PIC12F635 datasheet(45/196 Pages) MICROCHIP

45 / 196 page

Preliminary

DS41232B-page 43

PIC12F635/PIC16F636/639

4.2.3

ULTRA LOW-POWER WAKE-UP

The Ultra Low-Power Wake-up (ULPWU) on RA0 allows

a slow falling voltage to generate an interrupt-on-change

on RA0 without excess current consumption. The mode

is selected by setting the ULPWUE bit (PCON<5>). This

enables a small current sink which can be used to

discharge a capacitor on RA0.

To use this feature, the RA0 pin is configured to output

‘1’ to charge the capacitor, interrupt-on-change for RA0

is enabled and RA0 is configured as an input. The

ULPWUE bit is set to begin the discharge and a SLEEP

instruction is performed. When the voltage on RA0 drops

below VIL, an interrupt will be generated which will cause

the device to wake-up. Depending on the state of the

GIE bit (INTCON<7>), the device will either jump to the

interrupt vector (0004h) or execute the next instruction

when the interrupt event occurs. See Section 4.2.2

“Interrupt-on-change” and Section 12.9.3 “PORTA

Interrupt” for more information.

This feature provides a low power technique for

periodically waking up the device from Sleep. The time-

out is dependent on the discharge time of the RC circuit

on RA0. See Example 4-2 for initializing the Ultra Low

Power Wake-up module.

The series resistor provides overcurrent protection for the

RA0 pin and can allow for software calibration of the time-

out (see Figure 4-1). A timer can be used to measure the

charge time and discharge time of the capacitor. The

charge time can then be adjusted to provide the desired

interrupt delay. This technique will compensate for the

affects of temperature, voltage and component accuracy.

The Ultra Low-Power Wake-up peripheral can also be

configured as a simple Programmable Low-Voltage

Detect or temperature sensor.

EXAMPLE 4-2:

ULTRA LOW-POWER

WAKE-UP INITIALIZATION

Note:

For

information,

refer

the

Application

Note

AN879,

“Using

the

Microchip

Ultra

Low-Power

Wake-up

Module” (DS00879).

BCF

STATUS,RP0

;Bank 0

BCF

STATUS,RP1

;

BSF

PORTA,0

;Set RA0 data latch

MOVLW

H’7’

;Turn off

MOVWF

CMCON0

; comparators

BSF

STATUS,RP0

;Bank 1

BCF

STATUS,RP1

;

BCF

TRISA,0

;Output high to

CALL

CapDelay

; charge capacitor

BSF

PCON,ULPWUE

;Enable ULP Wake-up

BSF

IOCA,0

;Select RA0 IOC

BSF

TRISA,0

;RA0 to input

MOVLW

B’10001000’

;Enable interrupt

MOVWF

INTCON

; and clear flag

SLEEP

;Wait for IOC

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