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barkerben
Joined: 19 Jan 2006
Posts: 22
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| ZX4120 GPS module |
 Posted: Wed Feb 01, 2006 6:01 pm |
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Hi,
I had a ZX4120 GOS module sending serial data at 9600 baud 8N1 directly to a PC via a level converter. This part worked fine.
However, when I instead tried to read the serial data using a PIC18F452 , I ran into problems. I connected the TX line of the GPS directly to the RX line of the PIC (TTL levels), and then sent serial data from the PIC to the PC (via a level converter). I can send data from the PIC to the PC fine - tested with "hello world" etc.
I have simplified my parser down to the stage where it extracts the sentance it wants, then sends it unmodified on to the PC. However, the PC receives garbage. I suspect the problem may be with the baud rate, or could it e something to do with inverted levels between the two devices...?
My code, if anyone can bear to look, is as follows...
Cheers,
Ben
| Code: |
#include <18F452.h>
#define GPS_RX PIN_C7
#define PC_TX PIN_D2
#define LED PIN_B0
#define RESET PIN_B7
#fuses HS,NOWDT,NOPROTECT,NOLVP
#use Delay(Clock=20000000)
#use rs232(baud=9600, rcv=GPS_RX,stream=GPS) //GPS data from GPS unit
#use rs232(baud=9600, xmit=PC_TX,stream=PC) //TX line to level converter and PC
CHAR c; //Read serial characters into here
INT buffer_index; //Keep track of position in buffer
boolean found; //flag to determine if we have found the NMEA sentance we want
CHAR GPRMC [60]; //buffer to hold the desired sentance for parsing
void parse(); //function to do the parsing
void main() {
boolean i;
output_low(RESET); //reset GPS unit
output_high(LED);
delay_ms(4000);
output_high(RESET);
output_low(LED);
while(TRUE){
int b;
fprintf(PC,"$GPRMC"); //buffer doesn't contain this, so print it directly
for(b=0;b<60;b++){ //loop through array elemts printing them to the PC via link
fprintf(PC,"%c",GPRMC[b]);
}
fprintf(PC,"\n");
delay_ms(2000);
}
}
void parse()
{
buffer_index = 0;
found=FALSE; //initialy, we haven't found the sentance
c=fgetc(GPS); //read a character from the serial line
WHILE (!found){
WHILE (c != '$') //keep on reading until we find a '$' - the start of a sentance
{
c = fgetc (GPS);
}
c = fgetc (GPS);
IF (c == 'G')
{
c = fgetc (GPS);
IF (c == 'P')
{
c = fgetc (GPS); //run through subsequent characters to check if it is the one we want
IF (c == 'R')
{
c = fgetc (GPS);
IF (c == 'M')
{
c = fgetc (GPS);
IF (c == 'C')
{
c = fgetc (GPS);
found=TRUE;
//If we get here, we have identified NMEA sentance we want. Read it into a buffer.
WHILE (c != '$')
{
c = fgetc (GPS);
GPRMC[buffer_index] = c;
buffer_index++;
}
}
}
}
}
}
}
}
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Wed Feb 01, 2006 6:56 pm |
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At a minimum, you should reconfigure your GPS stream so that it uses
the hardware UART. Add the items shown in bold below.
| Quote: |
define GPS_TX PIN_C6
define GPS_RX PIN_C7
#use rs232(baud=9600, xmit=GPS_TX, rcv=GPS_RX, ERRORS, stream=GPS) |
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Donlaser
Joined: 17 Sep 2005
Posts: 12
Location: Trenque Lauquen, Argentina
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| Posted: Wed Feb 01, 2006 7:36 pm |
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Add a call to the parse function in the main loop. The garbage that you get in the pc is the uninitialized content of GRTMC buffer.
while(TRUE){
int b;
parse(); // <<<<<< add this line <<<<<<<
fprintf(PC,"$GPRMC"); //buffer doesn't contain this, so print it directly
for(b=0;b<60;b++){ //loop through array elemts printing them to the PC via link
fprintf(PC,"%c",GPRMC[b]); |
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Guest
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| Posted: Thu Feb 02, 2006 3:59 am |
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Oh dear - I must have been suffering from "it's-too-late-at-night-itus"!
How can I have left out the call to the parsing function!!! It works fine now -
thanks for your efforts.
Cheers,
Ben |
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