PIC12F635/PIC16F636/639
4.2.3
ULTRA LOW-POWER WAKE-UP
EXAMPLE 4-2:
ULTRA LOW-POWER
WAKE-UP INITIALIZATION
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.
BANKSELPORTA
;
BSF
MOVLW
MOVWF
PORTA,0
H’7’
CMCON0
;Set RA0 data latch
;Turn off
; comparators
;
;Output high to
; charge capacitor
BANKSELTRISA
BCF
CALL
BSF
TRISA,0
CapDelay
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 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
PCON,ULPWUE ;Enable ULP Wake-up
BSF
IOCA,0
;Select RA0 IOC
;RA0 to input
B’10001000’ ;Enable interrupt
BSF
TRISA,0
MOVLW
MOVWF
SLEEP
NOP
INTCON
; and clear flag
;Wait for IOC
;
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.
Note:
For more information, refer to the
Application Note AN879, “Using the
Microchip Ultra Low-Power Wake-up
Module” (DS00879).
© 2007 Microchip Technology Inc.
DS41232D-page 51
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