FXOS8700CQ datasheet(19/86 Pages) FREESCALE

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Freescale Semiconductor, Inc.

Embedded Functionality

FXOS8700CQ is a low-power, digital output 6-axis sensor with both I

2C and SPI interfaces. Extensive embedded functionality is

provided to detect inertial and magnetic events at low-power, with the ability to notify the host processor of an event via either of

the two programmable interrupt pins. The embedded functionality includes:

•

8-bit or 14-bit accelerometer data which includes high-pass filtered data, and 8 or 16-bit magnetometer data

•

Four different oversampling options for the accelerometer output data, and eight for the magnetometer. The oversampling

settings allow the end user to optimize the resolution versus power trade-off in a given application.

•

A low-noise accelerometer mode that functions independently of the oversampling modes for even higher resolution

•

Low-power auto-wake/sleep function for conserving power in portable battery powered applications

•

Accelerometer pulse detection circuit which can be used to detect directional single and double taps

•

Accelerometer directional motion and freefall event detection with programmable threshold and debounce time

•

Acceleration transient detection with programmable threshold and debounce time. Transient detection can employ either

a high-pass filter or use the difference between reference and current sample values.

•

Orientation detection with programmable hysteresis for smooth transitions between portrait/landscape orientations

•

Accelerometer vector magnitude change event detection with programmable reference, threshold, and debounce time

values

•

Magnetic threshold event detection with programmable reference, threshold, and debounce time

•

Magnetometer vector magnitude change event detection with programmable reference, threshold and debounce time

values

•

Magnetic min/max detection circuit which can also be used for autonomous calibration of magnetic hard-iron offset

Many different configurations of the above functions are possible to suit the needs of the end application. Separate application

notes are available to further explain the different configuration settings and potential use cases.

7.1

Factory calibration

FXOS8700CQ's integrated accelerometer and magnetometer sensors are factory calibrated for sensitivity and offset on each

axis. The trim values are stored in Non-Volatile Memory (NVM). On power-up, the trim parameters are read from NVM and

applied to the internal compensation circuitry. After mounting the device to the PCB, the user may further adjust the

accelerometer and magnetometer offsets through the OFF_X/Y/Z and M_OFF_X/Y/Z registers, respectively. For more

information on device calibration, refer to Freescale application note, AN4069.

7.2

8-bit or 14-bit accelerometer data

The measured acceleration data is stored in the OUT_X_MSB, OUT_X_LSB, OUT_Y_MSB, OUT_Y_LSB, OUT_Z_MSB, and

OUT_Z_LSB registers as 2’s complement 14-bit numbers. The most significant 8-bits of each axis are stored in the OUT_X, Y,

Z_MSB registers, so applications needing only 8-bit results simply read these three registers and ignore the OUT_X,Y, Z_LSB

registers. To do this, the f_read mode bit in CTRL_REG1 must be set.

When the full-scale range is set to 2g, the measurement range is -2g to +1.999g, and each count corresponds to 0.244 mg at

±14-bits resolution. When the full-scale is set to 8g, the measurement range is -8g to +7.996g, and each count corresponds to

0.976 mg. The resolution is reduced by a factor of 64 if only the 8-bit results are used ([f_read] = 1). For further information on

the different data formats and modes, please refer to Freescale application note AN4076.

7.3

Accelerometer low-power modes versus high-resolution modes

FXOS8700CQ can be optimized for lower power or for higher resolution of the accelerometer output data. High resolution is

achieved by setting the lnoise bit in register 0x2A. This improves the resolution (by lowering the noise), but be aware that the

dynamic range becomes limited to ±4g when this bit is set. This will affect all internal embedded functions (scaling of thresholds,

etc.) and reduce noise. Another method for improving the resolution of the data is through oversampling. One of the oversampling

schemes of the output data can activated when CTRL_REG2[mods] = 0b10 which will improve the resolution of the output data

without affecting other internal embedded functions or limiting the dynamic range.

There is a trade-off between low power and high resolution. Low power can be achieved when the oversampling rate is reduced.

When CTRL_REG2[mods] = 0b10, the lowest power is achieved, at the expense of higher noise. In general, the lower the

selected ODR and OSR, the lower the power consumption. For more information on how to configure the device in low-power or

high-resolution modes and understand the benefits and trade-offs, please refer to Freescale application note AN4075.

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