FXLS8471Q Datasheet, PDF (22/75 Pages) Freescale Semiconductor, Inc – Linear Accelerometer

8
Embedded Functionality
FXLS8471Q is a low-power, digital output 3-axis acceleration sensor with both I²C and SPI interface options. Extensive
embedded functionality is provided to detect inertial events at low power, with the ability to notify the host processor via either of
the two programmable interrupt pins. The embedded functionality includes:
• 8-bit or 14-bit accelerometer data with an option for high-pass filtered output data
• Four different oversampling options for the output data. The oversampling settings allow the end user to optimize the
resolution versus power consumption 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 and landscape orientations
• Accelerometer vector-magnitude change event detection with programmable reference, threshold, and debounce time
values
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.
8.1 Factory calibration
FXLS8471Q is factory calibrated for sensitivity and offset on each axis. The trim values are stored in Non-Volatile Memory (NVM).
On startup, 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 offsets through the OFF_X/Y/Z registers. For more information on
accelerometer calibration, refer to Freescale application note AN4069.
8.2 8-bit or 14-bit 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 2 g, the measurement range is -2 g to +1.999 g, and each count corresponds to 0.244 mg at
±14-bits resolution. When the full-scale is set to 8 g, the measurement range is -8 g to +7.996 g, 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 (CTRL_REG1[f_read] = 1). For further
information on the different data formats and modes, please refer to Freescale application note AN4076.
8.3 Low-power modes versus high-resolution modes
FXLS8471Q can be optimized for lower power or 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 fixed at ±4 g 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 be activated when CTRL_REG2[mods] = 2’b10 which will improve the resolution of the output
data without affecting the internal embedded functions or fixing 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] = 2’b10, 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.
FXLS8471Q
22
Sensors
Freescale Semiconductor, Inc.