Introduction
The Global Positioning System (GPS) uses signals from satellites in space and ground stations on Earth to accurately determine a location.
GPS receives Radio Frequency signals sent by satellites and ground stations, which it uses to calculate its exact position. GPS itself does not transmit any information.
The received signals contain timestamps indicating when they were sent. By measuring the time difference between transmission and reception, and knowing the signal speed, GPS calculates the distance to each satellite. Using data from three or more satellites, the GPS can triangulate its precise position.
For more details on GPS and how to use it, refer to the GPS Receiver Module section in sensors and modules.
The GPS receiver module communicates with a microcontroller or PC terminal via USART.
For information on using UART with the 8051 microcontroller, refer to the UART in 8051 topic in the 8051 Inside section.
GPS Receiver Module
Interfacing Diagram
GPS module Interfacing with 8051
Example
Now, let’s interface a GPS receiver module with the 8051 microcontroller to display Time, Latitude, and Longitude on a 20×4 LCD.
In this setup, the AT89S52 microcontroller reads data serially from the GPS receiver using USART communication at 9600 baud.
The microcontroller parses the “$GPGGA” string from the GPS data to extract information on time, latitude, and longitude.
Latitude and Longitude are displayed on the 20×4 LCD in Degree Decimal format.
Program
/*
GPS Information extraction using 8051
http://www.electronicwings.com
*/
#include<reg52.h>
#include<string.h>
#include<stdio.h>
#include<stdlib.h>
#include "LCD_20x4_H_file.h"
#include "USART_Interrupt.h"
unsigned long int get_gpstime();
float get_latitude(unsigned char);
float get_longitude(unsigned char);
float get_altitude(unsigned char);
void convert_time_to_UTC(unsigned long int);
float convert_to_degrees(float raw);
#define GGA_Buffer_Size 80
#define GGA_Pointers_Size 20
char idata GGA_Buffer[GGA_Buffer_Size]; /* to store GGA string */
char idata GGA_CODE[3];
idata unsigned char GGA_Pointers[GGA_Pointers_Size]; /* to store instances of ',' */
char CommaCounter;
idata char Data_Buffer[15];
volatile unsigned int GGA_Index;
unsigned char IsItGGAString = 0;
void main(void) {
unsigned long int Time;
float Latitude,Longitude,Altitude;
char GPS_Buffer[15];
LCD_Init();
UART_Init();
Ext_int_Init(); /* enable interrupt */
while (1){
memset(GPS_Buffer,0,15);
LCD_String_xy(1,0,"UTC Time: ");
Time = get_gpstime(); /* Extract Time */
convert_time_to_UTC(Time); /* convert time to UTC */
LCD_String(Data_Buffer);
LCD_String(" ");
LCD_String_xy(2,0,"Lat: ");
Latitude = get_latitude(GGA_Pointers[0]); /* Extract Latitude */
Latitude = convert_to_degrees(Latitude); /* convert raw latitude in degree decimal*/
sprintf(GPS_Buffer,"%.05f",Latitude);/* convert float value to string */
LCD_String(GPS_Buffer); /* display latitude in degree */
memset(GPS_Buffer,0,15);
LCD_String_xy(3,0,"Long: ");
Longitude = get_longitude(GGA_Pointers[2]);/* Extract Latitude */
Longitude = convert_to_degrees(Longitude);/* convert raw longitude in degree decimal*/
sprintf(GPS_Buffer,"%.05f",Longitude);/* convert float value to string */
LCD_String(GPS_Buffer); /* display latitude in degree */
memset(GPS_Buffer,0,15);
LCD_String_xy(4,0,"Alt: ");
Altitude = get_altitude(GGA_Pointers[7]); /* Extract Latitude */
