How to interface GPS with AVR microcontroller (ATmega16)

Developed By: 

Akshay Daga

GPS modem is a device which receives signals from satellite and provides information about latitude, longitude, altitude, time etc. The GPS navigator is more famous in mobiles to track the road maps. The GPS modem has an antenna which receives the satellite signals and transfers them to the modem. The modem in turn converts the data into useful information and sends the output in serial RS232 logic level format. The information about latitude, longitude etc is sent continuously and accompanied by an identifier string.

This article shows how to interface the GPS modem with ATmega16 and extract the location (latitude and longitude) from the GPGGA string and display it on LCD.

The connection of GPS modem with AVR microcontroller (ATmega 16) is shown in the circuit diagram. The ground pin of max 232 and serial o/p of GPS modem is made common. Pin2 of MAX232 is connected to pin 3 of GPS modem and pin 3 of max 232 is connected to pin 2 of modem. This type of connection is called a serial cross cable.

 

Pin 2 of MAX232	Pin 3 of GPS Modem
Pin 3 of MAX232	Pin 2 of GPS Modem
Pin 5 Ground Pin of MAX232	Pin 5 Ground of GPS Modem

The commonly available GPS modem gives output in serial (RS232) form. The output consists of a series of string.

 

String format:

The following is an example of the output string from the GPS module with its explanation. This output strings contains information about latitude, longitude, time etc and will always start with $GPGGA. Refer NMEA Standards for more details on string formats.

An example string has been given and explained below:

 $GPGGA,100156.000,2650.9416,N,07547.8441,E,1,08,1.0,442.8,M,-42.5,M,,0000*71

1.      A string always start from ‘$’ sign

2.      GPGGA :Global Positioning System Fix Data

3.      ‘,’ Comma indicates the separation between two values

4.      100156.000 : GMT time as 10(hr):01(min):56(sec):000(ms)

5.      2650.9416,N: Latitude 26(degree) 50(minutes) 9416(sec) NORTH

6.      07547.8441,E: Longitude 075(degree) 47(minutes) 8441(sec) EAST

7.      1 : Fix Quantity 0= invalid data, 1= valid data, 2=DGPS fix

8.      08 :  Number of satellites currently viewed.

9.      1.0: HDOP

10.  442.8,M : Altitude (Height above sea level in meter)

11. -42.5,M :         Geoids height

12.    __ , DGPS data

13. 0000 : DGPS data

14. *71 : checksum

 

The following algorithm is used to extract the latitude and longitude information from the GPS module using $GPGGA string and display it on a LCD:

1.      Get data in UDR and check weather that data is equal to $. If the data matches go to step(2) else get a new data.

2.      Get data byte by byte and check if the received byte is equal to GPGGA

3.      If the step (2) matches completely then  go to step (4) else go back to step(1)

4.      Leave first comma and wait till second comma (since we not looking for time).

5.      Start taking data in an array lati_value[ ] till the next comma.

6.      Get latitude direction in lati_dir

7.      Do the same for longitude

8.      Display the values on LCD and go back to step (1).

 

Circuit:

Code:

// Program to get latitude and longitude value from GPS modem and display it on LCD:

/*

LCD DATA port----PORT A

signal port------PORT B

rs-------PB0

rw-------PB1

en-------PB2

*/

 

#define F_CPU 12000000UL

 

#include<avr/io.h>

#include<util/delay.h>

 

#define USART_BAUDRATE 4800

#define BAUD_PRESCALE (((F_CPU / (USART_BAUDRATE * 16UL))) - 1)

 

 

#define LCD_DATA PORTA //LCD data port

 

#define ctrl PORTB

#define en PB2 //enable signal

#define rw PB1 //read/write signal

#define rs PB0 //resister select signal

 

void LCD_cmd(unsigned char cmd);

void init_LCD(void);

void LCD_write(unsigned char data);

void LCD_write_string(unsigned char *str);

 

void usart_init();

unsigned int usart_getch();

