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The first instruments were based on the Nintendo Gameboy Advance (GBA) as a platform. It is easy to develop for - the hardware has been reverse engineered, and there are GNU based free software development environments with libraries to ease the pain of developing graphical user interfaces. I used the GBA multi-player link port to interface with GPS modules and digital pressure sensors. The application software is on a flash memory cartridge that fits into the game ROM slot. So there is no need to modify the GBA hardware at all.
I then did some experiments with the Charmed Labs XPort 2 embedded GBA development system. This is a card with an FPGA and flash memory that fits into the game ROM slot. The FPGA can be reconfigured with your own add-on hardware logic, and there are a large number of I/O pins on the card available for interfacing to the outside world. With this I was able to develop a GPS/altimeter/vario with GPS tracklog save capability.
The XPort 2 system is however not suitable for outdoors applications with exposure to heat, humidity, dust and hard knocks. And the FPGA eats into battery life.
I then tried out the Nintendo DS Lite. This needs a BIOS reflash for homebrew code execution using the GBA slot. After this is done, you can execute code from a flash cartridge in the GBA slot to take advantage of the new hardware - ARM9 CPU + ARM7 CPU, two screens with improved resolution, touch screen, more RAM, upgraded graphics and audio hardware, etc.
I used a DSerial2 interface card (unfortunately, no longer available) in the DS cartridge slot to interface with a GPS and pressure sensor, and developed a mapping altimeter/variometer application. Unfortunately, the system had a fatal flaw - unlike the standard GBA and the earlier GBA SP models which used a reflective LCD Screen, the Nintendo DS Lite uses a backlit screen. This is washed out in bright sunlight, making it useless for paragliding/hanggliding applications. The use of the backlight decreases battery life as well. So that ended up as an expensive experiment ...
NOTE : I found that the newest Gameboy Advance SP units have a backlit screen as well. So if you do plan to construct any GBA projects, make sure you test the display outdoors or get yourself an older GBA SP model with the reflective screen.
I am now developing small form factor, low cost, low power instruments, using inexpensive microcontrollers, LCD displays from cellphones and standard cellphone batteries.
Click on the image links below for circuit schematics, project notes and source code.
Be warned that even though the circuits may look very simple, you will need some experience with soldering fine-pitch SMD components, hardware bring-up, and firmware development. There are no step-by-step instructions, so you will have to use your own experience and effort to fill in the blanks. Google is your immediate and obliging friend. I, on the other hand, am a distant acquaintance. Of course, feedback and bug reports are always welcome. So far, most of the people contacting me to report successful replication of these projects have been hardware designers - some with little or no knowledge of C programming. I'm more of a software guy myself as far as experience goes, so your mileage may vary ...
Finally, here's a list of online stores where you can source most of the components for these projects :
Dipmicro - prototyping boards (SOIC & SSOP), SMD ceramic and tantalum caps
Tayda Electronics- SMD resistors, diodes, PCB mount pushbutton & slide switches, inductors
Futurlec- solar cells, SSOP/SOIC/LQFP breakout boards, ICs, micro-controllers
Future Electronics- regulators, micro-controllers
Digikey - ICs, micro-controllers, BMP085 pressure sensor
Electronic Goldmine- copper clad boards, SMD resistors & capacitors, ICs, PCB drill bits, solar cells
Microchip Direct- PIC micro-controllers, PICKit 2 programmer
Sparkfun- BMP085 pressure sensor, breakout boards, ICs, programmers etc.
DealExtreme- LiPolymer batteries, GBA flash cards