HMR3300-D00-232 Datasheet, PDF (5/11 Pages) Honeywell Accelerometers – Digital Compass Solutions

HMR3200/3300
Other nasty magnetic environments are man-made AC and DC magnetic fields created from nearby motors and high
current conductors. These fields should also require compass or source relocation when possible. In some cases, ferrous
metal shielding may help if the shield material is thin and far enough away from the compass.
Hard-iron distortion can be calibrated out, and is composed of soft-irons that are also magnetized and create remnant
(stray) magnetic flux. Classic hard-iron distortion typically comes from large vehicle chassis components and engine
blocks that have up to ±2 gauss on the parts. Locating the compass away from hard and soft-irons is the first line of
defense to preserve accuracy, and the calibration routine will null out the remaining hard-iron influences.
Calibration Procedure
For the HMR3200, one complete turn in a level plane is the best way to expose the sensors to all headings to compute
the calibration offsets. Since the compass collects data at a 15 samples per second rate, a sample per degree of rotation
or slower is a good guideline. If slow turns are not possible, multiple faster turns are a good substitute. The goodness of
the calibration or the amount of hard-iron present is found by checking the Xof, Yof, and Zof values after the calibration
routine is complete. In known clean magnetic environments, the horizontal values (XY = level, YZ = upright) should be
±200 ADC counts or less in these offset variables. Sending these Xof, Yof, and Zof values back to zero returns the
compass to the factory calibration state. The vertical axis values can be zero set or ignored for the HMR3200.
For the HMR3300, the above described level turns will calibrate the XY axis’, but the Z-axis must also be calibrated as
well One full rotation with as much pitch and roll variation included as application allows. If only mild pitch and roll
variations are possible, complete the level rotations, exit the calibration routine, and force the Zof value to zero. Some
accuracy maybe lost in this zeroing, but the mild tilt would likely never cause serious tilt-compensation heading error.
UART COMMUNICATION PROTOCOL
HMR3200/HMR3300 modules communicate through ASCII characters with the * or # characters as start bytes. The data
bit format is 1 Start, 8 Data, 1 Stop, and No parity bits. Factory baud rate is set to 19,200. Asynchronous communication
has the complete menu of commands. Synchronous communication is limited to direct heading queries and no other
commands.
POWER-ON/RESET
The compasses require a hard power-on transition on the power supply voltage to serve as an internal hardware reset
and clock-start. Some bench power supplies may create a soft-start condition and the HMR3200/HMR3300 my react with
a constant-on LED illumination if not reset suddenly. An in-line power supply switch (mechanical or electrical) may be
required when prototyping to avoid soft-starts.
Upon application of power or after a Reset Command, the HMR3200/HMR3300 will run about an 800 milli-second
initialization routine to set the onboard hardware and grab EEPROM variables and shadow them in controller RAM
locations for operation. The LED will illuminate during the routine and extinguish upon completion.
INITIAL STATUS OUTPUT
The HMR3200/HMR3300 will begin sending ASCII characters immediately after the initialization routine ends and the LED
extinguishes. The first line of text will be the model number of the compass and the internal firmware revision number. For
the HMR3300, a second response string will be sent, starting with a # character and either the N, W or A characters. The
#N response indicates normal operation and is the always expected response from the HMR3200. The #W and #A
responses are only for the HMR3300, and indicate the low temperature warning and alarm environments had been
encountered. These responses will be reset to normal when the user sends the pitch and roll re-zero commands to re-
calibrate the MEMS accelerometer for best tilt-compensation performance and accurate tilt indications.
After initialization, the compasses automatically begin streaming heading or magnetometer output data at 15Hz
(HMR3200) or 8Hz (HMR3300). Users must send a start/stop command (*S) to exit continuous streaming data, and to get
the controller’s full attention to the next commands.
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