PIC12F1822-ISLASHMF datasheet(309/398 Pages) MICROCHIP

309 / 398 page

 2010 Microchip Technology Inc.

Preliminary

DS41413A-page 309

PIC12F/LF1822/16F/LF1823

26.1

Analog MUX

The capacitive sensing module can monitor up to four

inputs for the PIC12F/LF1822 (CPSCH<3:0>) and up

to eight inputs for the PIC16F/LF1823 (CPSCH<7:0>).

See Register 26-2 for details. The capacitive sensing

inputs are defined as CPS<7:0>, as applicable to

device. To determine if a frequency change has

occurred the user must:

• Select the appropriate CPS pin by setting the

appropriate CPSCH 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.

26.2

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.

26.3

Voltage References

The capacitive sensing oscillator uses voltage refer-

ences to provide two voltage thresholds for oscillation.

The upper voltage threshold is referred to as Ref+ and

the lower voltage threshold is referred to as Ref-.

The user can elect to use fixed voltage references,

which are internal to the capacitive sensing oscillator,

or variable voltage references, which are supplied by

the Fixed Voltage Reference (FVR) module and the

Digital-to-Analog Converter (DAC) module.

When the fixed voltage references are used, the VSS

voltage determines the lower threshold level (Ref-) and

the VDD voltage determines the upper threshold level

(Ref+).

When the variable voltage references are used, the

DAC voltage determines the lower threshold level

(Ref-) and the FVR voltage determines the upper

threshold level (Ref+). An advantage of using these ref-

erence sources is that oscillation frequency remains

constant with changes in VDD.

Different oscillation frequencies can be obtained

through the use of these variable voltage references.

The more the upper voltage reference level is lowered

and the more the lower voltage reference level is

raised, the higher the capacitive sensing oscillator

frequency becomes.

Selection between the voltage references is controlled

by the CPSRM bit of the CPSCON0 register. Setting

this bit selects the variable voltage references and

clearing this bit selects the fixed voltage references.

Please see Section 14.0 “Fixed Voltage Reference

(FVR)” and Section 16.0 “Digital-to-Analog Converter

(DAC) Module” for more information on configuring the

variable voltage levels.

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