HMC5843
To change the measurement mode to continuous measurement mode, after the 8.3 milli-second power-up time send the
three bytes:
0x3C 0x02 0x00
This writes the 00 into the second register or mode register to switch from single to continuous measurement mode
setting. With the data rate at the factory default of 10Hz updates, a 100 milli-second delay should be made by the I2C
master before querying the HMC5843 data registers for new measurements. To clock out the new data, send:
0x3D, and clock out DXRA, DXRB, DYRA, DYRB, DZRA, DZRB located in registers 3 through 8. The HMC5843 will
automatically re-point back to register 3 for the next 0x3D query, expected 100 milli-seconds or later. All six data registers
must be read properly before new data can be placed in any of these data registers.
SELF TEST OPERATION
To check the HMC5843 for proper operation, a self test feature in incorporated in which the sensor offset straps are
excited to create a nominal field strength (bias field) to be measured. To implement this self test, the least significant bits
(MS1 and MS0) of configuration register A are changed from 00 to 01.
Then, by placing the mode register into single-conversion mode (0x01), two data acquisition cycles will be made on each
magnetic vector. The first acquisition will be a set pulse followed shortly by measurement data of the external field. The
second acquisition will have the offset strap excited in the positive bias mode (~5.5mA in offset straps) to create about a
0.55 gauss self test field plus the external field. The first acquisition values will be subtracted from the second acquisition,
and the net measurement will be placed into the data output registers.
If the configuration register B is left at the factory default value of 0x20, values around +715 ADC counts (0.55Ga * 1300
counts/Ga) will be placed in the data output registers. To leave the self test mode, change MS1 and MS0 bit of the
configuration register A back to 0x00. Also change the mode register if single-conversion mode is not the intended next
mode of operation.
SCALE FACTOR CALIBRATION
Using the method described above in section SELF TEST OPERATION, user can scale sensor s’ sensitivity to match each
other. Since placing device in positive bias mode (or alternatively negative bias mode) applies the same artificial field on
all three axes, the resulting ADC measurement in data output registers can be used to scale the sensors.
Alternatively, the built-in self test can be used to periodically compensate the scaling errors due to temperature variations.
A compensation factor can be found by comparing the self test outputs with the ones obtained at a known temperature.
For example, if the self test output is 715 at room temperature and 730 at the current temperature then a scale factor of
(715/730) should be applied to all current magnetic readings. A temperature sensor is not required to use this method.
EXTERNAL CAPACITORS
The two external capacitors should be ceramic type construction with low ESR characteristics. The exact ESR values are
not critical but values less than 200 milli-ohms are recommended. Reservoir capacitor C1 is nominally 4.7 μF in
capacitance, with the set/reset capacitor C2 nominally 0.22 μF in capacitance. Low ESR characteristics may not be in
many small SMT ceramic capacitors (0402), so be prepared to up-size the capacitors to gain Low ESR characteristics.
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