LIS3DH datasheet(16/42 Pages) STMICROELECTRONICS

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Terminology and functionality

LIS3DH

16/42

Doc ID 17530 Rev 1

3.2.2

Self-test

Self-test allows to check the sensor functionality without moving it. The self-test function is

off when the self-test bit (ST) is programmed to ‘0‘. When the self-test bit is programmed to

‘1‘ an actuation force is applied to the sensor, simulating a definite input acceleration. In this

case the sensor outputs exhibit a change in their DC levels which are related to the selected

full scale through the device sensitivity. When self-test is activated, the device output level is

given by the algebraic sum of the signals produced by the acceleration acting on the sensor

and by the electrostatic test-force. If the output signals change within the amplitude

specified inside Table 3, then the sensor is working properly and the parameters of the

interface chip are within the defined specifications.

3.2.3

6D / 4D orientation detection

The LIS3DH include 6D / 4D orientation detection.

6D / 4D orientation recognition: In this configuration the interrupt is generated when the

device is stable in a known direction. In 4D configuration Z axis position detection is disable.

3.3

Sensing element

A proprietary process is used to create a surface micro-machined accelerometer. The

technology allows carrying out 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. 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 moulding 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 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 by a low-noise capacitive amplifier which

converts the capacitive unbalancing of the MEMS sensor into an analog voltage that is

finally available to the user by 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.

The LIS3DH features a Data-Ready signal (RDY) which indicates when a new set of

measured acceleration data is available thus simplifying data synchronization in the digital

system that uses the device.

The LIS3DH may also be configured to generate an inertial Wake-Up and Free-Fall interrupt

signal accordingly to a programmed acceleration event along the enabled axes. Both Free-

Fall and Wake-Up can be available simultaneously on two different pins.

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