved

                                     //leap year
    if (leapint){
        one();
    }else {
        zero();
    }
    zero();
    zero();                         //leap sec warn
    zero();                         //dst??
    zero();                         //dst
    marker();                       //frame bit P0
}

void gettime(void){
    
    hourch[0] = wstr[6];      //HHMMSS
    hourch[1] = wstr[7];

    minch[0] = wstr[8];
    minch[1] = wstr[9];

    secch[0] = wstr[10];
    secch[1] = wstr[11];

    daych[0] = wstr[56];     //DAY1  DDMMYY
    daych[1] = wstr[57];     //DAY2

    monthch[0] = wstr[58];     //MONTH1
    monthch[1] = wstr[59];     //MONTH2

    yearch[0] = wstr[60];     //YEAR1
    yearch[1] = wstr[61];     //YEAR2

    uart_write(wstr);

    uart_write(" ");

   hourint = atoi(hourch);
   minuteint = atoi(minch);
   secondint= atoi(secch);

   dayint = atoi(daych);
   monthint = atoi(monthch);
   yearint = atoi(yearch);

   year4int = yearint + 2000;

   doy = get_yday(monthint, dayint, yearint);

   ltoa(doych,doy,10);

   uart_write(doych);

    uart_write("\r");
}

void lon(void){

}

void determine_mode(void){  //determine lat or long mode

}


int main(void) {

    // set up oscillator control register, using internal OSC at 8MHz.
    OSCCONbits.IRCF = 0x06; //set OSCCON IRCF bits to select OSC frequency 8MHz
    OSCCONbits.SCS = 0x02; //set the SCS bits to select internal oscillator block
    __delay_ms(70);         //lets think about life a bit before proceeding..
    __delay_ms(70);
    __delay_ms(70);

    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

    pwm_init();

    ready = 0;

    while (1) {
    __delay_ms(10);
    isrcall = 0;
    ping = 0;
    if (ready){
        LATCbits.LATC2 = 1;
    }

        if (new_rx == 1)            //got our string...
        {
            if (strstr(ctxt, "GPRMC"))  // && ready
            {
                gprmc = 1;
                strncpy((char*)wstr, (char*)ctxt, sizeof(ctxt));
                gettime();
            }
            new_rx=0;              //finished with GPS string
        }


    if (gprmc){
        sendtime();
    }
    gpgga = 0;
    gprmc = 0;
    }
}

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.

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.

Project: GPS2 Click with the PIC Clicker

I’m working on a GPS-based WWVB simulator and I’m pretty close to finished. I wanted to share up to the point where I add on the WWVB circuits and have it stripped down to just the PIC development board and GPS module.

This is on a Mikrelektronika PIC Clicker 18F47J53 development board and a QUECTEL L30 “GPS2” Click board (Mikroe’s module boards built for MikroeBus).

The code is below, I’ll leave the rest of the explanation in the video.

The good stuff..
/* * File: main.c * Author: Charles M Douvier * Contact at: http://iradan.com / 0xEE.net / @chasxmd * Created on April 4th, 2014 * * Target Device: PIC Click / 18F47J53 * * Project: Mikroe.com GPS2 test * Using a PIC Click Dev board and GPS2 click module I am reading the NMEA * string and declaring a "lock" by turning on the LED on RA1 * * * Version: * 0.1 First build I could prove I had GPS lock * * * */ #ifndef _XTAL_FREQ //blah blah not the right way I don't care #define _XTAL_FREQ 8000000 //8Mhz 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 OSC=INTOSC, WDTEN=OFF, CP0=OFF //Internal OSC, No WDT and No code protect
#pragma config IESO=OFF, FCMEN=OFF
#pragma config XINST = OFF
#pragma config PLLDIV = 1 //Divide by 1
#pragma config STVREN = ON //stack overflow/underflow reset enabled
#pragma config XINST = OFF //Extended instruction set disabled
#pragma config CPUDIV = OSC1 //No CPU system clock divide