sprintf(GPS_Buffer,"%.2f",Altitude);/* convert float value to string */
LCD_String(GPS_Buffer); /* display latitude in degree */
}
}
unsigned long int get_gpstime(){
unsigned char index;
unsigned char Time_Buffer[15];
unsigned long int _Time;
/* parse Time in GGA string stored in buffer */
for(index = 0;GGA_Buffer[index]!=','; index++){
Time_Buffer[index] = GGA_Buffer[index];
}
_Time= atol(Time_Buffer); /* convert string of Time to integer */
return _Time; /* return integer raw value of Time */
}
float get_latitude(char lat_pointer){
unsigned char lat_index = lat_pointer+1; /* index pointing to the latitude */
unsigned char index = 0;
char Lat_Buffer[15];
float _latitude;
/* parse Latitude in GGA string stored in buffer */
for(;GGA_Buffer[lat_index]!=',';lat_index++){
Lat_Buffer[index]= GGA_Buffer[lat_index];
index++;
}
_latitude = atof(Lat_Buffer); /* convert string of latitude to float */
return _latitude; /* return float raw value of Latitude */
}
float get_longitude(unsigned char long_pointer){
unsigned char long_index;
unsigned char index = long_pointer+1; /* index pointing to the longitude */
char Long_Buffer[15];
float _longitude;
long_index=0;
/* parse Longitude in GGA string stored in buffer */
for( ; GGA_Buffer[index]!=','; index++){
Long_Buffer[long_index]= GGA_Buffer[index];
long_index++;
}
_longitude = atof(Long_Buffer); /* convert string of longitude to float */
return _longitude; /* return float raw value of Longitude */
}
float get_altitude(unsigned char alt_pointer){
unsigned char alt_index;
unsigned char index = alt_pointer+1; /* index pointing to the altitude */
char Alt_Buffer[12];
float _Altitude;
alt_index=0;
/* parse Altitude in GGA string stored in buffer */
for( ; GGA_Buffer[index]!=','; index++){
Alt_Buffer[alt_index]= GGA_Buffer[index];
alt_index++;
}
_Altitude = atof(Alt_Buffer); /* convert string of altitude to float */
return _Altitude; /* return float raw value of Altitude */
}
void convert_time_to_UTC(unsigned long int UTC_Time)
{
unsigned int hour, min, sec;
hour = (UTC_Time / 10000); /* extract hour from integer */
min = (UTC_Time % 10000) / 100; /* extract minute from integer */
sec = (UTC_Time % 10000) % 100; /* extract second from integer*/
sprintf(Data_Buffer, "%d:%d:%d", hour,min,sec); /* store UTC time in buffer */
}
float convert_to_degrees(float raw){
double value;
float decimal_value,temp;
long int degrees;
float position;
value = raw;
/* convert raw latitude/longitude into degree format */
decimal_value = (value/100);
degrees = (int)(decimal_value);
temp = (decimal_value - (int)decimal_value)/0.6;
position = (float)degrees + temp;
return position; /* return float value in degrees */
}
void Serial_ISR() interrupt 4
{
unsigned char received_char;
if(RI){
EA = 0; /* Disable global interrupt */
received_char = SBUF;
if(received_char =='$'){ /* check for '$' */
GGA_Index = 0;
IsItGGAString = 0;
CommaCounter = 0;
}
else if(IsItGGAString == 1){ /* if true save GGA info. into buffer */
if(received_char == ',' )
GGA_Pointers[CommaCounter++] = GGA_Index; /* store instances of ',' in buffer */
GGA_Buffer[GGA_Index++] = received_char;
}
else if(GGA_CODE[0] == 'G' && GGA_CODE[1] == 'G' && GGA_CODE[2] == 'A'){ /* check for GGA string */
IsItGGAString = 1;
GGA_CODE[0] = 0; GGA_CODE[1] = 0; GGA_CODE[2] = 0;
}
else{
GGA_CODE[0] = GGA_CODE[1]; GGA_CODE[1] = GGA_CODE[2]; GGA_CODE[2] = received_char;
}
RI = 0; /* Clear RI flag */
EA = 1; /* enable interrupt */
}
}