Raspberry Pi Compass : Info requested

I’m at the early stages of writing a simple electronic compass application for the Raspberry Pi. I was wondering if anyone’s already done this, or sussed out some of the software/hardware issues. I’ve yet to buy any hardware or write any software. Hoping to produce a straight forward simple desktop application which I’ll make freely available, so any code shared will be going towards community benefit.
I’ve dropped out of programming for a couple of years but thought this would be a nice project to get me back into the flow…
Aiming to use BBC Basic with embedded assembly language where necessary.
With best wishes,
Martin

I’ve ordered some of these 3-Axis Compass Boards off eBay; Amazed that they cost less than £2 each.
The identical item seems to be sold at anything between £2 and up to £16 at various places around the web.
http://www.ebay.co.uk/itm/292002460722
Still seeking info on how best to interface them with the Pi GPIO, in particular how to grab the info coming off the board and into the Pi….

Still seeking info on how best to interface them with the Pi GPIO, in particular how to grab the info coming off the board and into the Pi….

https://cdn-shop.adafruit.com/datasheets/HMC5883L_3-Axis_Digital_Compass_IC.pdf possibly.

Thanks Steve, I printed that off earlier today so we’re hurtling down the same path. It’s the software needed to access the GPIO pins on the Raspberry Pi side of things in the correct manner that I was hoping someone had already sussed out. Probably need a tiny assembly language routine as I doubt it can be done from Basic.
There’s a comprehensive set of pin out diagrams for the various Raspberry Pi plus a library in python at the following link, so the info is about and I’ll probably end up picking through that if nothing else surfaces…
http://www.raspberrypi-spy.co.uk/2012/06/simple-guide-to-the-rpi-gpio-header-and-pins/

The board you linked to looks like it’s interfaced by I2C, so you should just be able to use the OS_IICOp SWI to talk to them, unless I’m missing something?

The board’s 7-bit IIC address is &1E so its 8-bit address is &3C/&3D for write/read. Pins 3 and 5 on the 40 pin header are the clock and data pins for IIC on the Pi, using 3.3V logic on the Pi but using 1.8V logic on the chip. Problem.

using 3.3V logic on the Pi but using 1.8V logic on the chip. Problem.

The eBay listing for the module rather than the bare chip suggested that the module worked on 3-5V, so hopefully less of a problem.