Summary of How to interface GPS with AVR microcontroller (ATmega16)
GPS modem receives satellite signals and outputs serial RS232 data (NMEA sentences) containing time, latitude, longitude, altitude, and related info. This article explains interfacing a GPS modem to an ATmega16 via a MAX232 level shifter (crossed TX/RX, common ground), parsing the GPGGA NMEA sentence to extract latitude and longitude, and displaying them on an LCD. An example GPGGA string is dissected field-by-field to show time, coordinates, fix status, satellite count, altitude, and checksum.
Parts used in the GPS with ATmega16 project:
- GPS modem
- ATmega16 microcontroller
- MAX232 level shifter
- LCD display
- Serial cross cable connections (TX/RX wiring)
- Common ground connection
- Power supply for GPS modem and microcontroller
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.
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Pin 2 of MAX232
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Pin 3 of GPS Modem
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Pin 3 of MAX232
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Pin 2 of GPS Modem
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Pin 5 Ground Pin of MAX232
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Pin 5 Ground of GPS Modem
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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
For more detail: How to interface GPS with AVR microcontroller (ATmega16)
- How is the GPS modem connected to the ATmega16?
Via a MAX232 level shifter with serial cross connections (MAX232 pin 2 to GPS pin 3 and MAX232 pin 3 to GPS pin 2) and a common ground. - What serial format does the GPS modem output use?
The GPS modem outputs serial RS232 data in NMEA sentence format, such as GPGGA. - How do you extract latitude and longitude from the GPS output?
By parsing the GPGGA NMEA string fields that contain latitude and longitude values. - What does a GPGGA string start with?
A GPGGA string always starts with the $ character followed by GPGGA. - What information does the GPGGA sentence provide?
It provides GMT time, latitude, longitude, fix status, number of satellites, HDOP, altitude, geoid height, DGPS data, and checksum. - How is fix validity indicated in GPGGA?
By the fix quantity field where 0=invalid data, 1=valid data, and 2=DGPS fix. - What are the coordinate formats in the sample GPGGA string?
Latitude is given as degrees and minutes (e.g., 2650.9416,N) and longitude as degrees and minutes (e.g., 07547.8441,E). - What component handles RS232 to TTL conversion?
The MAX232 level shifter converts RS232 signals to logic levels suitable for the microcontroller.
