HMR3300 Datasheet HMR3200, HMR3200-DEMO-232, HMR3300-D00-232 | P4-P6

HMR3200/3300

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pair of sensors for compassing (flat or upright). The HMR3300 uses all three magnetic sensors plus includes an

accelerometer to provide tilt (pitch and roll) sensing relative to the board’s horizontal (flat) position.

The HMR3200/HMR3300 circuit starts with Honeywell HMC1021Z and HMC1022 single and two-axis magnetic sensors

providing X, Y, and Z axis magnetic sensing of the earth’s field. The HMC1022 provides the horizontal components (X and

Y), and HMC1021Z provides the vertical (Z) axis component of the incident magnetic field into cartesian magnitudes at

orthognal angles. These sensors are supplied power by a constant current source to maintain best accuracy over

temperature. The sensor output voltages and constant current sensor supply voltage are provided to a multiplexed 16-bit

Analog to Digital Converter (ADC) integrated circuit. A microcontroller integrated circuit periodically queries the

multiplexed ADC and performs the offset corrections and computes the heading. This microcontroller also performs the

external serial data interface and other housekeeping functions such as the calibration routine. An onboard EEPROM

integrated circuit is employed to retain necessary data variables for best performance.

For the HMR3200, the three magnetic sensors (XYZ) are included and no accelerometer is present. The *L (level) and *U

(upright) are available for horizontal and vertical circuit board orientations respectively. At level, the XY sensors are used

to compute heading; and upright, the YZ sensors are used to compute heading.

For the HMR3300, an additional pair of data inputs from the ±2g accelerometer (Analog Devices ADXL213) is received by

the microcontroller. These tilt inputs (pitch and roll) are added to sensor data inputs to form a complete data set for a three

dimensional computation of heading. If the board is held horizontal, the pitch and roll angles are zero and the X and Y

sensor inputs dominate the heading equation. When tilted, the Z magnetic sensor plus the accelerometer’s pitch and roll

angles enter into heading computation.

The power supply for the HMR3200/HMR3300 circuit is regulated +5 volt design allowing the user to directly provide the

regulated supply voltage or a +6 to +15 volt unregulated supply voltage. If the unregulated supply voltage is provided,

then the linear voltage regulator integrated circuit drops the excess supply voltage to a stable +5 volts. The power supply

is a dual ground (analog and digital) system to control internal noise and maximize measurment accuracy.

ELECTRICAL INTERFACE PINOUT

Pin Number Pin Name Description

1 SCK Synchronous Data Clock (Pulled high in UART mode and left open)

2 RX Receive Data, 5V CMOS Input

3 TX Transmit Data, 5V CMOS Output

4 CS Chip Select (Pulled high in UART mode and left open)

5 Cal Calibration Input (No connection normally, consult for details)

6 +5 +5 Volt Regulated Power Input (SW1 must be fully CCW)

7 GD Logic and Power Return (Ground)

8 V+ Unregulated Power Input (+6 to +15 volts, factory default, SW1 must be CW)

APPLICATION NOTES

When To Calibrate

The HMR3200/HMR3300 comes with an optional user hard-iron calibration routine to null modest intensity hard-iron

distortion. For many users in cleaner magnetic environments, the factory calibration will be better and yield more accurate

readings than after a user calibration.

The calibration routine is not cure-all for nasty magnetic environments. If a needle compass is thrown off from true

readings, then it is very likely the HMR3200 and HMR3300 will have poor accuracy too. Most compass error sources

come from ferrous metals (steel, iron, nickel, cobalt, etc.) located too close to the compass location and are known as

soft-irons creating soft-iron distortion. Soft-iron distortion will change the intensity and direction of the magnetic fields on

any nearby compass, and the calibration routine can not remove these flux concentration and bending errors. A good rule

of thumb is to keep soft-irons at least two largest dimensions away from the compass. For example, a half-inch stainless

steel panhead bolt should be at least an inch away from the HMC1021Z and HMC1022 sensor locations.

HMR3200/3300

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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.

HMR3200/3300

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OPERATIONAL COMMANDS

Syntax: *X<cr><lf> Sends command for an operational mode change. The * prefix indicates command type.

A #I response indicates an invalid command was sent.

Heading Output Command

*H<cr><lf>

Selects the Heading output mode (factory set default). This configuration is saved in non-volatile memory. All data

are in decimal degrees.

Response Format: Heading<cr><lf> (HMR3200 only and 15 times per second)

Eg: 235.6<cr><lf>

Response Format: Heading, Pitch, Roll<cr><lf> (HMR3300 only and 8 times per second)

Eg: 123.4, 18.6, -0.5<cr><lf>

Magnetometer Output Command

*M<cr><lf>

Selects the magnetometer output mode. This configuration is saved in non-volatile memory. All data are in signed

decimal values with user calibration offsets included.

Response Format: MagX, MagY, MagZ<cr><lf>

Eg: 1256, -234, -1894<cr><lf>

Compass Orientation (HMR3200 only)

*L<cr><lf>

Heading calculation is done assuming the compass is level (XY).

*U<cr><lf>

Heading calculation is done assuming the compass is upright (connector end down) (YZ).

These orientation commands are saved in non-volatile memory.

Starting and Stopping Data Output

*S<cr><lf>

The data output will toggle between Start and Stop each time this command is issued (factory set default is Start,

first Start/Stop command will stop data output). Continuous data streaming control. Most commands require the

compass to be in a stop condition for the controller to immediately execute the desired command.

Query Output

*Q<cr><lf>

Query for a single output response string in the currently selected mode (Magnetometer/Heading). The *Q

commands are allowed only in Stop data mode. Query commands allow the user to slow the data flow by

requesting each response string.

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