MMC212xMG Dual-axis Magnetic Sensor from MEMSIC used in HDMM01 breakout from Pollin

The MMC212xMG/HDMM01 is a dual-axis magnetic sensor with I2C interface. It can measure magnetic field with a full range of ±5 gausses and a sensitivity of 512counts/gauss @3.0 V at 25°C. Use the MMC212xMG (About Modules) module for it.

You can wire this up as follows:

Note: Connect I2C pull up resistors on SCL and SDA to 3V3. There are no resistors on the HDMM01 breakout. Rp selection guide: 4.7Kohm for a short I²C bus length (less than 10cm), and 10Kohm for less than 5cm I²C bus.

Restore Sensor Characteristics

The influence of a strong magnetic field (more than 5.5 gausses) could change the sensor characteristics. It is possible to restore the original sensor characteristics with a strong restoring magnetic field which can be generated internally by the sensor. The following command will do this. It's not necessary to execute this command on every reboot but should be done occasionally.

I2C3.setup( { scl: A8, sda: B4 } );
var mmc = require("MMC212x").connect( I2C3 );
mmc.restoreSensorCharacteristic();

Calibration

The sensor is not calibrated and must be calibrated for the first time and later from time to time.

You have to rotate the sensor horizontally during the following endless loop and try to find the minimal and maximal values of x and y.

I2C3.setup( { scl: A8, sda: B4 } );
var mmc = require("MMC212x").connect( I2C3 );

var xMax, xMin, yMax, yMin;
function startMeasuring() {
  mmc.getMagneticValue( function(x,y) {

    xMin = Math.min( xMin || x, x );
    xMax = Math.max( xMax || x, x );
    yMin = Math.min( yMin || y, y );
    yMax = Math.max( yMax || y, y );

    print( "mmc.calibrate(" + xMin + ", " + xMax + ", " + yMin + ", " + yMax + " ); // x=" + x + " y=" + y );

    startMeasuring();
  } );
}

startMeasuring();

After calling the dumped command the sensor is calibrated. The sensor does not remember the min and max values. So you have to call the method calibrate() every time on program startup:

Now you should place the last output to , e.g. mmc.calibrate(1925, 2151, 1919, 2147 );

I2C3.setup( { scl: A8, sda: B4 } );
var mmc = require("MMC212x").connect( I2C3 );
mmc.calibrate(1925, 2151, 1919, 2147 ); // call on every startup

Read raw magnetic sensor values

The uncalibrated magnetic values of the x- and y-axis can be read asynchronously.

I2C3.setup( { scl: A8, sda: B4 } );
var mmc = require("MMC212x").connect( I2C3 );

mmc.getMagneticValue( function(x,y) {
  console.log( "x = " + x + "    y = " + y );
} );

Calculate the angle between magnetic north and the +y-axis of the sensor

On Page 4 of the datasheet is a picture with the sensing directions +x and +y. The module can calculate from the raw sensor values (x,y) the angle between the magnetic north and the +y-axis of the sensor with the following convention:

• The angle is increasing in clockwise direction.
• +y-axis pointing to north → 0°
• +y-axis pointing to east → 90°
• +y-axis pointing to south → 180°
• +y-axis pointing to west → 270°

I2C3.setup( { scl: A8, sda: B4 } );
var mmc = require("MMC212x").connect( I2C3 );
mmc.calibrate(1925, 2151, 1919, 2147 ); // values from calibration

mmc.getMagneticValue( function(x,y) {
  console.log( " angle = " + mmc.getAngle(x,y));
} );

Types with different I²C-addresses

There are four types which only differ by their I²C addresses:

I2C3.setup( { scl: A8, sda: B4 } );

var mmc = require("MMC212x").connect( I2C3 ); // MMC2120MG, default, use address 0x30
var mmc = require("MMC212x").connect( I2C3, 0x30 ); // MMC2120MG
var mmc = require("MMC212x").connect( I2C3, 0x32 ); // MMC2121MG
var mmc = require("MMC212x").connect( I2C3, 0x34 ); // MMC2122MG
var mmc = require("MMC212x").connect( I2C3, 0x36 ); // MMC2123MG

Buying

• Pollin

Links

• Datasheet from Memsic
• Datasheet from Pollin
• Breakout HDMM01 (in German)

References

/* Start a new measuring
 * When the measured value is available the callback function will be called
 * with the measured result.
 * @param callback The callback
 */
MMC212x.prototype.getMagneticValue = function (callback) { ... }

/* Calculate the angle between north and the current sensor direction (+y-axis).
 * The angle is increasing in clockwise direction.
 *  +y-axis pointing to north →   0°
 *  +y-axis pointing to east  →  90°
 *  +y-axis pointing to south → 180°
 *  +y-axis pointing to west  → 270°
 * @param x Sensor value of the x-axis - get from getMagneticValue(..)
 * @param y Sensor value of the y-axis - get from getMagneticValue(..)
 * @returns {number} Angle between magnetic north and the current sensor orientation
 */
MMC212x.prototype.getAngle = function (x, y) { ... }

/* After calling restoreSensorCharacteristic() you should calibrate the module.
 *
 * This method may be called only 10ms after reset.
 */
MMC212x.prototype.restoreSensorCharacteristic = function () { ... }

/* Calibrates internal ellipse parameter with the minimum and maximum of the x- and y-axis.
 *
 * Explaination:
 * If the sensor will be rotated the values of (x,y) will form an ellipse. These method
 * calculates the center of the ellipse and the scaling factor from x to y.
 *
 * @param xMin minimum value of the x-axis
 * @param xMax maximum value of the x-axis
 * @param yMin minimum value of the y-axis
 * @param yMax maximum value of the y-axis
 */
MMC212x.prototype.calibrate = function (xMin, xMax, yMin, yMax) { ... }

/* Creates a new MMC212x sensor instance
 * @constructor
 * @param i2c Instance of the I2C Class, e.g. I2C1
 * @param i2cAddress optional alternate I²C address, default 0x30
 */
exports.connect = function (i2c, i2cAddress) { ... }