Datasheet

PIC16F688
DS41203E-page 36 © 2009 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 consump-
tion. 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 4.2.3 “INTERRUPT-ON-
CHANGE” and Section 11.3.3 “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 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 ANSEL ;
BCF ANSEL,0 ;RA0 to digital I/O
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 ;