MPR121 datasheet(28/57 Pages) FREESCALE

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MPR121

Sensors

Freescale Semiconductor

FILTER DELAY LIMIT (FDL)

Figure 27. Filter Delay Limit Register

Additionally there are different conditions in the system that affects how these registers operate. These are rising data, falling

data or touched data. When the data changes between these conditions, the current filter process is cancelled and all filter

counters return to zero.

The operation of the filter is in the relationship between the 2nd filter data and the baseline filter value. The occurrence of a

touch will also change the operation of the system. The touch generation process is described in the application note AN3892.

The falling data system is enabled any time the 2nd filter data is less than the baseline filter data. The rising data system is enabled

any time the 2nd filter data is greater than the baseline filter data. The following cases describe the baseline system when it is not

changing between the three states mentioned above.

Case 1

Small incremental changes to the system represent long term slow (environmental) changes in the system. The MHD setting

regulates this case by allowing any data that is less than two times the MHD to pass the filter. Thus, if the baseline is 700 and

the data is 701 with a MHD of one, then the baseline filter would increase to equal the data for the next cycle.

Figure 28. Max Half Delta

543210

FDL

Reset:

000000

= Unimplemented

Table 25. Filter Delay Limit Register Field Descriptions

Field

Description

7:0

FDL

Filter Delay Limit – The Filter Delay Limit determines the rate of operation of the

filter. A larger number makes it operate slower.

00000000 Encoding 0 – Sets the Filter Delay Limit to 1

00000001 Encoding 1 – Sets the Filter Delay Limit to 2

11111111 Encoding 255 – Sets the Filter Delay Limit to 255

MHD = 1

Baseline

Data

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