#define _XTAL_FREQ 8000000 //defined for delay

char ctxt[120]; //buff NMEA string
volatile unsigned int ping, isrcall, index, reading, new_rx;
int ready, gpgga, gprmc; //gps associated vars
//char *rxdata;
//volatile unsigned int uart_data; // use ‘volatile’ qualifer as this is changed in ISR

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

if (PIR3bits.RC2IF) // see if interrupt caused by incoming data
{
isrcall = 0x01;
char temp;
temp = RCREG2; // 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”
}
}
//PIR3bits.RC2IF = 0; // clear interrupt flag
}
//RCSTA2bits.FERR = 0; //Clear errors
//RCSTA2bits.OERR = 0;
}

/*
* Set up my ports
*/
void init_io(void) {
// This code before the TRIS setup is for switching the RX2/TX2 to proper pins for the dev board
INTCONbits.GIE = 0; //no interruptions please
EECON2 = 0x55;
EECON2 = 0xAA;
PPSCONbits.IOLOCK = 0; //turn off PPS write protect

//PPS Info:
//RX2DT2R: EUSART2 Synchronous/Asynchronous Receive (RX2/DT2) to the Corresponding RPn Pin bits
//RP22 000[RX2DT2R4 RX2DT2R3 RX2DT2R2 RX2DT2R1 RX2DT2R0]
//TX2/CK2 6 EUSART2 Asynchronous Transmit/Asynchronous Clock Output
//RP23 000 [RP6R4 RP6R3 RP6R2 RP6R1 RP6R0]
//RD5 RX
//RD6 TX
//sample:
// Assign RX2 To Pin RP0
//MOVLW 0x00 MOVWF RPINR16, BANKED
// Assign TX2 To Pin RP1
//MOVLW 0x06 MOVWF RPOR1, BANKED

RPINR16 = 0x16; //Pin 22 / RD5
RPOR23 = 0x06; //Pin 23 / RD6

EECON2 = 0x55;
EECON2 = 0xAA;
PPSCONbits.IOLOCK = 1; //write protect PPS

LATA = 0x00;

TRISAbits.TRISA0 = 0; //LED1
TRISAbits.TRISA1 = 0; //LED2
TRISAbits.TRISA2 = 0; //POWER ON

TRISBbits.TRISB0 = 0; // HEADER
TRISBbits.TRISB1 = 0; // HEADER
TRISBbits.TRISB2 = 0; // RST
TRISBbits.TRISB3 = 0; // CS
TRISBbits.TRISB4 = 0; // SCK
TRISBbits.TRISB5 = 1; // MISO

LATC = 0x00;

TRISCbits.TRISC0 = 0; // HEADER/DEBUGGING PIN
TRISCbits.TRISC1 = 0; // HEADER/DEBUGGING PIN
TRISCbits.TRISC2 = 0; // HEADER
TRISCbits.TRISC3 = 0; // output
TRISCbits.TRISC6 = 1; // PWM (WAKE UP)
TRISCbits.TRISC7 = 1; // MOSI

TRISDbits.TRISD0 = 1; // SCL
TRISDbits.TRISD1 = 1; // SCA
TRISDbits.TRISD4 = 1; // INT
TRISDbits.TRISD5 = 1; // RX
TRISDbits.TRISD6 = 0; // TX

TRISEbits.TRISE0 = 1; // HEADER
TRISEbits.TRISE1 = 1; // HEADER
TRISEbits.TRISE2 = 1; // HEADER
}
void uart_xmit(unsigned int mydata_byte) {
while(!TXSTA2bits.TRMT); // make sure buffer full bit is high before transmitting
TXREG2 = mydata_byte; // transmit data
}

void serial_init(void)
{
//4800 8N1
// calculate values of SPBRGL and SPBRGH based on the desired baud rate
//- SPEN bit (RCSTA2) must be set (= 1)
//- TRIS bit for RPn2/RX2/DT2 = 1
//- TRIS bit for RPn1/TX2/CK2 = 0 for
//Asynchronous and Synchronous Master modes

