Datasheet

PIC16F684
DS41202F-page 34 © 2007 Microchip Technology Inc.
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.
To use this feature, the RA0 pin is first configured to output
‘1’ to charge the capacitor. Then 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, the device will wake-up and execute the next
instruction. If the GIE bit of the INTCON register is set, the
device will call the interrupt service routine (0004h). See
Section 4.2.3 “Interrupt-on-Change” and
Section 12.4.3 “PORTA Interrupt-on-Change” 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 AN879,
“Using the Microchip Ultra Low-Power
Wake-Up Module” Application Note
(DS00879).
BANKSEL CMCON0 ;
MOVLW H’7’ ;Turn off
MOVWF CMCON0 ;comparators
BANKSEL ANSEL ;
BCF ANSEL,0 ;RA0 to digital I/O
BCF TRISA,0 ;Output high to
BANKSEL PORTA ;
BSF PORTA,0 ;charge capacitor
CALL CapDelay ;
BANKSEL PCON ;
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