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    GPS Car Computer


    The GPS Car Computer can display a wide range of GPS related information.  The feature list includes a speedometer with an over speed alarm, distance and time to destination, fuel economy meter, time, altitude, heading, compass and latitude/longitude.

    A USB 2.0 interface allows you to run software on your laptop that will pinpoint your position on a moving map.  You can also load updated firmware via the USB interface.

    The physical size of the electronics (display + printed circuit board) is small enough that it can fit into many dashboards and (with a suitable front panel) will look as if it is part of the vehicle’s standard instrumentation.


    This project was described in the January 2010 and February 2010 editions of Silicon Chip magazine. This web page is intended to provide updates to the firmware and errata/notes as they are discovered for people who have already built it.

    If you have not read the article and are interested in building the GPS Car Computer you should read my web page on building the device.

    The original name for the project was GPS Display and later it was changed to GPS Car Computer. So, you can expect to find a few references to the old name in the supporting material. An example is an early front panel design shown in the photo above


    The illustration of the 6 pin mini DIN connector has the function of pins 1 and 5 swapped. Pin 1 is the headlight control and pin 5 is the fuel injector input. The correct illustration is shown on the right.

    Instruction Sheet

    The GPS Car Computer Instruction Sheet provides a compact description of the GPS Car Computer’s features and operation. It is intended to be printed on both sides of a single sheet of paper so that you can stuff it into your glovebox

    New Firmware

    A new version of the firmware (version 1.4) is available for download at the end of this page.  The difference from V1.2 and 1.3 is minor - a number of changes have been made in the startup sequence of the microcontroller to fix an intermittent issue where the micro can hang during powerup. 

    This issue seems to occur with only a few microcontrollers but it causes the Car Conputer to hang on startup with the display either blank or showing just a few characters.  This update is recommended for all users of the Car Computer but particually if your unit is suffering from this issue.

    A new feature introduced in V1.2 (and is included in V1.4) allows you to configure the display for different units of speed, distance and altitude. Other than this small change the new version is identical to the original version published by Silicon Chip and used by Altronics in their kit.

    Using this version you can choose to display distance and speed as:

    And for altitude you can choose from:

    To configure the distance/speed units press the SET button when you are on the speed display. You can then select the units (kmh/knots/mph) using the UP/DOWN buttons. Press SET a second time and you will get the over speed setting screen. Finally, pressing SET a third time will take you back to the speed display. The distance to destination display will also automatically adopt the same units. You use a similar process to configure the altitude units. Press the SET button when you are on the compass/altitude display and then select the units (m/ft) - then press SET a second time to reach the compass/heading setting.

    This version is an update and is intended to be loaded via USB from your computer.

    A handy feature of the GPS Car Computer is that you can re flash or update the firmware over the USB interface. This means that you are not forced to dismantle the unit and pull out the chip to update it (difficult if you have soldered it in place) . You also do not need a programmer and this is especially useful if you have built the device from a kit with a pre programmed chip.

    The update includes the PC software needed to reflash over USB and it will run on Windows XP, Vista or Win7 in either the 32 and 64 bit variants. Detailed instructions are also included. The download also contains the original version (V1.1) so that you can go back to it if you so wish (for testing or whatever).

    Common Faults

    The Car Computer hangs on startup showing either a blank screen (sometimes with a horizontal line) or a few characters (such as "SHOW").  This is an issue with the startup sequence of the micro and has been fixed in Version 1.4 available from the download section at the bottom of this page.

    The push button switches can be damaged during construction by being forced to one side or the other resulting in the switch being permanently closed. Another issue with the switches is that the shaft of one or more of them can be forced to one side by the position of the holes in the front panel, also resulting in the switch being permanently closed. The result is that when you apply power the firmware will see the closed switch and enter the reset or reflashing modes, both of which leave the LCD blank leading you to believe that the device is dead. To check for this fault simply take the electronics out of the box and check the operation of each switch with a multimeter set to the continuity range.

    If your Car Computer is stuck at startup waiting for enough satellites there are a few things that you can try:

    1. Wait 20 minutes or more for the GPS module to get started. GPS satellites transmit a sequence of information that enables the EM 408 to calculate the details of the satellite’s orbit and, from that, its location on the planet’s surface . This data takes 12.5 minutes to transmit so a “factory fresh” module will take at least this time before it can get a valid position fix. The module also saves this data in memory, which is supported by a super capacitor, so that when power is reapplied to the module it can quickly find its location without waiting for the complete set of data. But, if it has been a long time since the module was last used, the capacitor might have lost its charge - in that case you will have to wait for another 12.5 or more minutes.
    2. Reorientate the GPS module and make sure that it is not indoors or blocked from seeing the sky. The most sensitive orientation for the module is horizontally with the antenna on top of the module pointed to the sky.
    3. Use an external antenna. The gold coloured connector on the side of the module is for this. You need an active GPS antenna with a MMCX connector suitable for a 3 volt module. There are quite a few to choose from, just Google for "GPS antenna MMCX". I have tested this one from from DealExtreme and it worked well. Another example is SparkFun who also sell the GPS module. They have an antenna with a magnetic mount for US$15. This has a SMA connector so you will also need an interface cable MMCX to SMA for US$10.


    Free GPS Software - An Up-to-Date List

    BetterSoftware GPS - Software that plots your position on a moving map (free—they ask for a donation)

    OpenStreetMap - A free editable map of the whole world (works with BetterSoftware GPS) (free)

    OziExplorer GPS Mapping Software and Hema Maps (both cost $$$)

    NMEA Monitor - Show the output from the USB interface (free)

    VisualGPS - Show the output from the USB interface (free)

    PuTTY - Terminal Emulation Software (free)

    Earth Bridge - Will interface the GPS Car Computer to Google Earth (free)


    GPS Car Computer Instruction Sheet DOWNLOAD
    Firmware Version 1.4 Update (load via USB - full instructions are included)
    Adds configurable units for speed, distance and altitude.  Fixed startup issue.
    Silicon Chip USB Serial Port Driver (for USB interface to a PC) DOWNLOAD
    Source to Version 1.4.  Requires the CCS compiler to build. DOWNLOAD