Terminology and functionality
LIS3DH
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.
2
The acceleration data may be accessed through an I C/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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Doc ID 17530 Rev 1
STMICROELECTRONICS [ ST ]
