PIC16F1946 datasheet(331/448 Pages) MICROCHIP

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Preliminary

DS41414C-page 331

PIC16(L)F1946/1947

26.2.4.2

Timer1

To select Timer1 as the timer resource for the CPS

module, set the TMR1CS<1:0> of the T1CON register

to ‘11’. When Timer1 is chosen as the timer resource,

the capacitive sensing oscillator will be the clock

source for Timer1. Because the Timer1 module has a

gate control, developing a time base for the frequency

measurement can be simplified by using the Timer0

overflow flag.

It is recommend that the Timer0 overflow flag, in

conjunction with the Toggle mode of the Timer1 Gate, be

used to develop the fixed time base required by the soft-

ware portion of the CPS module. Refer to Section 21.12

“Timer1 Gate Control Register”

for additional informa-

tion.

TABLE 26-2:

TIMER1 ENABLE FUNCTION

26.2.5

SOFTWARE CONTROL

The software portion of the CPS 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 capac-

itive sensing oscillator due to an additional capac-

itive load.

• Set the frequency threshold.

26.2.5.1

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.

26.2.5.2

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.

26.2.5.3

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 on the software required for CPS module.

TMR1ON

TMR1GE

Timer1 Operation

Off

Count Enabled by input

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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