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.
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.
