Datasheet

© 2007 Microchip Technology Inc. DS41232D-page 51
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 of the PCON
register. 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 V
IL, an interrupt will be generated which will cause
the device to wake-up. Depending on the state of the
GIE bit of the INTCON register, 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 more information, refer to the
Application Note AN879, “Using the
Microchip Ultra Low-Power Wake-up
Module” (DS00879).
BANKSEL PORTA ;
BSF PORTA,0 ;Set RA0 data latch
MOVLW H’7’ ;Turn off
MOVWF CMCON0 ; comparators
BANKSEL TRISA ;
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
NOP ;