Data Sheet

Sensors

Freescale Semiconductor 11

MMA8452Q

5Functionality

The MMA8452Q is a low-power, digital output 3-axis linear accelerometer with a I

C interface and embedded logic used to

detect events and notify an external microprocessor over interrupt lines. The functionality includes the following:

• 8-bit or 12-bit data which includes High Pass Filtered data

• 4 different oversampling options for compromising between resolution and current consumption based on application

requirements

• Additional Low Noise mode that functions independently of the Oversampling modes for higher resolution

• Low Power and Auto-WAKE/SLEEP for conservation of current consumption

• Single/Double tap with directional information 1 channel

• Motion detection with directional information or Freefall 1 channel

• Transient/Jolt detection based on a high pass filter and settable threshold for detecting the change in acceleration above

a threshold with directional information 1 channel

• Portrait/Landscape detection with trip points fixed at 30° and 60° for smooth transitions between orientations.

All functionality is available in 2g, 4g or 8g dynamic ranges. There are many configuration settings for enabling all the different

functions. Separate application notes have been provided to help configure the device for each embedded functionality.

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.