MPR121 datasheet(30/57 Pages) FREESCALE

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MPR121

Sensors

Freescale Semiconductor

After this averaging the filter reacts to Cases 1, 2, and 3.

Figure 32.

ADVANCED CASES

With an understanding of the basic cases, more advanced cases can be discussed. In a touch sensor system, we can take

advantage of some known properties to improve the functionality of the filter. These include direction of change, touch occurrence

and the rate of touch. The first four cases are still utilized but more functionality is added. The following cases described how

different settings are useful as opposed to what exactly the settings do, like cases 1-4.

Case 5

The direction of change for a touch in the system is always negative. Thus the system takes advantage of this by allowing for

varying parameters for different directions of change. Since a touch can only be in the decreasing direction, it is usually best to

set the decreasing filter to be slower that the increasing one. This allows for automatic recovery from a bad baseline reading.

A. As the touch occurs, the baseline is decreased slowly due to a non solid touch, but due to the slow

reaction, a touch is still detected.

B. The baseline quickly snaps back to the initial value by having fast filtering in the positive direction

C. The repeated touch is easily handled since the baseline quickly adjusted; if it was slow, the second

touch would have resulted in a possible false negative for a touch detection.

Figure 33.

Case 1

Case 2

Baseline

Data

Averaged

Data

Case 1

MHD = 1

NCL = 3

NHD = 1

FDL = 4

Baseline

Data

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