MPR121 datasheet(29/57 Pages) FREESCALE

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MPR121

Sensors

Freescale Semiconductor

Case 2

Changes that are larger than double the MHD are regarded as noise and accounted for by the values of the NHD and NCL.

Any data outside the MHD is rejected by the filter however sequential values that fall into this category are counted and if enough

sequential data exists then the baseline will be adjusted.

In this case, the NCL regulates how many sequential data points must be seen before the data is changed. When the count

is reached, the baseline is incremented by the NHD.

Figure 29.

Case 3

When the data is inconsistent but greater than double than MHD the baseline will not vary. Each time a transition takes place,

the filter counters are reset, thus the fact that the data is oscillating around the baseline means that the noise is rejected and the

baseline will not vary.

Figure 30.

Case 4

Low frequency changes to the data can trick the filter in some instances. The FDL is also available to slow down the overall

system. This is done by taking an average of the specified number of values before running them through the baseline filter.

Figure 31.

Baseline

Data

3 counts

NHD added to

baseline

into effect

Case 1 comes

MHD = 1

NCL = 3

NHD = 1

Baseline

Data

MHD = 1

NCL = 3

NHD = 1

2 < NCL

1 < NCL

2 < NCL

Data

Averaged

Data

MPR121 Datenblatt(PDF) 29 Page - Freescale Semiconductor, Inc