MPR121 datasheet(31/57 Pages) FREESCALE

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MPR121

Sensors

Freescale Semiconductor

Case 6

The system needs the capability to handle environment changes that appear very similar to actual touches. In Case 5, the

touch was a real touch, but slow enough that initially it is thought better for the baseline not to change at all.

A. The decrease is the interface being cleared with a wet rag, causing a relatively slow capacitance

change. The baseline accurately tracks this slow change.

B. The baseline begins to increase as the interface becomes dry.

C. A delta from the new baseline allows a touch to be accurately detected.

Figure 34.

Case 7

This case is when a touch is occurring. While the baseline system does not detect a touch, it is obviously an important part of

the process. The baseline can be set to slowly calibrate a touch from the system preventing keys from becoming stuck. Only the

NHD, NCL and FDL are necessary since the value can never be less than double the MHD.

A. The touch is detected which disengages the increasing/decreasing baseline filter but leaves it

enabled with very slow filtering

B. Even though the touch has not been released it times out and is eventually rejected.

C. Normal baseline filter is engaged.

Figure 35.

Baseline

Data

Baseline

Data

MPR121 数据表(PDF) 31 Page - Freescale Semiconductor, Inc