Saturday, May 10, 2014

Going from Arduino TWI to BeagleBone Black Bonescript

It's handy for a robot to know what orientation it is holding, and for that a Magnetometer can give you where north is at relative to the chip orientation. For this I used the HMC5883. I started out with a cut down minimal version of code running on Arduino and then started porting that over to Bonescript for the BeagleBone Black to execute.

The HMC5883 only needs power (3v3), ground, and the two wires of a TWI.

One of the larger API changes of the port is the Arduino line:

Wire.requestFrom( HMC5883_ADDRESS_MAG, 6 );

Which takes the TWI address of the chip you want 6 bytes from. It has to be prefixed by another TWI transmission to set the read byte address to start from where you would like, in this case HMC5883_REGISTER_MAG_OUT_X_H_M = 0x03.

On the Bonescript side this becomes the following. The TWI address isn't used because the wire object already knows that, but you pass the register address to start reading from directly to the request to read bytes:

self.wire.readBytes( HMC5883_REGISTER_MAG_OUT_X_H_M,  
function(err, res) { ... });

I pushed the code up to github bbbMagnetometerHMC5883. Note that getting "north" requires both a proper declination angle for your location and also some modification to allow for how your magneto chip is physically oriented on your robot. Actual use of the chip is really easy with the async style of nodejs.

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