GPS over I2C!

We’ve written a simple I2C interface for I2GPS (available for sale here). If you’ve ever combined GPS to Arduino you have probably used TinyGPS by Mikal Hart.  We use it too and we love it.

When you want to incorporate GPS into a larger project a couple of issues arise that I2GPS solves.  The first issue is that the GPS object must constantly be “fed” new data.  This leads to ubiquitous calls to gps.feed() throughout the code.  If you have other time critical functions (like updating an LED matrix), the juggling of tasks can get overly complex.  The second is the size of the code can cramp the size of your main program.  We ran into both of these problems with ClockTHREEjr.

I2GPS solves these problems with by offloading the memory and processing burden to a second processor.  The I2GPS slave code needs only to read the GPS, and respond to I2C requests to data.

I’ve modeled the interface after the DS3231, the real time clock included with I2GPS, the same clock as the Chronodot from Macetech.  So from the client side, things look a lot like reading the Chronodot.  Here is a snippet of client code (not updated).  The DS3231 is still available though the I2C interface.

// read 32 bytes from address 0, store in "gps_data" byte array
  gps_raw_read((uint8_t)0, (uint8_t)32, gps_data);

// Parse response
  Serial.print("UNIX TIME:");
  Serial.println(" 1/1000th DEG");
  Serial.println(" 1/1000th DEG");
  Serial.println(" CM");

Wall clock time is also available starting at address 0x1C.

gps_raw_read(0x1C, 6, ymdhms);

Serial.print("CALENDAR TIME:");
Serial.print(ymdhms[1], DEC); // Month
Serial.print(ymdhms[2], DEC); // Day
Serial.print(ymdhms[0], DEC); // Year

Serial.print(" ");
Serial.print(ymdhms[3], DEC); // Hour
Serial.print(ymdhms[4], DEC); // Minute
Serial.print(ymdhms[5], DEC); // Second

The I2GPS has several physical interface options. Shown above is the “Buckler” (small shield) interface that plugs directly into the analog side of the Arduino.

The complete data structure memory map is available as a google spreadsheet (for the latest) or below in static html.  I’ve put a couple of field to log the state of analog and digital pins, but supporting code has not been implemented yet.  Available data include: unix time (and wall clock time), latitude, longitude, altitude, course, speed, fix age.


Programming ClockTHREEjr_v2

At some point, you may wish to re-program your ClockTHREEjr_v2 or maybe you are building a clock from scratch.  This post will walk you through the steps.


  1. Download and install Arduino for your target platform.  (More details can be found here.)
    • Navigate to the Arduino download site:
    • Download Arduino for your platform.
    • Unzip the file to the directory of your choice.  (We will refer to whatever directory you choose as <arduino_dir>).  I use c:\arduino\ on Windows or /home/justin/arduino/ on Linux so that (for arduino 1.0 for instance) the path extracted executable is c:\arduino\arduino-1.0\arduino.exe on Windows or /home/justin/arduino/arduino-1.0/arduino on Linux.
  2. Download and install ClockTHREE source code
    • Download the latest ClockTHREE library source code file from here:
    • Unzip the library source code file in your home directory. I use “My Documents\sketechbook\” on windows and /home/justin/sketchbook/ on linux.  We will refer to the directory you choose as <sketchbook>.
  3. IMPORTANT: Point Arduino to your newly extracted library files.
    • Start arduino (look here if you have trouble with this step)
    • Click File->Preferences to pull up the preferences menu.
    • Leave “Use external editor” unchecked if you plan on using Arduino as your editor.
    • Click the “Browse” button and navigate to your <sketchbook> directory from step 2.
    • Click “OK”.
    • Close and restart Arduino
    • Click “File->Sketchbook->libraries”.  If “ClockTHREE” is listed you’ve done well.
  4. Using a text editor, edit source code:
    • On or about line 32 of <sketchbook>/libraries/ClockTHREE/examples/ClockTHREE_04/ClockTHREE_04.ino, comment out all but one of the following lines depending on language and hardware.
    // #include "dutch_v1.h"
    // #include "english_v0.h"
    // #include "english_v2.h"
    #include "english_v3.h"
    // #include "french_v1.h"
    // #include "german_v1.h"
    // #include "german_v3.h"
    // #include "german_v5.h"
    // #include "hebrew_v1.h"
  5. Compile and upload
    • Connect the FTDI cable to ClockTHREE (mind color labels) and to your computer.
    • Click Tools->Boards->Duemilanove
    • Select USB port Tools->Serial Port (this may take some trial and error if you have several devices connected).
    • Compile and upload <sketchbook>/libraries/ClockTHREE/examples/ClockTHREE_04/ClockTHREE_04.ino.
      Click the arrow button, second from the right.  
  6. Congratulations!  That is it.  Now you can customize the code to your hearts content!