Digilent Pmod CMPS2 Reference Manual - Page 11

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Pmod CMPS2™ Reference Manual

67. Cycle the SCL line to receive the data bits from the X, Y, and Z registers in the SDA line, providing an ACK

between each data byte. The Pmod CMPS2 address pointer automatically moves to each consecutive

byte. End the communication by sending a NACK followed by a STOP command.

I2CReadMultiple(6); //read six bytes, sending an ACK to the slave device betwe

en each byte received and a NACK after the last byte

68. Each of the readings will contain the external magnetic field H in addition to offset associated with the

current put through the coil by the RESET action.

69. The offset associated with the device can be calculated and then subtracted from future measurements to

obtain the actual magnetic field.

The other method of calibration is to remove the hard iron bias introduced to the magnetometer by the

surrounding environment by measuring (either continually or for a set time) all of the possible magnetometer

readings in a Cartesian sphere and keep track of the highest and lowest readings for each axis. The offset can then

be calculated in the same fashion as described above with the SET and RESET actions. The advantage of this

method is when the Pmod CMPS2 is continually changing locations or when other external magnetic fields may be

introduced into the test environment.

2.4.2 Data Conversion

The Memsic MMC34160PJ provides data for each axis in units of Gauss, but the information is generally more

legible when presented as a compass heading. The method for converting Gauss units to a compass heading is

provided below.

Calculate the real Gauss value for the X and Y axes from the amount of LSBs returned where the LSB value

by default is 0.48828125 mG, resulting in 2048 LSBs per Gauss.

2. Calculate the direction D by first checking to see if the X Gauss data is equal to 0 to prevent divide by 0

zero errors in the future calculations. If the X Gauss data is 0, check to see if the Y Gauss data is less than

0. If Y is less than 0 Gauss, the direction D is 90 degrees; if Y is greater than or equal to 0 Gauss, the

direction D is 0 degrees.

3. If the X Gauss data is not zero, calculate the arctangent of the Y Gauss and X Gauss data and convert from

polar coordinates to degrees.

4. If the direction D is greater than 360 degrees, subtract 360 degrees from that value.

5. If the direction D is less than 0 degrees, add 360 degrees to that value.

6. The compass heading can then be determined by the direction value D:



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= ∗ 0.48828125

If D is greater than 337.25 degrees or less than 22.5 degrees – North

If D is between 292.5 degrees and 337.25 degrees – North-West

If D is between 247.5 degrees and 292.5 degrees – West

If D is between 202.5 degrees and 247.5 degrees – South-West

If D is between 157.5 degrees and 202.5 degrees – South

2 = − +

= 1 +

= arctan (

) ∗

180

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