read GPS NMEA data using PIC18F452

Ricky · Joined: 19 Sep 2005 Posts: 1

Hi,
I wish to read off NMEA data off OEM GPS receiver using a PIC18F452 and display it on LCD. Can someone please give me hints on how to do this in terms of code writing or good links? Any help will be much appreciated.

Thanks in advance,
Ricky

EmbdFreak · Joined: 28 Jul 2005 Posts: 23

Well, as u know the output of GPS on RS-232 are NMEA strings followed by the data each one holds. Each string starts with a '$' sign followed by several information fields. There are about 10-15 NMEA strings an average GPS gives (depends on model). Now you have which string contains the data u need, e.g. 'GPRMC' gives u latitude, longitude, lat hem, long hem, UTC date,time etc.. . each one is separated by a comma. So u need to implement a kind of parser which will parse this data until a CR,LF is detected, which ends the string. Ur program design should look for '$',upon recieval count number of commas and keep storing/displaying your field of interest. As for LCD, use lcd.c with any hd44780 based lcd display.
I hope this gets u going,

Regards,

Douglas Kennedy · Joined: 07 Sep 2003 Posts: 755 Location: Florida

The GPS sentences are spoken by most GPS receivers about once every second without regard to the state the PIC is in ( asynchronously). You will need a circular buffer to buffer a complete sentence ( 40 to 60 bytes). Look at postings for writing an interrupt driven circular buffer based on an RDA interrupt when a char is received. Do all of the preparation for display to the LCD in your main code and leave the ISR ( interrupt service routine) to filter out unwanted sentences and place good data in the buffer.

pe1icq · Joined: 29 Aug 2005 Posts: 12

I've recently worked on an update of an implementation on the 16f628, but it was written for the HiTech compiler.
http://sharon.esrac.ele.tue.nl/users/pe1rxq/aprstracker/aprstracker.html
This implementation processes GPGGA and GPVTG sentences.

You could also have a look at SOTT (version 1.0). It is a similar application but written using the CCS compiler. This one works with GPRMC sentences.
You can find it in the files area on http://groups.yahoo.com/group/TinyTrak/

//Arno

Guest · Posted: Mon Sep 19, 2005 9:48 am

http://www.vermontficks.org/pic.htm

Look at the GPS repeater project, the code is in CCS C.