 

unsigned char value,i,lati_value[15],lati_dir, longi_value[15], longi_dir, alti[5] ;

 

int main(void)

{

DDRA=0xff; //LCD_DATA port as out put port

DDRB=0x07; //ctrl as out put

init_LCD(); //initialization of LCD

_delay_ms(50); // delay of 50 mili seconds

LCD_write_string("we at");

LCD_cmd(0xC0);

usart_init(); // initialization of USART

while(1)

{

value=usart_getch();

if(value=='$')

{

value=usart_getch();

if(value=='G')

{

value=usart_getch();

if(value=='P')

{

value=usart_getch();

if(value=='G')

{

value=usart_getch();

if(value=='G')

{

value=usart_getch();

if(value=='A')

{

value=usart_getch();

if(value==',')

{

value=usart_getch();

while(value!=',')

{

value=usart_getch();

}

lati_value[0]=usart_getch();

value=lati_value[0];

for(i=1;value!=',';i++)

{

lati_value[i]=usart_getch();

value=lati_value[i];

}

lati_dir=usart_getch();

value=usart_getch();

while(value!=',')

{

value=usart_getch();

}

longi_value[0]=usart_getch();

value=longi_value[0];

for(i=1;value!=',';i++)

{

longi_value[i]=usart_getch();

value=longi_value[i];

}

longi_dir=usart_getch();

LCD_cmd(0x01);

_delay_ms(1);

LCD_cmd(0x80);

_delay_ms(1000);

i=0;

while(lati_value[i]!='\0')

{

LCD_write(lati_value[j]);

j++;

}

LCD_write(lati_dir);

LCD_cmd(0xC0);

_delay_ms(1000);

i=0;

while(longi_value[i]!='\0')

{

LCD_write(longi_value[i]);

i++;

}

LCD_write(longi_dir);

_delay_ms(1000);

 

}

}

}

}

}

}

}

}

}

 

void init_LCD(void)

{

LCD_cmd(0x38); //initialization of 16X2 LCD in 8bit mode

_delay_ms(1);

 

LCD_cmd(0x01); //clear LCD

_delay_ms(1);

 

LCD_cmd(0x0E); //cursor ON

_delay_ms(1);

 

LCD_cmd(0x80); // ---8 go to first line and --0 is for 0th position

_delay_ms(1);

return;

}

 

 

void LCD_cmd(unsigned char cmd)

{

LCD_DATA=cmd;

ctrl =(0<<rs)|(0<<rw)|(1<<en);

_delay_us(40);

ctrl =(0<<rs)|(0<<rw)|(0<<en);

//_delay_ms(50);

return;

}

 

 

void LCD_write(unsigned char data)

{

LCD_DATA= data;

ctrl = (1<<rs)|(0<<rw)|(1<<en);

_delay_us(40);

ctrl = (1<<rs)|(0<<rw)|(0<<en);

//_delay_ms(50);

return ;

}

 

 

void usart_init()

{

   

UCSRB |= (1<<RXCIE) | (1 << RXEN) | (1 << TXEN);   // Turn on the transmission and reception circuitry

UCSRC |= (1 << URSEL) | (1 << UCSZ0) | (1 << UCSZ1); // Use 8-bit character sizes

 

UBRRL = BAUD_PRESCALE; // Load lower 8-bits of the baud rate value into the low byte of the UBRR register

UBRRH = (BAUD_PRESCALE >> 8); // Load upper 8-bits of the baud rate value into the high byte of the UBRR register

}

 

 

unsigned int usart_getch()

{

 

while ((UCSRA & (1 << RXC)) == 0); // Do nothing until data have been recieved and is ready to be read from UDR

return(UDR); // return the byte

}

 

void LCD_write_string(unsigned char *str) //take address vaue of the string in pionter *str

{

int i=0;

while(str[i]!='\0') // loop will go on till the NULL charaters is soon in string 

{

LCD_write(str[i]); // sending data on CD byte by byte

i++;

}

return;

}