PIC16F688
7.3.3
COMPARATOR INPUT SWITCH
7.5
Comparator Interrupt Operation
The inverting input of the comparators may be switched
between two analog pins in the following modes:
The comparator interrupt flag is set whenever there is
a change in the output value of the comparator.
Changes are recognized by means of a mismatch
circuit which consists of two latches and an exclusive-
or gate (see Figure 7-2 and Figure 7-3). One latch is
updated with the comparator output level when the
CMCON0 register is read. This latch retains the value
until the next read of the CMCON0 register or the
occurrence of a Reset. The other latch of the mismatch
circuit is updated on every Q1 system clock. A
mismatch condition will occur when a comparator
output change is clocked through the second latch on
the Q1 clock cycle. The mismatch condition will persist,
holding the CxIF bit of the PIR1 register true, until either
the CMCON0 register is read or the comparator output
returns to the previous state.
• CM<2:0> = 001(Comparator C1 only)
• CM<2:0> = 010(Comparators C1 and C2)
In the above modes, both pins remain in analog mode
regardless of which pin is selected as the input. The CIS
bit of the CMCON0 register controls the comparator
input switch.
7.4
Comparator Response Time
The comparator output is indeterminate for a period of
time after the change of an input source or the selection
of a new reference voltage. This period is referred to as
the response time. The response time of the
comparator differs from the settling time of the voltage
reference. Therefore, both of these times must be
considered when determining the total response time
to a comparator input change. See the Comparator and
Voltage Reference specifications in Section 14.0
“Electrical Specifications” for more details.
Note:
A write operation to the CMCON0 register
will also clear the mismatch condition
because all writes include
a
read
operation at the beginning of the write
cycle.
Software will need to maintain information about the
status of the comparator output to determine the actual
change that has occurred.
The CxIF bit of the PIR1 register is the comparator
interrupt flag. This bit must be reset in software by
clearing it to ‘0’. Since it is also possible to write a ‘1’ to
this register, a simulated interrupt may be initiated.
The CxIE bit of the PIE1 register and the PEIE and GIE
bits of the INTCON register must all be set to enable
comparator interrupts. If any of these bits are cleared,
the interrupt is not enabled, although the CxIF bit of the
PIR1 register 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 read or write of CMCON0. This will end the
mismatch condition. See Figures 7-6 and 7-7
b) Clear the CxIF interrupt flag.
A persistent mismatch condition will preclude clearing
the CxIF interrupt flag. Reading CMCON0 will end the
mismatch condition and allow the CxIF bit to be
cleared.
© 2007 Microchip Technology Inc.
DS41203D-page 59
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