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. The macro program stored in the serial EEPROM simply prepares the data for, and calls upon the internal ROM functions to do all the heavy lifting. Alternatively, the steps can be directed by the users host controller thus eliminating the need for the EEPROM. All ROM functions are accessible as commands sent to the serial interface from the host or are callable from the user’s macro program. Figure 1 is a schematic of a tilt-compensated compass application. This one includes the EEPROM for macro storage, which means the microcontroller only needs to issue a start measurement command and then retrieve the resulting heading, pitch and roll values directly in radians or degrees when the conversion is complete.

This implementation uses miniature inductive magnetometer sensors (PNI corporation). With a single resistor (RG), the SSP1492 sensor interface produces the DC bias currents necessary to operate these sensors in their optimal linear region.

The tri-axial accelerometer (Kionix KXP84) with a serial interface is easily interfaced to the SSP1492 through its General Purpose I/O (GPIO) and a driver written and stored on the macro program memory storage device – the external serial EEPROM. When instructed to execute a reading, the SSP1492 macro program, which is typically uploaded into the SSP1492 memory at power-up, gathers the 3 magnetic (B) and 3 gravity (G) magnitudes (for X,Y and Z). These create a 3-dimensional gravity and 3-dimensional magnetic vector.

Reviewing your geometry a bit, remember that two vectors create a plane and in this case the plane created from the G and B vectors is a plane that is always perpendicular to the ground projecting directly up and down, North and South. A cross product of two vectors will produce a unit vector that is perpendicular to the plane of those two vectors. By calculating the cross product of the B and G vectors will result in a truly horizontal East vector. Next, much like the projection of a flag pole on the ground from sun light, a dot product function is used to find the projections of this truly horizontal East vector on the actual magnetic vector to find the actual orientation of the entire unit. The SSP1492 contains these math functions, in fact the macro function, or host controller, only has to make one function call (send one command) to produce the tilt compensated heading and one more calls to produce the unit's pitch and roll orientation.

Calibration of the sensors is an important step to producing accurate results. The SSP1492 has this covered as well. Gain and offset compensation values for each sensor can be stored into the SSP1492’s own EEPROM space. The linear fit function stored in ROM can be performed on each sensor. This function uses compensation values stored at predetermined locations in the SSP1492’s EEPROM. The Linear Fit function is automatically called upon by another function used in this application that orchestrates the bulk of the compass acquisition duties called Autopilot. This Autopilot function prepares the data and setting and calls other sub functions including the sensor acquisition, linear fitting, tilt compensation vector math and pitch-roll calculation functions.

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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!