AIS2DW12 datasheet(21/60 Pages) STMICROELECTRONICS

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AIS2DW12

Terminology and functionality

With this feature the system may be efficiently switched from 12.5 Hz to full performance

depending on user-selectable positioning and acceleration events, thus ensuring power

saving and flexibility.

The stationary/motion detection function only recognizes the device’s sleep state.

When the stationary/motion detection function is activated by setting the STATIONARY bit in

WAKE_UP_DUR (35h), the AIS2DW12 detects acceleration below a fixed threshold but

does not change either ODR or operating mode after sleep state detection.

The Activity/Inactivity recognition and stationary/motion detection functions are activated by

writing the desired threshold in the WAKE_UP_THS (34h) register. The high-pass filter is

automatically enabled.

If the device is in sleep (inactivity/stationary) mode, when at least one of the axes exceeds

the threshold in WAKE_UP_THS (34h), the device goes into a sleep-to-wake state (as

wake-up).

For the activity/inactivity function, the device, in a wake-up state, will return to the operating

mode and ODR before sleep state detection.

Activity/Inactivity, stationary/motion detection threshold and duration can be configured in

the following control registers:

WAKE_UP_THS (34h)

WAKE_UP_DUR (35h)

3.2.5

Offset management

The user can manage offset in the output or for wakeup detection using dedicated

embedded hardware (see Section 5.1: Block diagram of filters).

3.3

Sensing element

A proprietary process is used to create a surface micromachined accelerometer. The

technology allows processing suspended silicon structures which are attached to the

substrate in a few points called anchors and are free to move in the direction of the sensed

acceleration. In order to be compatible with the traditional packaging techniques, a cap is

placed on top of the sensing element to avoid blocking the moving parts during the molding

phase of the plastic encapsulation. When an acceleration is applied to the sensor the proof

mass displaces from its nominal position, causing an imbalance in the capacitive half-

bridge. This imbalance is measured using charge integration in response to a voltage pulse

applied to the capacitor.

At steady-state the nominal value of the capacitors are a few pF and when an acceleration

is applied, the maximum variation of the capacitive load is in the fF range.

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