MMA8451Q datasheet(16/53 Pages) FREESCALE

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Freescale Semiconductor

MMA8451Q

5.7

Transient Detection

The MMA8451Q has a built-in high pass filter. Acceleration data goes through the high pass filter, eliminating the offset (DC)

and low frequencies. The high pass filter cut-off frequency can be set by the user to four different frequencies which are

dependent on the Output Data Rate (ODR). A higher cut-off frequency ensures the DC data or slower moving data will be filtered

out, allowing only the higher frequencies to pass. The embedded Transient Detection function uses the high pass filtered data

allowing the user to set the threshold and debounce counter. The transient detection feature can be used in the same manner as

the motion detection by bypassing the high pass filter. There is an option in the configuration register to do this. This adds more

flexibility to cover various customer use cases.

Many applications use the accelerometer’s static acceleration readings (i.e., tilt) which measure the change in acceleration

due to gravity only. These functions benefit from acceleration data being filtered with a low pass filter where high frequency data is

considered noise. However, there are many functions where the accelerometer must analyze dynamic acceleration. Functions

such as tap, flick, shake and step counting are based on the analysis of the change in the acceleration. It is simpler to interpret

these functions dependent on dynamic acceleration data when the static component has been removed. The Transient Detection

function can be routed to either interrupt pin through bit 5 in CTRL_REG5 register (0x2E). Registers 0x1D – 0x20 are the

dedicated Transient Detection configuration registers. The source register contains directional data to determine the direction of

the acceleration, either positive or negative. For details on the benefits of the embedded Transient Detection function along with

specific application examples and recommended configuration settings, please refer to Freescale application note, AN4071.

5.8

Tap Detection

The MMA8451Q has embedded single/double and directional tap detection. This function has various customizing timers for

setting the pulse time width and the latency time between pulses. There are programmable thresholds for all three axes. The tap

detection can be configured to run through the high pass filter and also through a low pass filter, which provides more customizing

and tunable tap detection schemes. The status register provides updates on the axes where the event was detected and the

direction of the tap. For more information on how to configure the device for tap detection please refer to Freescale application

note AN4072.

5.9

Orientation Detection

The MMA8451Q incorporates an advanced algorithm for orientation detection (ability to detect all 6 orientations) with

configurable trip points. The embedded algorithm allows the selection of the mid point with the desired hysteresis value.

The MMA8451Q Orientation Detection algorithm confirms the reliability of the function with a configurable Z-lockout angle.

Based on known functionality of linear accelerometers, it is not possible to rotate the device about the Z-axis to detect change in

acceleration at slow angular speeds. The angle at which the device no longer detects the orientation change is referred to as the

“Z-Lockout angle”. The device operates down to 14° from the flat position.

For further information on the configuration settings of the orientation detection function, including recommendations for

configuring the device to support various application use cases, refer to Freescale application note, AN4068.

Figure 8 and Figure 9 show the definitions of the trip angles going from Landscape to Portrait and then also from Portrait to

Landscape.

Figure 7. Landscape/Portrait Orientation

Top View

Earth Gravity

Pin 1

Xout @ 0g

Yout @ -1g

Zout @ 0g

Xout @ 1g

Yout @ 0g

Zout @ 0g

Xout @ 0g

Yout @ 1g

Zout @ 0g

Xout @ -1g

Yout @ 0g

Zout @ 0g

Side View

FRONT

Xout @ 0g

Yout @ 0g

Zout @ 1g

BACK

Xout @ 0g

Yout @ 0g

Zout @ -1g

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