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) with this

application note.

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. When MODS = 11 the lowest power is achieved. The lowest power is achieved 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: PulseDetection, 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.