41594B datasheet(295/372 Pages) MICROCHIP

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Preliminary

DS41594B-page 295

PIC12LF1840T48A

27.5

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.

27.6

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.

27.6.1

TIMER0

To select Timer0 as the timer resource for the CPS:

• 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 20.0 “Timer0 Module” for

additional information.

27.6.2

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 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 CPS module. Refer to

Section 21.12 “Timer1 Gate Control Register” for

additional information.

TABLE 27-2:

TIMER1 ENABLE FUNCTION

27.7

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.

27.7.1

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.

27.7.2

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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