OPERATIONAL EXAMPLES
The HMC5883L has a fairly quick stabilization time from no voltage to stable and ready for data
 retrieval. The nominal 56 milli-seconds with the factory default single measurement mode means
 that the six bytes of magnetic data registers (DXRA, DXRB, DZRA, DZRB, DYRA, and DYRB) are
 filled with a valid first measurement.
To change the measurement mode to continuous measurement mode, after the 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 15Hz updates,
 a 67 milli-second typical delay should be allowed by the I2C master before querying the HMC5883L 
data registers for new measurements. To clock out the new data, send:
0x3D, and clock out DXRA, DXRB, DZRA, DZRB, DYRA, and DYRB located in registers 3 through 8. 
The HMC5883L will automatically re-point back to register 3 for the next 0x3D query. 
All six data registers must be read properly before new data can be placed in any of these data 
registers.
Below is an example of a (power-on) initialization process for “continuous-measurement mode”:
1. Write CRA (00) – send 0x3C 0x00 0x70 (8-average, 15 Hz default, normal measurement)
2. Write CRB (01) – send 0x3C 0x01 0xA0 (Gain=5, or any other desired gain)
3. Write Mode (02) – send 0x3C 0x02 0x00 (Continuous-measurement mode)
4. Wait 6 ms or monitor status register or DRDY hardware interrupt pin
5. Loop
Send 0x3D 0x06 (Read all 6 bytes. If gain is changed then this data set is using previous gain)
Convert three 16-bit 2’s compliment hex values to decimal values and assign to X, Z, Y, respectively.
Send 0x3C 0x03 (point to first data register 03)
Wait about 67 ms (if 15 Hz rate) or monitor status register or DRDY hardware interrupt pin
End_loop
Below is an example of a (power-on) initialization process for “single-measurement mode”:
1. Write CRA (00) – send 0x3C 0x00 0x70 (8-average, 15 Hz default or any other rate, normal measurement)
2. Write CRB (01) – send 0x3C 0x01 0xA0 (Gain=5, or any other desired gain)
3. For each measurement query:
Write Mode (02) – send 0x3C 0x02 0x01 (Single-measurement mode)
Wait 6 ms or monitor status register or DRDY hardware interrupt pin
Send 0x3D 0x06 (Read all 6 bytes. If gain is changed then this data set is using previous gain)
Convert three 16-bit 2’s compliment hex values to decimal values and assign to X, Z, Y, respectively.
HMC5883L
www.honeywell.com 19
SELF TEST OPERATION
To check the HMC5883L 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 self test, the least significant bits (MS1 and MS0) of configuration register A are changed
 from 00 to 01 (positive bias) or 10 (negetive bias).
Then, by placing the mode register into single or continuous-measurement mode, 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 (about 10 mA) in the positive bias mode for X, Y, and Z axes to create about a 1.1 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.
Since self test adds ~1.1 Gauss additional field to the existing field strength, using a reduced gain 
setting prevents sensor from being saturated and data registers overflowed. For example, if the 
configuration register B is set to 0xA0 (Gain=5), values around +452 LSb (1.16 Ga * 390 LSb/Ga) 
will be placed in the X and Y data output registers and around +421 (1.08 Ga * 390 LSb/Ga) will be
 placed in Z data output register. To leave the self test mode, change MS1 and MS0 bit of the 
configuration register A back to 00 (Normal Measurement Mode). Acceptable limits of the self test 
values depend on the gain setting. Limits for Gain=5 is provided in the specification table.
Below is an example of a “positive self test” process using continuous-measurement mode:
1. Write CRA (00) – send 0x3C 0x00 0x71 (8-average, 15 Hz default, positive self test measurement)
2. Write CRB (01) – send 0x3C 0x01 0xA0 (Gain=5)
3. Write Mode (02) – send 0x3C 0x02 0x00 (Continuous-measurement mode)
4. Wait 6 ms or monitor status register or DRDY hardware interrupt pin
5. Loop
Send 0x3D 0x06 (Read all 6 bytes. If gain is changed then this data set is using previous gain)
Convert three 16-bit 2’s compliment hex values to decimal values and assign to X, Z, Y, respectively.
Send 0x3C 0x03 (point to first data register 03)
Wait about 67 ms (if 15 Hz rate) or monitor status register or DRDY hardware interrupt pin
End_loop
6. Check limits –
If all 3 axes (X, Y, and Z) are within reasonable limits (243 to 575 for Gain=5,
 adjust these limits basing on the gain setting used. See an example below.) Then
All 3 axes pass positive self test
Write CRA (00) – send 0x3C 0x00 0x70 (Exit self test mode and this procedure)
Else
If Gain<7
Write CRB (01) – send 0x3C 0x01 0x_0 (Increase gain setting and retry, skip the next data set)
Else
At least one axis did not pass positive self test
Write CRA (00) – send 0x3C 0x00 0x70 (Exit self test mode and this procedure)
End If
Below is an example of how to adjust the “positive self” test limits basing on the gain setting:
1. If Gain = 6, self test limits are:
Low Limit = 243 * 330/390 = 206
High Limit = 575 * 330/390 = 487
2. If Gain = 7, self test limits are:
Low Limit = 243 * 230/390 = 143
High Limit = 575 * 230/390 = 339