2009 Microchip Technology Inc.
DS40044G-page 67
PIC16F627A/628A/648A
10.6
Comparator Interrupts
The comparator interrupt flag is set whenever there is
a change in the output value of either comparator.
Software will need to maintain information about the
status of the output bits, as read from CMCON<7:6>, to
determine the actual change that has occurred. The
CMIF bit, PIR1<6>, is the comparator interrupt flag.
The CMIF bit must be reset by clearing ‘0’. Since it is
also possible to write a ‘1’ to this register, a simulated
interrupt may be initiated.
The CMIE bit
(PIE1<6>)
and the
PEIE
bit
(INTCON<6>) must be set to enable the interrupt. In
addition, the GIE bit must also be set. If any of these
bits are clear, the interrupt is not enabled, though the
CMIF bit will still be set if an interrupt condition occurs.
The user, in the interrupt service routine, can clear the
interrupt in the following manner:
a)
Any write or read of CMCON. This will end the
mismatch condition.
b)
Clear flag bit CMIF.
A mismatch condition will continue to set flag bit CMIF.
Reading CMCON will end the mismatch condition and
allow flag bit CMIF to be cleared.
10.7
Comparator Operation During
Sleep
When a comparator is active and the device is placed
in Sleep mode, the comparator remains active and the
interrupt is functional if enabled. This interrupt will
wake-up the device from Sleep mode when enabled.
While the comparator is powered-up, higher Sleep
currents than shown in the power-down current
specification will occur. Each comparator that is
operational will consume additional current as shown in
the comparator specifications. To minimize power
consumption while in Sleep mode, turn off the
comparators, CM<2:0> = 111, before entering Sleep. If
the device wakes up from Sleep, the contents of the
CMCON register are not affected.
10.8
Effects of a Reset
A device Reset forces the CMCON register to its Reset
state. This forces the Comparator module to be in the
comparator Reset mode, CM<2:0> = 000. This ensures
that all potential inputs are analog inputs. Device current
is minimized when analog inputs are present at Reset
time. The comparators will be powered-down during the
Reset interval.
10.9
Analog Input Connection
Considerations
A simplified circuit for an analog input is shown in
digital output, they have reverse biased diodes to VDD
and VSS. The analog input therefore, must be between
VSS and VDD. If the input voltage deviates from this
range by more than 0.6V in either direction, one of the
diodes is forward biased and a latch-up may occur. A
maximum
source
impedance
of
10 k
Ω
is
recommended for the analog sources. Any external
component connected to an analog input pin, such as
a capacitor or a Zener diode, should have very little
leakage current.
Note:
If a change in the CMCON register
(C1OUT or C2OUT) should occur when a
read operation is being executed (start of
the Q2 cycle), then the CMIF (PIR1<6>)
interrupt flag may not get set.