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Sheet目录3893

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PIC16(L)F722A/723A

DS41417B-page 118

2010-2012 Microchip Technology Inc.

14.1

Analog MUX

The capacitive sensing module can monitor up to 8

inputs. The capacitive sensing inputs are defined as

CPS<7:0>. To determine if a frequency change has

occurred the user must:

Select the appropriate CPS pin by setting the

CPSCH<2:0> bits of the CPSCON1 register

Set the corresponding ANSEL bit

Set the corresponding TRIS bit

Run the software algorithm

Selection of the CPSx pin while the module is enabled

will cause the capacitive sensing oscillator to be on the

CPSx pin. Failure to set the corresponding ANSEL and

TRIS bits can cause the capacitive sensing oscillator to

stop, leading to false frequency readings.

14.2

Capacitive Sensing Oscillator

The capacitive sensing oscillator consists of a constant

current source and a constant current sink, to produce

a triangle waveform. The CPSOUT bit of the

CPSCON0 register shows the status of the capacitive

sensing oscillator, whether it is a sinking or sourcing

current. The oscillator is designed to drive a capacitive

load (single PCB pad) and at the same time, be a clock

source to either Timer0 or Timer1. The oscillator has

three different current settings as defined by

CPSRNG<1:0> of the CPSCON0 register. The different

current settings for the oscillator serve two purposes:

Maximize the number of counts in a timer for a

fixed time base

Maximize the count differential in the timer during

a change in frequency

14.3

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.

14.4

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.

14.4.1

TIMER0

To select Timer0 as the timer resource for the capacitive

sensing module:

Set the T0XCS bit of the CPSCON0 register

Clear the T0CS 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 11.0 “Timer0 Module” for

additional information.

14.4.2

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 using

either:

The Timer0 overflow flag

The Timer2 overflow flag

The WDT overflow flag

It is recommended that one of these flags, in conjunc-

tion with the toggle mode of the Timer1 gate, is used to

develop the fixed time base required by the software

portion of the capacitive sensing module. Refer to

Section 12.0 “Timer1 Module with Gate Control” for

additional information.

TABLE 14-1:

TIMER1 ENABLE FUNCTION

Note:

The fixed time base can not be generated

by the timer resource the capacitive

sensing oscillator is clocking.

TMR1ON

TMR1GE

Timer1 Operation

Off

Count Enabled by input

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