MPR121 datasheet(11/57 Pages) FREESCALE

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MPR121

Sensors

Freescale Semiconductor

AN3889: MPR121 Capacitance Sensing Settings

INTRODUCTION

Touch acquisition takes a few different parts of the system in order to detect touch. The first stage of this process is to capture

the pad capacitance. Freescale’s MPR121 utilizes the principle that a capacitor holds a fixed amount of charge at a specific elec-

tric potential. Both the implementation and the configuration will be described in this application note.

Figure 3. Data Flow in the MPR121

CAPACITANCE MEASUREMENT

The basic measurement technique used by the MPR121 is to charge up the capacitor C on one electrode input with a DC

current I for a time T (the charge time). Before measurement, the electrode input is grounded, so the electrode voltage starts from

0 V and charges up with a slope, Equation 1, where C is the pad capacitance on the electrode (Figure 4). All of the other elec-

trodes are grounded during this measurement. At the end of time T, the electrode voltage is measured with a 10 bit ADC. The

voltage is inversely proportional to capacitance according to Equation 2

.The electrode is then discharged back to ground at the

same rate it was charged.

Equation 1

Equation 2

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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