Digital Accelerometer Packs Intelligence Into Tiny Package

Aug. 15, 2012
Freescale's tiny 2 mm by 2mm MMA8652FC Xtrinsic Accelerometer handles many sensing chores automatically. It uses a 32 entry FIFO with low and high pass filtering as well as additional data processing support to handle analysis without host intervention.

Freescale has been pushing higher integration with its 12-axis MEMS sensor included in the USB-based Windows 8 Xtrinsic Sensor (see Sensor Fusion Or Sensor Confusion?). That device has a 3D accelerometer, 3D magnetometer, and 3D gyro, plus an ambient light sensor and a barometer/thermometer and a micro to front end to a USB host. At the other end of the spectrum is the tiny Freescale MMA8652FC Xtrinsic Accelerometer (Fig. 1). This 2 mm by 2 mm chip has an I2C digital interface and it targets compact mobile applications that may not required the range of interfaces found in the higher end device.

Figure 1. Freescale MMA8652FC Xtrinsic Accelerometer includes logic to handle a range of analysis chores without host intervention.

The MMA865xFC comes in a 12- and a lower cost, 10-bit ADC version. The 12-bit device is rated at 1 mg/LSB. The device has less than 1 bit of noise or 150 micro g/root Hertz. Both operate from 1.5 to 800 Hz.

The chips have two configurable interrupts. This allows the host to select from a range of features from simple data ready to flagging special events. The devices also have advanced auto-wake and sleep functions with a debounce counter that allow detection multiple motions.

This array of embedded functions provides fixed function, microcontroller-like support that can minimize host support requirements. There is a 32 data point FIFO with timed-based watermark. The data can be run through a low or high pass filter. This allows features like freefall motion, transient, fast motion, and jolt detection. It can detect single, double and directional taps by itself. It handles shake detection through motion thresholding. There is enhanced orientation support with hysteresis and Z-lockout.

The MMA865xFC has a very low power mode that uses only 7 µA. There are separate supply voltages for the sensor and the host interface. It can automatically switch operating and sleep modes based on conditions and system settings. The settings can also switch from slow to fast recording allowing advanced data recording without host intervention.

Developers can check out the accelerometers using Freescale's MMA865xFC Sensor Toolbox (Fig. 2). It handles both the 10- and 12-bit sensors.

Figure 2. Freescale MMA865xFC Sensor Toolbox provides a way for developers to evaluate the accelerometer.

The tiny 2 mm by 2 mm DFN package supports visual solder joint inspection. It has leads that go up the side of the package and the solder flows up the leads changing the color when there is a good solder joint. This was derived from Freescale's automotive applications. Like the automotive components, the MMA865xFC falls under Freescale's long term support so expect it to be around awhile.

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