PIC12F1822-ISLASHMF datasheet(311/398 Pages) MICROCHIP

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Preliminary

DS41413A-page 311

PIC12F/LF1822/16F/LF1823

26.5

Timer Resources

To measure the change in frequency of the capacitive

sensing oscillator, a fixed time base is required. For the

period of the fixed time base, the capacitive sensing

oscillator is used to clock either Timer0 or Timer1. The

frequency of the capacitive sensing oscillator is equal

to the number of counts in the timer, divided by the

period of the fixed time base.

26.6

Fixed Time Base

To measure the frequency of the capacitive sensing

oscillator, a fixed time base is required. Any timer

resource or software loop can be used to establish the

fixed time base. It is up to the end user to determine the

method in which the fixed time base is generated.

26.6.1

TIMER0

To select Timer0 as the timer resource for the capacitive

sensing module:

• Set the T0XCS bit of the CPSCON0 register.

• Clear the TMR0CS bit of the OPTION register.

When Timer0 is chosen as the timer resource, the

capacitive sensing oscillator will be the clock source for

Timer0. Refer to Section 19.0 “Timer0 Module” for

additional information.

26.6.2

TIMER1

To select Timer1 as the timer resource for the

capacitive sensing 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 con-

junction with the Toggle mode of the Timer1 Gate, be

used to develop the fixed time base required by the

software portion of the capacitive sensing module.

Refer to Section 20.12 “Timer1 Gate Control Regis-

ter” for additional information.

TABLE 26-2:

TIMER1 ENABLE FUNCTION

26.7

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 capac-

itive sensing oscillator due to an additional capac-

itive load.

• Set the frequency threshold.

26.7.1

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.

26.7.2

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.

Note:

The fixed time base can not be generated

by the timer resource that the capacitive

sensing oscillator is clocking.

TMR1ON

TMR1GE

Timer1 Operation

Off

Count Enabled by input

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