Datasheet

© 2008 Microchip Technology Inc. DS41262E-page 63
PIC16F631/677/685/687/689/690
4.2.4 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.
Follow these steps to use this feature:
a) Charge the capacitor on RA0 by configuring the
RA0 pin to output (= 1).
b) Configure RA0 as an input.
c) Enable interrupt-on-change for RA0.
d) Set the ULPWUE bit of the PCON register to
begin the capacitor discharge.
e) Execute a SLEEP instruction.
When the voltage on RA0 drops below V
IL, an interrupt
will be generated which will cause the device to
wake-up and execute the next instruction. If the GIE bit
of the INTCON register is set, the device will then call
the interrupt vector (0004h). See Section 4.4.2 “Inter-
rupt-on-change” and Section 14.3.3
“PORTA/PORTB 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.
A series resistor between RA0 and the external
capacitor provides overcurrent protection for the
RA0/AN0/C1IN+/ICSPDAT/ULPWU 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 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
BSF STATUS,RP1 ;Bank 2
BCF ANSEL,0 ;RA0 to digital I/O
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
BCF STATUS,RP0 ;Bank 0
SLEEP ;Wait for IOC
NOP ;