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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

AN4071

Sensors

2 Freescale Semiconductor, Inc.

1.2 Summary

A. The high-pass filter is used to eliminate the static acceleration when the HPF_OUT bit is set.

B. Different cutoff frequencies from the high-pass filter affect the output data and the embedded functions using the

high-pass filter.

C. The high-pass filtered data is useful for writing algorithms which require an analysis of the change in acceleration

and simplifies the data analysis by eliminating the static acceleration.

D. The transient function is interrupt driven and saves on current consumption and unnecessary processing because

the logic is internal to the sensor.

E. An algorithm for directional shake can be reliably detected by analyzing the (high-pass filter) data.

F. The high-pass filter data can be used for image stability applications, compensating for the jitter of the user’s hands

and/or from the environment.

G. The transient function can be used to detect a change in acceleration that exceeds a set threshold. This is slightly

different than the motion detection function.

H. The transient threshold and debounce counter can be changed in either standby or active mode to allow for

adjustments after the part has transitioned from wake to sleep mode.

I. Note the MMA8453Q does not have the option for high-pass filtered data, although it does contain the transient

function and the tap function uses the high-pass filter.

2.0 MMA8451, 2, 3Q Consumer 3-axis Accelerometer 3 by 3 by 1 mm

The MMA8451, 2, 3Q has a selectable dynamic range of ±2g, ±4g, ±8g. The device has 8 different output data rates, selectable

high-pass filter cutoff frequencies, and high-pass filtered data. The available resolution of the data and the embedded features is

dependant on the specific device.

Note: The MMA8450Q has a different memory map and has a slightly different pinout configuration.

Figure 1. MMA8451, 2, 3Q Consumer 3-axis Accelerometer 3 by 3 by 1 mm

141516

876

VDD

VDDIO

BYP

SCL

GND

INT1

GND

INT2

SA0

SDA

MMA845xQ

16-Pin QFN

(Top View)

AN4071

Sensors

Freescale Semiconductor, Inc. 3

2.1 Output Data, Sample Rates and Dynamic Ranges of all Three Products

2.1.1 MMA8451Q

1. 14-bit data

2g (4096 counts/g = 0.25 mg/LSB) 4g (2048 counts/g = 0.5 mg/LSB) 8g (1024 counts/g = 1 mg/LSB)

2. 8-bit data

2g (64 counts/g = 15.6 mg/LSB) 4g (32 counts/g = 31.25 mg/LSB) 8g (16 counts/g = 62.5 mg/LSB)

3. Embedded 32 sample FIFO (MMA8451Q)

2.1.2 MMA8452Q

1. 12-bit data

2g (1024 counts/g = 1 mg/LSB) 4g (512 counts/g = 2 mg/LSB) 8g (256 counts/g = 3.9 mg/LSB)

2. 8-bit data

2g (64 counts/g = 15.6 mg/LSB) 4g (32 counts/g = 31.25 mg/LSB) 8g (16 counts/g = 62.5 mg/LSB)

2.1.3 MMA8453Q Note: No HPF Data

1. 10-bit data

2g (256 counts/g = 3.9 mg/LSB) 4g (128 counts/g = 7.8 mg/LSB) 8g (64 counts/g = 15.6 mg/LSB)

2. 8-bit data

2g (64 counts/g = 15.6 mg/LSB) 4g (32 counts/g = 31.25 mg/LSB) 8g (16 counts/g = 62.5 mg/LSB)

3.0 Comparison of Motion Detection and Transient Detection

The transient function (with HPF enabled) is different from the motion detection embedded function in MMA8451, 2, 3Q. Refer

to Freescale application note AN4070 for details on the motion detection. The intended use of these functions is different.

A. The embedded motion function triggers an interrupt when any of the enabled axes has exceeded a set acceleration

threshold.

• Motion detection is useful when the device may be tilted in an orie

ntation and the interest is to know if the

acceleration in one axis has crossed a specified threshold amount measured from 0g.The motion detection

analyzes change in threshold considering both static and dynamic acceleration.

B. The transient detection embedded function compares whether any o

f the enabled axes has exceeded a set change

in acceleration disregarding the static acceleration. The data that is compared to the threshold to be exceeded is

high-pass filtered data.

• Transient detection is useful wh

en the device may be tilted in an orientation (i.e., there is some non-zero reading

on one or more of X, Y and Z) and the interest is more on the dynamic acceleration crossing a specified threshold

amount in a short amount of time (frequency being the reciprocal of time) regardless of what the current value is.

Example

If a change in acceleration of 0.5g in any axe

s was the target for an application, then it would be challenging to use the em-

bedded motion detection function for detecting this change. The reason for this can be better understood with an example:

Assume that the device was oriented such that X = 0.24g, Y = 0.44g and Z = 0.82g; it would be challenging to detect a 0.5g

change in acceleration in any of the axes using the motion detect function since the new readings have to be outside the range

X = -0.26g to 0.74g, Y = -0.06g to 0.94g and Z = 0.32g to 1.32g. Because only a single threshold setting can be chosen for each

axes when using the motion detect embedded function and because of the need to detect acceleration outside a range, it would,

therefore, be seen as impossible to choose a threshold that would detect a 0.5 change in acceleration for any of the axes.

3.1 Use Case 1

Use Case 1 Transient: Unknown Orientation with Specific Acceleration Change for Detection

A use case example for transient detection may be when the device

containing the accelerometer is in an environment of an

unknown orientation or changing orientation and a small amplitude, but quick disturbance must be detected, e.g., when the de-

vice is accelerating through 0g to +1g to -1g and a subtle impact force (change in acceleration of 0.5g) is used as an indication

to wake the device or as a motion signature of an event. The accelerometer reading combines the gravity and the linear

acceleration reading. It is much simpler to decouple the two by eliminating the effect of gravity. Using the example above, assume

that the device was oriented such that at given time T, X = 0.24g, Y = 0.44g and Z = 0.82g. If transient detection is enabled on

all

3 axes with a threshold of 0.5g and an ODR of 100 Hz, any time that X goes outside of the range -0.26g to 0.74g within

T + 10 ms,

a transient detect interrupt would be generated. Eliminating the static acceleration on the device simplifies the condi-

tion to focus on the change in acceleration only, since the orientation is of no consequence in this situation. Figure 2 demonstrates

that the sensor detects both the static and the dynamic acceleration. Decoupling the static from the dynamic acceleration can

simplify the analysis.

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