Velocity meter with GPS module(u-blox RCB-4H) and PIC16F877A

sonealx · Joined: 03 Jun 2010 Posts: 4

Hello everybody;

I want to use the GPVTG string in NMEA protocol. Example;

$GPVTG,77.52,T,,M,0.004,N,0.008,K,A*06

The 0.008 shows the speed in terms of km/h. My project is about that. Firstly I want to take this speed information and send it to the LCD. I know how to show any number or word in LCD but;

My problem is that I don't know how to read and process the NMEA data strings which are sent from GPS module with PIC.

I am also new in codes but;

Can anyone help me about reading NMEA string with CCS C software codes??

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

This may not be for a beginner.
Your GPS will repeat sentences when it wants. This means you will need a circular buffer fed by an isr char by char. The sentence $GPVTG should be gated into the buffer and all others rejected. A flag is set in the isr when the $GPVTG is fully received. Next you will write a parsing routine that will break the sentence down between commas. If speed is used in any meaningful way you will need to calculate the crc for the sentence in that way you know you have valid data. These parsing and crc routines will be in your main code.
Caveat
This information is not to be used for any navigational purposes or for any other purposes in which your safety or the safety of others may be at risk.