Data extraction from Trimble Lassen iQ GPS module

[tom83]

Hi,

I'm trying to extract a string of data containing the latitude and longitude from my Trimble Lassen iQ GPS module to be sent to a PIC (16f876A) which would then transmit the data to be displayed on Hyperterminal.
I've written a program but it doesn't work.

#include<16F876A.h>
#include<stdio.h>
#use delay (CLOCK=20000000)
#fuses hs, noprotect, nowdt, nolvp //do i need to include brownout?
#use rs232 (BAUD=4800,XMIT=PIN_C6,RCV=PIN_C7, parity=N) //connection between GPS and PIC
#use rs232 (BAUD=4800,XMIT=PIN_C5,RCV=PIN_C4, parity=N) //connection between PIC and PC
#byte PORTC=7

void init();
void main();
void read_RS232();
int i = 0;
int j = 0;
char string[73];
char latitude[13];
char longitude[14];


void main(){

init();
set_tris_C(0x90); // is this correct?
while(1){
output_high(pin_C6); // is this line correct?
read_RS232();
output_low(pin_C7); // is this line correct?

for (i=0; i<12; i++){
latitude[i] = string[j+17];
j++;
}
latitude[12] = '\0';
j=0;
for (i=0; i<13; i++){
longitude[i] = string[j+30];
j++;
}
longitude[13] = '\0';
j=0;
printf("Latitude: %S\n\r", latitude);
printf("Longitude: %S\n\r", longitude);
}
}


void read_RS232(){

for(i=0; i<73; i++){ // is this correct?
string[i] = getc();
}
string[72] = '\0'; //is this correct?
}


void init(){
output_bit(pin_C7,0); //is this line correct?
}

I'm still a novice in C-programming and PIC, so the program might look terrible to some of you. Please give me some guidelines on how to improve my program. Or if some of you have successfully tried retrieving data from the Trimble Lassen iQ using PIC16f876A, i would really appreciate it if you can show me a sample of your program. Thanks so much!!

Regards,
Tom

[sjbaxter]

Tom,

First, I think you need to do a search on the internet for NMEA 0183 to find out what you should expect to get out of a GPS unit. NMEA 0183 is the serial format used for most GPS sensor outputs (using RS232). The serial output contains a number of 'sentences' each of which contain position, satellite, speed and time information.

it looks something like:

[Douglas Kennedy]

You'll need an interrupt routine to stuff into a circular buffer the inbound sentence from the GPS
Hint search this forum for circular buffer.

In your main routine loop until your interrupt routine signals a sentence has been received then begin parsing the sentence.
To keep the buffer size down reject unwanted sentences from the GPS by
testing the first few chars to get the ones you want.
You'll learn alot by working through this yourself.
Remember RS232 is asynchronous so when you look at your code in main know that at any point in your program the GPS can and will be sending data..that's why you need the interrupt to service the GPS and then return to your main code.

[rnielsen]

This routine looks at one particular sentence and retrieves the Speed information from it. It detects a '$' character, which indicates the begining of a new sentence. Then looks to see if a 'C' is sent as part of the sentence header. This indicates that this sentence contains the speed information. It then, simply, counts the comma's and when it gets to the speed info it stuffs it into the variable SBUF which is used in another part of the program.

You can modify the code a bit to grab the information you need and place it into a variable where you can use it elsewhere.

[tom83]

Sigh, i really appreciate all your advice...but too bad i don't really understand your suggestions or the correct way to implement them. Anyway, thanks again.

[kender]