The SSP1492 (Product Brief)can interface with the sensors and perform the high level math functions necessary to create a tilt compensated Compass application. The internal 8051 microprocessor engine along with sensor acquisition and math function built into the SSP1492's internal ROM does most of the work.

It will be shown in this example that an SSP1492 can interface directly to and provide all signal processing needs to derive a calibrated angular displacement signal from a Honeywell HMC1512 angular displacement sensor using a minimal number of external components.

Magneto resistive (MR) Sensors are resistive devices that change resistance depending on the orientation of an external magnetic field. The resistance of an MR device is highest when a magnetic field is aligned in parallel with its established "easy" axis, which is set at the time of manufacturing and is lowest when the magnetic field is perpendicular to its "easy" axis. This creates a resistance profile that peaks and dips twice per rotation. The MR sensors resistive response to the angular moment of a magnetic field is:



MR sensors are used for a host of application including angular displacement sensing. In this application brief, the SSP1492 is used to measure the angular position of an external magnetic rotating above a Honeywell HMC1512 MR sensor. The SSP1492's frequency mode data converter can directly measure the MR sensor's resistance by connecting the sensor as the R element in SSP1492's RC oscillator -called the sensor oscillator. The HMC1512 contains 8 MR sensors configured in two Wheatstone bridges that are physically rotated 45° from one another but because of the MR sensor's cos²0 magnetic field angular response this creates two signals that are 90° out of phase from one another. These two signals in quadrature can be decoded into a single angular unit using a four-quadrant arctangent function or ATAN2. Before the quadrature signals can be processed by an arctangent function they must be normalized to one another. This requires a calibration step to gather the MR sensors minimum and maximum resistances of both axis to compute a compensating gain and offset that will normalize the two. Figure 1 below graphically summarizes typical signals discussed. In this Figure, the two MR sensor bridge output signals are named Sine and Cosine.

The SSP1492 can perform all the features mentioned above to provide a single calibrated angular output value. As shown in Sensor Platforms Application Note 001, the SSP1492's network of analog switches and built-in math functions can algebraically determine the deflection value of a resistive (or capacitive) bridge with the measurement of each individual element within the bridge. Furthermore, the two bridge outputs can be normalized using the SSP1492's built-in linear fit function which uses coefficients stored in an internal EEPROM. The user can upload the coefficients into the EEPROM at time of calibration. The SSP1492 also has an ATAN2 function to compute the angular displacement of the magnetic field. The SSP1492 supports and performs most of it's math functions in 32-bit IEEE floating point format. The user may wish to add final offset and scaling to the output to suit their needs by using the SSP1492's floating point math functions. The output can also be converted to integer format after all necessary processing is complete.

All SSP1492 signal processing functions and hardware peripherals are accessible as commands that are sent to the SSP1492 through either the I2C or SPI interface. Alternatively, a small program to orchestrate the process can be stored in an inexpensive serial EEPROM (as small as 1K-byte) and downloaded automatically as a macro program. is a schematic of MR sensor solution using the Honeywell HMC1512. This implementation includes the EEPROM for macro storage, which means the host need only issue a start measurement command and then retrieve the resulting angular position when the conversion is complete. Code used in the macro program for this application example is available upon request.

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Create an entire smart sensor solution within hours with only a sensor and as few as 2 external passive components. Internal ROM already contains ready-to-use signal processing functions. No programming is required to operate system as a slave from I2C or SPI interface. Click here to learn more or for an Evaluation Kit!