Sparkfun: HMC5883L Magnetometer

Overview
The [https://www.sparkfun.com/products/10530? HMC5883L Magnetometer] is a sensor that measures the magnetic field vector in three dimensions. the Magnetometer uses a 400KHz I2C bus to communicate. The breakout board comes with filtering capacitors and four pins for the I2C, Vcc and ground.

Connecting to the Bone
The Beagle Bone can be connected to the magnetometer via the I2C bus, but the bone only has 100kHz I2C and communication is limited. Ground and Vcc on the breakout board should be connected to pins 1 and 2 respectively on the bone's P9 header, and the SCL and SDA pins should be connected to one of the I2C pairs on the bone. 4.7k resistors should be connected between SCL and Vcc and between SDA and Vcc. I used I2C3 (P9, pins 19 and 20). The address of the magnetometer can be found by using the i2cdetect command from the shell. I get: beagle$ i2cdetect -y -r 3 0 1  2  3  4  5  6  7  8  9  a  b  c  d  e  f 00:          -- -- -- -- -- -- -- -- -- -- -- -- -- 10: -- -- -- -- -- -- -- -- -- -- -- UU -- -- 1e -- 20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --  40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --  50: -- -- -- -- UU UU UU UU -- -- -- -- -- -- -- -- 60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 70: -- -- -- -- -- -- -- --

The address should be 0x1e

Communicating with the HMC5883L Using the Shell
the HMC5883L has 13 8-bit registers:

The sensor values for each axis are 16-bit and are stored across 2 registers each.

beagle$ i2cget -y 3 0x1e 3

beagle$ i2cget -y 3 0x1e 4

will get the two halves of the 16-bit signed measurement along the +x axis.

By default the magnetometer takes a single sample and enters idle mode.The lowest two bits of the mode register control the measurement mode. Setting these to 0x01 will cause the device to take one measurement and then enter idle mode again. Setting to 0x00 should cause the device to enter continuous measurement mode,

beagle$ i2cset -y 3 0x1e 2 1 will cause a single measurement to be taken.

The new values for each axis can be read from registers 03-08 as shown above. By default the measurements correspond to 1090 LSB/Gauss, but this can be changed by modifying the gain in Configuration Register B.

Note: reading a value of 0xF000 (-4096) in a data output register indicates an overflow or underflow error in the ADC or a math error.

Using C
the I2C code from exercise 12 should work for the magnetometer, but it produces an error trying to read from or write to the device. This is probably because the magnetometer I2C operates at 400Khz and the bone uses 100kHz, so anything other than the simple shell commands produces corrupted data due to timing errors.

Example Code
The following shell script sets the magnetometer to continuous measurement mode and then reads and displays the sensor values every half seconds assuming the sensor is connected to I2C bus 3.

cleanup { # Release the GPIO port echo "" exit } trap cleanup SIGINT # call cleanup on Ctrl-C XM=0 XL=0 YM=0 YL=0 ZM=0 ZL=0 i2cset -y 3 0x1e 2 0 while [ "1" = "1" ]; do  XM=`i2cget -y 3 0x1e 3` XL=`i2cget -y 3 0x1e 4` ZM=`i2cget -y 3 0x1e 5` ZL=`i2cget -y 3 0x1e 6` YM=`i2cget -y 3 0x1e 7` YL=`i2cget -y 3 0x1e 8` echo -n -e "X_MSB=${XM} X_LSB=${XL} Y_MSB=${YM} Y_LSB=${YL} Z_MSB=${ZM} Z_LSB=${ZL}\r" # sleep for a while sleep 0.5 done cleanup # call the cleanup routine
 * 1) read the sensor value of a magnetometer
 * 1) set to continuous measurement mode
 * 1) Read forever
 * 1) read sensor values