PIC16F631/677/685/687/689/690
FIGURE 8-4:
COMPARATOR
8.4
Comparator Interrupt Operation
INTERRUPT TIMING W/O
CMxCON0 READ
The comparator interrupt flag can be 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 8-2 and Figure 8-3). One latch is
updated with the comparator output level when the
CMxCON0 register is read. This latch retains the value
until the next read of the CMxCON0 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. At this point the two mismatch
latches have opposite output levels which is detected
by the exclusive-or gate and fed to the interrupt
circuitry. The mismatch condition persists until either
the CMxCON0 register is read or the comparator
output returns to the previous state.
Q1
Q3
CxIN+
TRT
Cxout
Set CxIF (edge)
CxIF
reset by software
FIGURE 8-5:
COMPARATOR
INTERRUPT TIMING WITH
CMxCON0 READ
Q1
Q3
CxIN+
TRT
Note 1: A write operation to the CMxCON0
register will also clear the mismatch
condition because all writes include a read
operation at the beginning of the write
cycle.
Cxout
Set CxIF (edge)
CxIF
cleared by CMxCON0 read
reset by software
2: Comparator interrupts will operate correctly
regardless of the state of CxOE.
The comparator interrupt is set by the mismatch edge
and not the mismatch level. This means that the inter-
rupt flag can be reset without the additional step of
reading or writing the CMxCON0 register to clear the
mismatch registers. When the mismatch registers are
cleared, an interrupt will occur upon the comparator’s
return to the previous state, otherwise no interrupt will
be generated.
Note 1: If a change in the CMxCON0 register
(CxOUT) should occur when a read
operation is being executed (start of the
Q2 cycle), then the CxIF of the PIR1
register interrupt flag may not get set.
2: When either comparator is first enabled,
bias circuitry in the Comparator module
may cause an invalid output from the
comparator until the bias circuitry is
stable. Allow about 1 μs for bias settling
then clear the mismatch condition and
Software will need to maintain information about the
status of the comparator output, as read from the
CMxCON0 register, or CM2CON1 register, to determine
the actual change that has occurred.
interrupt
flags
before
enabling
comparator interrupts.
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, an interrupt can be generated.
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.
DS41262D-page 96
© 2007 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
