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High pass filtered data and transient detection using the MMA8450Q

Posted: 08 Mar 2010 ?? ?Print Version ?Bookmark and Share

Keywords:MMA8450Q transient detection? high pass filter? pass filtered data?

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 analysing 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 MMA8450Q has a built in high pass filter. The output data goes through the high pass filter, eliminating the offset (DC) and low frequencies (well below the cut-off). 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 MMA8450Q also contains 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.

View the PDF document for more information.

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