PIR3bits.RC2IF=0; // make sure receive interrupt flag is clear

TXSTA2bits.BRGH=1; // select low speed Baud Rate (see baud rate calcs below)
TXSTA2bits.TX9=0; // select 8 data bits
TXSTA2bits.TXEN = 1; // enable transmit
RCSTA2bits.SPEN=1; // serial port is enabled
RCSTA2bits.RX9=0; // select 8 data bits
RCSTA2bits.CREN=1; // receive enabled
SPBRG2=104; // here is calculated value of SPBRGH and SPBRGL
SPBRGH2=0;

__delay_ms(50); // give time for voltage levels on board to settle
uart_xmit(‘R’); // transmit some data for testing
}

int main(void) {
ping = 0;
new_rx = 0;
isrcall = 0;
ready = 0;
gpgga = 0;

init_io();
serial_init();

// set up oscillator control register, using internal OSC at 8MHz.
OSCCONbits.IRCF = 0x07; //set OSCCON IRCF bits to select OSC frequency 8MHz
OSCCONbits.SCS = 0x02; //set the SCS bits to select internal oscillator block
__delay_ms(50); //lets think about life a bit before proceeding..

RCONbits.IPEN = 0;
PIE3bits.RC2IE = 1; //Enable RX2 Interrupt
INTCONbits.PEIE = 1; // Enable peripheral interrupt
INTCONbits.GIE = 1; // enable global interrupt

LATBbits.LATB2 = 0; //GPS Reset
__delay_ms(74);
LATBbits.LATB2 = 1; //pull out of reset

LATAbits.LATA0 = 1; //startup heartbeat LED
__delay_ms(50);
LATAbits.LATA0 = 0;

LATCbits.LATC1 = 1; //proves my ISR RC1 output works
__delay_us(35);
LATCbits.LATC1 = 0;

LATCbits.LATC2 = 1; //proves my New Byte RC2 output works
__delay_us(35);
LATCbits.LATC2 = 0;

ADCON0 = 0b00000000; //don’t need any ADC
ADCON1 = 0b00000000; //speed Vref=AVdd, VssRef=AVss

/* Disable for the time being
* This is TIMER code, untested.
INTCONbits.TMR0IE = 0;
TMR0=0;
T0CONbits.T08BIT = 1;
T0CONbits.T0CS = 0;
T0CONbits.PSA = 0;
T0CONbits.T0PS = 0x04;
INTCONbits.TMR0IF = 0;
T0CONbits.TMR0ON = 1;
*/

while (!PORTCbits.RC6) { //in warmup?
LATAbits.LA2 = 0;
__delay_ms(3);
LATAbits.LA2 = 1; //Turn GPS On
__delay_ms(10);
}

LATAbits.LA2 = 1; //Ensure GPS is On

while (1) {
isrcall = 0;
ping = 0;
gpgga = 0;

if (RCSTA2bits.OERR)
{
RCSTA2bits.CREN=0; //DS39964B-page 347
__delay_us(2);
RCSTA2bits.CREN=1; //Overrun error (can be cleared by clearing bit, CREN)
}

if (new_rx == 1) //got our string…
{
if (strstr(ctxt, “GPGGA”))
{
gpgga = 1;
}
new_rx=0; //finished with GPS string
}
// uart_xmit(‘x’); // this was a test, it works.
if (isrcall) { //testing bits
LATAbits.LATA0 = 1; // $ Detect!
__delay_us(10); //
LATAbits.LATA0 = 0;
}

if (gpgga) {
LATCbits.LATC2 = 1; //GPGGA detect
__delay_us(1); //
LATCbits.LATC2 = 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
LATCbits.LATC1 = 1; //DEBUGGING
__delay_us(1); //
LATCbits.LATC1 = 0;
}
else if (ctxt[42] != ‘1’) //this is the 43rd bit but we didn’t drop the $ into the buffer
{
ready = 0; //
LATAbits.LATA1 = 0; //LOCK LED
}
}
}
}

Category: GPS

WWVB signal generated from GPS

Like this:

GPS on the PIC 18F26K22

Like this:

Project: GPS2 Click with the PIC Clicker

Like this:

Share this:

Like this:

Share this:

Like this:

Share this:

Like this: