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2010-2012 Microchip Technology Inc.

DS41417B-page 119

PIC16(L)F722A/723A

14.5

Software Control

The software portion of the capacitive sensing module

is required to determine the change in frequency of the

capacitive sensing oscillator. This is accomplished by

the following:

Setting a fixed time base to acquire counts on

Timer0 or Timer1

Establishing the nominal frequency for the

capacitive sensing oscillator

Establishing the reduced frequency for the

capacitive sensing oscillator due to an additional

capacitive load

Set the frequency threshold

14.5.1

NOMINAL FREQUENCY

(NO CAPACITIVE LOAD)

To determine the nominal frequency of the capacitive

sensing oscillator:

Remove any extra capacitive load on the selected

CPSx pin

At the start of the fixed time base, clear the timer

resource

At the end of the fixed time base save the value in

the timer resource

The value of the timer resource is the number of

oscillations of the capacitive sensing oscillator for the

given time base. The frequency of the capacitive

sensing oscillator is equal to the number of counts on

in the timer divided by the period of the fixed time base.

14.5.2

REDUCED FREQUENCY

(ADDITIONAL CAPACITIVE LOAD)

The extra capacitive load will cause the frequency of the

capacitive sensing oscillator to decrease. To determine

the reduced frequency of the capacitive sensing

oscillator:

Add a typical capacitive load on the selected

CPSx pin

Use the same fixed time base as the nominal

frequency measurement

At the start of the fixed time base, clear the timer

resource

At the end of the fixed time base save the value in

the timer resource

The value of the timer resource is the number of oscil-

lations of the capacitive sensing oscillator with an addi-

tional capacitive load. The frequency of the capacitive

sensing oscillator is equal to the number of counts on

in the timer divided by the period of the fixed time base.

This frequency should be less than the value obtained

during the nominal frequency measurement.

14.5.3

FREQUENCY THRESHOLD

The frequency threshold should be placed midway

between the value of nominal frequency and the

reduced frequency of the capacitive sensing oscillator.

Refer to Application Note AN1103, “Software Handling

for Capacitive Sensing” (DS01103) for more detailed

information the software required for capacitive

sensing module.

Note:

For more information on general capacitive

sensing refer to Application Notes:

AN1101, “Introduction to Capacitive

Sensing” (DS01101)

AN1102, “Layout and Physical Design

Guidelines for Capacitive Sensing”

(DS01102)

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