AIS2IH datasheet(16/63 Pages) STMICROELECTRONICS

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3.2.4

Activity/Inactivity, stationary/motion detection functions

The activity/inactivity function recognizes the device’s sleep state and allows reducing system power

consumption.

When the activity/inactivity function is activated by setting the SLEEP_ON bit in WAKE_UP_THS (34h), the

AIS2IH automatically goes to 12.5 Hz ODR in the low-power mode previously selected by the LP_MODE[1:0] bits

in CTRL1 (20h) if the sleep state condition is detected and wakes up as soon as the interrupt event has been

detected, increasing the output data rate and bandwidth.

With this feature the system may be efficiently switched from low-power mode 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 AIS2IH detects acceleration below a fixed threshold but does not change either ODR or operating

mode (High-Performance mode or Low-Power 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 (HP or LP) 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

High tap/double-tap user configurability

The device embeds the possibility to select the following parameters:

•

single axis or multiple axes in TAP_THS_Z (32h)

•

axis priority in TAP_THS_Y (31h)

•

threshold value of each axis in TAP_THS_X (30h), TAP_THS_Y (31h), TAP_THS_Z (32h)

•

max time threshold between 2 consecutive taps for double-tap recognition, min time threshold between 2

consecutive taps to detect a new tap event in INT_DUR (33h)

3.2.6

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.

3.4

IC interface

The complete measurement chain is composed of a low-noise capacitive amplifier which converts the capacitive

unbalancing of the MEMS sensor into an analog voltage using an analog-to-digital converter.

The acceleration data may be accessed through an I²C/SPI interface thus making the device particularly suitable

for direct interfacing with a microcontroller.

AIS2IH

Sensing element

DS12421 - Rev 4

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