MMA8452Q
Sensors
12
Freescale Semiconductor, Inc.
5.1
Device Calibration
The device interface is factory calibrated for sensitivity and Zero-g offset for 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 circuitry. In normal use, further
calibration in the end application is not necessary. However, the MMA8452Q allows the user to adjust the Zero-g offset for each
axis after power-up, changing the default offset values. The user offset adjustments are stored in 6 volatile registers. For more
information on device calibration, refer to Freescale application note, AN4069
.
5.2
8-bit or 12-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 12-bit numbers. The most significant 8-bits of each axis are stored in OUT_X (Y,
Z)_MSB, so applications needing only 8-bit results can use these 3 registers and ignore OUT_X,Y, Z_LSB. To do this, the
F_READ bit in CTRL_REG1 must be set. When the F_READ bit is cleared, the fast read mode is disabled.
When the full-scale is set to 2g, the measurement range is -2g to +1.999g, and each count corresponds to 1g/1024
(1 mg) at 12-bits resolution. When the full-scale is set to 8g, the measurement range is -8g to +7.996g, and each count
corresponds to 1g/256 (3.9 mg) at 12-bits resolution. The resolution is reduced by a factor of 16 if only the 8-bit results are used.
For more information on the data manipulation between data formats and modes, refer to Freescale application note, AN4076.
There is a device driver available that can be used with the Sensor Toolbox demo board (LFSTBEB8451, 2, 3Q).
5.3
Low-Power Modes vs. High-Resolution Modes
The MMA8452Q can be optimized for lower power modes or for higher resolution of the output data. High resolution is
achieved by setting the LNOISE bit in Register 0x2A. This improves the resolution but be aware that the dynamic range is limited
to 4g when this bit is set. This will affect all internal functions and reduce noise. Another method for improving the resolution of
the data is by oversampling. One of the oversampling schemes of the data can activated when MODS = 10 in Register 0x2B
which will improve the resolution of the output data only. The highest resolution is achieved at 1.56 Hz.
There is a trade-off between low power and high resolution. Low Power can be achieved when the oversampling rate is
reduced. The lowest power is achieved when MODS = 11 or when the sample rate is set to 1.56 Hz. For more information on
how to configure the MMA8452Q in Low-Power mode or High-Resolution mode and to realize the benefits, refer to Freescale
application note, AN4075.
5.4
Auto-WAKE/SLEEP Mode
The MMA8452Q can be configured to transition between sample rates (with their respective current consumption) based on
four of the interrupt functions of the device. The advantage of using the Auto-WAKE/SLEEP is that the system can automatically
transition to a higher sample rate (higher current consumption) when needed but spends the majority of the time in the SLEEP
mode (lower current) when the device does not require higher sampling rates. Auto-WAKE refers to the device being triggered by
one of the interrupt functions to transition to a higher sample rate. This may also interrupt the processor to transition from a SLEEP
mode to a higher power mode.
SLEEP mode occurs after the accelerometer has not detected an interrupt for longer than the user definable time-out period.
The device will transition to the specified lower sample rate. It may also alert the processor to go into a lower power mode to save
on current during this period of inactivity.
The Interrupts that can WAKE the device from SLEEP are the following: Pulse Detection, Orientation Detection, Motion/Freefall,
and Transient Detection. Refer to AN4074, for more detailed information for configuring the Auto-WAKE/SLEEP.
5.5
Freefall and Motion Detection
MMA8452Q has flexible interrupt architecture for detecting either a Freefall or a Motion. Freefall can be enabled where the set
threshold must be less than the configured threshold, or motion can be enabled where the set threshold must be greater than
the threshold. The motion configuration has the option of enabling or disabling a high-pass filter to eliminate tilt data (static offset).
The freefall does not use the high-pass filter. For details on the Freefall and Motion detection with specific application examples
and recommended configuration settings, refer to Freescale application note, AN4070.
5.5.1
Freefall Detection
The detection of “Freefall” involves the monitoring of the X, Y, and Z axes for the condition where the acceleration magnitude
is
below
a user specified threshold for a user definable amount of time. Normally, the usable threshold ranges are between
±100 mg and ±500 mg.
