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首页LIS2DH12和LIS2DW12的单双击配置.pdf
LIS2DH12和LIS2DW12的单双击配置.pdf
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ST的三轴传感器 LIS2DH12和LIS2DW12的单双击 配置代码,寄存器配置,以及ODR的设置,还有阈值配置的算法,双击的中间间隔的算法
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May 2018
DT0101 Rev 1
1/14
www.st.com
D
T0101
Design
tip
Setting up single-tap and double-tap recognition
with ST’s MEMS
accelerometers
By Vladimir JANOUSEK, Zuzana JIRANKOVA,
and Petr STUKJUNGER
Main components
LIS2DW12 MEMS digital output motion sensor:
high-performance ultra-low-power 3-axis "femto" accelerometer
LIS2DH12 MEMS digital output motion sensor:
ultra-low-power high-performance 3-axis "femto" accelerometer
Purpose and benefits
This design tip explains how to enable and personalize the single-tap and double-tap
recognition feature of MEMS accelerometers from STMicroelectronics.
First we explain this embedded feature, what it does and how it can be parameterized.
Then we discuss the impact of its parameters on detection results. Finally, we show
using the two most frequently used ST accelerometers, LIS2DW12 and LIS2DH12,
exact settings and example source codes for implementing the single-tap and double-
tap recognition feature in applications.
Description
The single-tap and double-tap recognition feature allows detecting actions similar to the
single click and double click of a mouse. The difference in using a MEMS accelerometer
is that there is no need for a mechanical button to be pressed. This feature allows easy
implementation of user interfaces of wearable, portable and other devices. Replacing the
mechanical button brings more robust, user-friendly and cheaper design as well as the
possibility of a higher degree of system integration.
Single-tap is the action when the user taps with his finger on the device casing. It is
detected by the accelerometer as an acceleration shock. The accelerometer then
informs the host microcontroller of this finger tap by an interrupt signal. Figure 1. depicts
this flow.
Double-tap is basically a combination of two consecutive single-tap actions. Recognizing
two taps as a different action and linking different application behavior to single-tap and
double-tap events allow even more flexibility for controlling the application. Using an
MP3 player as an example, single-tap can be used to start/stop playback, while double-
tap can be used to move to the next song.
May 2018
DT0101 Rev 1
2/14
www.st.com
Figure 1. Tap event recognition flow
Parameterization
For single-tap recognition there are two parameters to set up – threshold and shock
time window.
Single-tap recognition interrupt is generated if the acceleration exceeds the preset
threshold and falls below within the shock time window - see Figure 2. below.
Figure 2. Single-tap recognition
(a) – Single-tap event recognized, (b) – Single-tap event not recognized, shock lasted too long
Threshold defines the intensity of the shock needed to generate a single-tap interrupt.
The amplitude of the shock is proportional to the force that the user applies in either a
positive or negative direction.
Shock time window sets the maximum allowed duration of a shock. During this
window the acceleration has to fall below the pre-selected threshold. Figure 2. depicts
functionality of the shock window parameter.
MEMS
accelerometer
Microcontroller
INT
Tap
Acceleration
Shock
Tap
Recognition
User action
handling
May 2018
DT0101 Rev 1
3/14
www.st.com
For double-tap recognition there are four parameters to set up. Besides the threshold
and shock time window which are the same as for single-tap recognition, there are also
the quiet time window and latency time window parameters.
Double-tap interrupt is generated if there are two consecutive single taps recognized.
The time between two single taps shall not be shorter than the quiet time window and
longer than latency time window - see Figure 3. below.
Quiet time window defines the period of time after the first detected single tap where
there should not be any other shock detected.
Latency time window defines the time period starting after the quiet time window
where the second single tap should happen in order to recognize this event as a double
tap.
Figure 3. Double-tap recognition
(a) – Double-tap event recognized, (b) – Double-tap event not recognized, Latency time window shorter
The sensitivity of tap recognition can be modified using the threshold. A lower threshold
means higher sensitivity and vice versa. However a threshold that is too low will lead to
too many false positives.
The shock time window helps to limit the number of false positives by avoiding long
shocks to be detected as a tap. A very short shock time window will decrease tap
detection sensitivity. Nevertheless a shock time window that is too long will increase the
number of false positives.
Quiet time and latency time windows used for double tap are usually not sensitive to
device placement and are rather subject to personalization.
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