[Unfinished]

The HMC5883L is a I2C controlled 3 axis digital compass IC manufactured by honeywell. 
Each axis provide analog data to a 12bit ADC, wich store his output into a register.  X,Y,Z data and all commands are merged into 8bit packets so, for send a command only is needed one packet send but for read X,Y,Z data two reads are neccesary.

HMC5883L module can be bought from ebay for about 3$ with free shipping.

Example with HMC5883L library in LCDBPV2 Example code changelog thread

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Links:

general
http://blog.oletronics.com/2012/01/hmc5883l-how-it-works.html

datasheet
http://www51.honeywell.com/aero/common/documents/myaerospacecatalog-documents/Defense_Brochures-documents/HMC5883L_3-Axis_Digital_Compass_IC.pdf

Arduino:
http://forum.arduino.cc/index.php?topic=141937.0http://www.loveelectronics.co.uk/Tutorials/8/hmc5883l-tutorial-and-arduino-library
http://www.control.aau.dk/~jdn/sketchbook/hmc5883l/hmc5883l.inoC Library
http://www.bildr.org/forum/viewtopic.php?f=29&t=650

picaxe
http://www.picaxeforum.co.uk/showthread.php?18912-HMC5883L-magnetometer&s=27297d6f2c27c4fab6ae5deee9ba9899http://www.i2cdevlib.com/docs/html/class_h_m_c5883_l.html

Raspberry pi forum
http://www.raspberrypi.org/phpBB3/viewtopic.php?t=32211&p=277739

Parallax
http://tymkrs.tumblr.com/post/22237323716/parallax-compass-module-3-axis-hmc5883l-29133

youtube videos
http://www.youtube.com/watch?v=nl2-jqMi0FE

other ebay sensor
http://www.ebay.com/sch/i.html?_odkw=L3G4200D&_sop=15&_osacat=0&_from=R40&_trksid=p2045573.m570.l1313.TR4.TRC2.A0&_nkw=L3G4200&_sacat=0

pic
http://www.ccsinfo.com/forum/viewtopic.php?p=179610

2 complement explanation
http://www.cs.uwm.edu/~cs151/Bacon/Lecture/HTML/ch03s09.html
http://electronics.stackexchange.com/questions/39296/calculating-range-of-twos-complement
http://www.exploringbinary.com/twos-complement-converter/
http://sandbox.mc.edu/~bennet/cs110/tc/tctod.html
http://www.ehow.com/how_8420505_convert-twos-complement-decimal.htmlhttp://math.stackexchange.com/questions/285459/how-can-i-convert-2s-complement-to-decimal
http://es.wikipedia.org/wiki/Complemento_a_dos
http://stackoverflow.com/questions/1049722/what-is-2s-complement
http://www.microchip.com/forums/m644250.aspx
http://www.ccsinfo.com/forum/viewtopic.php?t=25630
http://en.wikipedia.org/wiki/Two's_complement

c operators
http://es.wikipedia.org/wiki/Anexo:Operadores_de_C_y_C%2B%2B

tilt compensated
https://www.google.es/search?q=tilt-compensated-compass-arduino-tutorial&oq=tilt-compensated-compass-arduino-tutorial&aqs=chrome..69i57.225j0j4&sourceid=chrome&espv=210&es_sm=122&ie=UTF-8
http://forum.arduino.cc/index.php/topic,8573.0.htmldiy drones

http://www.diydrones.com/

arducopter
http://copter.ardupilot.com/

Calculating heading with tilted compass
http://www.timzaman.com/?p=1010

 

danirebollo.es