© 2010, 2011 Freescale Semiconductor, Inc. All rights reserved.
Document Number: AN4071
Rev 1, 10/2011
Freescale Semiconductor
Application Note
High-Pass Filtered Data and Transient
Detection Using the MMA8451, 2, 3Q
by:
Kimberly Tuck
Applications Engineer
1.0 Introduction
The acceleration readings from the sensor couple both
static and dynamic accelerations. Many tilt based applications
such as orientation detection, scrolling and tilt based gaming
apps use the accelerometer analyzing the change in static
acceleration due to gravity only. These functions are often
best when they’ve been low-pass filtered because the high-
frequency data is considered noise. There are many functions
where the accelerometer must analyze the dynamic
acceleration. Functions such as taps, flicks, shakes and step
counting are based on the analysis of the change in the
acceleration with no regard for the static acceleration. These
functions interpret the acceleration threshold change and
direction of motion. It is simpler to interpret the dynamic
acceleration data when the static component has been
removed.
The MMA8451, 2, 3Q has a built-in high-pass filter. In the
MMA8451, 2Q the output data can go through the high-pass
filter, eliminating the offset (DC) and low frequencies (well
below the cutoff). Note that the MMA8453Q does not have the
capability to read out data from the high-pass filter. The high-
pass filter cutoff frequency can be set by the user to four
different frequencies which are dependent on the Output Data
Rate (ODR) and the oversampling mode. A higher cutoff
frequency ensures the DC data or slower moving data will be
filtered out, allowing only the higher frequencies to pass. The
MMA8451, 2, 3Q contain an embedded transient function
which analyzes the high-pass filtered data allowing the user to
set a preset threshold with an added debounce counter to
ensure the event happens for a set duration. The threshold
represents a change in acceleration to be exceeded. This
embedded function can be routed to an interrupt pin to detect
jolts on different axes and will display the direction of the jolt.
TABLE OF CONTENTS
1.0 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
1.1 Key Words . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
1.2 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.0 MMA8451, 2, 3Q Consumer 3-axis Accelerometer 3 by 3 by 1 mm . . . . .2
2.1 Output Data, Sample Rates and Dynamic Ranges of all Three Products . . .3
2.1.1 MMA8451Q . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
2.1.2 MMA8452Q . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
2.1.3 MMA8453Q Note: No HPF Data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
3.0 Comparison of Motion Detection and Transient Detection . . . . . . . . . . . 3
3.1 Use Case 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
3.2 Use Case 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
4.0 Transient Settings and Interpretation of the Data . . . . . . . . . . . . . . . . . .5
4.1 High-Pass Filter Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
5.0 Using the HPF Data to Write Algorithms such as Directional Shake . . . 7
6.0 Configuring the Embedded Transient Detection . . . . . . . . . . . . . . . . . . .9
6.1 Register 0x1D:TRANSIENT_CFG Transient Configuration Register . . . . . . .9
6.2 Register 0x1F: TRANSIENT_THS Transient Threshold Register . . . . . . . . 10
6.3 Register 0x20: TRANSIENT_COUNT Transient Debounce Register . . . . . . 10
6.4 Register 0x1E:TRANSIENT_SRC Transient Source Register . . . . . . . . . . . 11
6.5 Configuring the Transient Detection to an Interrupt Pin. . . . . . . . . . . . . . . . . 11
6.6 Reading the System Interrupt Status Source Register . . . . . . . . . . . . . . . . .11
7.0 Details for Configuring the MMA8451, 2, 3Q for Transient Detection .12
Table 11.Registers of Importance for Setting up the Transient Detection and Using
the HPF
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1 Example Steps for Configuring Transient Detection . . . . . . . . . . . . . . . . . . . 13
1.1 Key Words
Accelerometer, Static Acceleration, Image Stabilization, High-
Pass Filter (HPF), Transient, HPF Data, 3-Axis
Accelerometer, Embedded Motion Detection, Embedded
Transient Detection, Vibration, Shake, Directional Shake,
Displacement, Velocity, Sensor, Interrupt, DBCNTM Bit, Jolt
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