PIC GPS device

PIC GPS device

PIC GPS unit in a small Really Useful storage box. I didnt drill any holes in the box for switches as I didnt want them to get turned ON by accident and I wanted the device to be splash proof for the mountains. You open the lid to turn it on and set the mode. The photo shows the LCD screen (middle left), three switches bottom left (ON/OFF, reset and mode change), batteries at top and the Parallax GPS unit on the middle right.

This is a simple PIC GPS unit based on a Parallax GPS module. The PIC sends a serial data request to the GPS module and it sends back the processed satellite data so that you can display it on a screen and data log for example. For example if you ask the GPS unit for 'number of satellites' (which can be requested by sending a few commands via a simple serial data link from the PIC) it will send back the number of satellites the unit is receiving. There are similar codes for requesting time, date, altitude and of course latitude and longitude. The number of satellites comes back as a single byte of data while the latitude and longatude data returns as 5 bytes. Generally the software fills an array with the data as it arrives from the module. My first attempts failed because I requested data too quickly and did not build in a short delay between requests.

I used a Really Useful storage box (0.35 litre) rather than a standard electronics project box for the prototype. Its easy to open up and adjust and is suitably transparent. Once the lid is on it will be splash proof for the mountains etc.

I am using a 40 pin PIC16F877 which is much more than I need for this task. However the PIC sits under the LCD screen so it does not take up extra space. I plan to store data in a compact flash card (CF) and perhaps also include a temperature sensor at a later date and then the extra ports in this PIC will come in handy. The basic idea for the project was inspired by a project by Thierry Duquesne in Elektor magazine (Elektor, July-August 2008).

The unit has two modes set by a toggle switch (right):
i) date, time (UTC), altitude and number of satellites
ii) latitude and longitude information

On the far top right of the LCD screen there a space to show that the unit is 'scanning for satellites' shown by a single character running through from . o O o . o O o . etc.

Note: the Parallax GPS module is no longer produced but can still be found on e-bay for example.
Newer versions exist that have different serial data speeds and slightly different request protocols etc.

Parallax GPS
Receiver Module
PDF datasheet
Parallax Elektor PIC page


Dr Jonathan Hare, The University of Sussex
Brighton, East Sussex. BN1 9QJ

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