MPR121 datasheet(26/57 Pages) FREESCALE

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MPR121

Sensors

Freescale Semiconductor

AN3891: MPR121 Baseline System

INTRODUCTION

Touch acquisition takes a few different parts of the system in order to detect touch. The baseline filter and touch detection are

tightly coupled. The purpose of the baseline filter is to “filter out touches” resulting in a system that is similar to a long term

average but also takes into account that one specific signature. A touch must have different properties than noise and

environmental change with respect to the filter response. This is accomplished through four register types that operate under

different conditions. These are Max Half Delta (MHD), Noise Half Delta (NHD), Noise Count Limit (NCL) and Filter Delay Limit

(FDL).

Figure 24. Data Flow in the MPR121

MAX HALF DELTA (NHD)

543210

MHD

Reset:

000000

= Unimplemented

Table 22. Max Half Delta Register Field Descriptions

Field

Description

5:0

MHD

Max Half Delta – The Max Half Delta determines the largest magnitude of

variation to pass through the third level filter.

000000 DO NOT USE THIS CODE

000001 Encoding 1 – Sets the Max Half Delta to 1

111111 Encoding 63 – Sets the Max Half Delta to 63

AFE AQUISITION

RAW

DATA

2nd

FILTER

1st

FILTER

BASELINE

FILTER

STATUS REGISTER

IRQ

TOUCH

1 - 32

μs

1 - 128

μs

4 - 2048

μs

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