Servo - November 2008

GPS SMA^BR^uilT^d ^a LOGGER

by Michael Simpson

I recently did a GPS series covering various GPS modules and their interfaces. A project that I have had in mind for a while was a small GPS Smart Logger. I call it a smart logger because in addition to the GPS data, you can log various other telemetry. You can also set the conditions and type of GPS data that gets logged.

When I started this project, I came up with the following requirements:

• Self-contained operation

• Able to operate on battery for 24 hours

• Able to detect low battery levels

• Log data must be written to SD memory card

• WAAS support

• Single button operation

• LED indicators

• 3.3V operation

• Ability to log additional data

• Must cost under $150 to build

I call this a smart logger because you have the ability to choose what does and does not get written to the SD memory card. You can also use one of the many analog-to-digital (A/D) lines on the DiosPro to log additional data.

I chose to operate the smart logger at 3.3V for a couple of reasons. First, both the SD card and the GPS module operate at that voltage. Second, the lower the operating voltage, the more run-time for a set of batteries.

The DiosPro microcontroller chip that I have chosen to use in this project will operate at 3.3V just as easily as it will at 5V. The DiosPro also supports an LVD library that is

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capable of detecting a low voltage drop. This is important to prevent log file corruption when the battery dies.

The wiring diagram for the project is shown in Schematic 1. I have used a couple of techniques to make hookup easier. Notice that the two LEDs are connected to two ports each. Ports 0 and 2 are held low, so this provides a ground for the LEDs. While this uses a port on the DiosPro, it does make the connection of the LEDs much simpler, as you will see later.

Another shortcut I have taken is to tie both leads on the record button to ports 6 and 7. The DiosPro has the ability to hold ports 0-7 high with a weak internal resistor. Port 6 is held low so that when the record button is pressed, this forces port 7 low, as well.

The EM 408 GPS connection is straightforward. I have simply connected it to the onboard UART. Notice how the Enable lead is also tied to VCC. This needs to be done in order for the GPS module to operate.

The SD memory card interface utilizes the Dios MMC library and is forced to use I/O ports 10, 11, and 12. The CS (Card Select) is configurable and can be set to any port. In this case, I have used port 13.