Sparkfun: HMC5883L Magnetometer

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Overview:  1, show a picture of the device that shows what pins it has.
Wiring:   1, Give a diagram that shows how the pull up resistors are used.
Code:     0,  i2cget can get 16bit values.  Check out the 'w' option.
git:    0 put in git
Demo:     0
Total:    2/10
Comments:  More details are needed.  I think someone else in the class would have trouble
reproducing what you have done.   I'd like to see a C code version working.  I'm not sure
the clock difference is the problem.

Overview

The 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: Vcc, ground, and the two I2C pins (clock and data).

Magnetometer.jpg

Connecting to the Bone

The Beagle Bone can be connected to the magnetometer via the I2C bus. 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. I used I2C bus 3 (pin 19 on header P9 connected to SCL and pin 20 on P9 to SDA) because it is configured out of reset. 4.7kΩ pullup resistors should be connected between SCL and Vcc and between SDA and Vcc.

Miniproject2.JPG

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:

Address Name Access
00 Configuration Register A Read/Write
01 Configuration Register B Read/Write
02 Mode Register Read/Write
03 Data Output X MSB Register Read
04 Data Output X LSB Register Read
05 Data Output Z MSB Register Read
06 Data Output Z LSB Register Read
07 Data Output Y MSB Register Read
08 Data Output Y LSB Register Read
09 Status Register Read
10 Identification register A Read
11 Identification register B Read
12 Identification register C Read

The sensor values for each axis are 16-bit and are stored across 2 registers each assuming the devise is connected to I2C bus 3.

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 (register 02) 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 where measurements are taken periodically at a sampling rate set by configuration register A (register 00),

beagle$ i2cset -y 3 0x1e 2 1

writes a 1 to register 02 and this 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.