Datasheet

© 2009 Microchip Technology Inc. DS39775C-page 343
PIC18F87J50 FAMILY
23.6 Comparator Interrupts
The comparator interrupt flag is set whenever any of
the following occurs:
- Low-to-high transition of the comparator
output
- High-to-low transition of the comparator
output
- Any change in the comparator output.
The comparator interrupt selection is done by the
EVPOL1:EVPOL0 bits in the CMxCON register
(CMxCON<4:3>).
In order to provide maximum flexibility, the output of the
comparator may be inverted using the CPOL bit in the
CMxCON register (CMxCON<5>). This is functionally
identical to reversing the inverting and non-inverting
inputs of the comparator for a particular mode.
An interrupt is generated on the low-to-high or high-to-
low transition of the comparator output. This mode of
interrupt generation is dependent on EVPOL<1:0> in
the CMxCON register. When EVPOL<1:0> = 01 or 10,
the interrupt is generated on a low-to-high or high-to-
low transition of the comparator output. Once the
interrupt is generated, it is required to clear the interrupt
flag by software.
When EVPOL<1:0> = 11, the comparator interrupt flag
is set whenever there is a change in the output value of
either comparator. Software will need to maintain infor-
mation about the status of the output bits, as read from
CMSTAT<1:0>, to determine the actual change that
occurred. The CMxIF bits (PIR2<6:5>) are the Compar-
ator Interrupt Flags. The CMxIF bits must be reset by
clearing them. Since it is also possible to write a ‘1’ to
this register, a simulated interrupt may be initiated.
Table 23-2 shows the interrupt generation with respect
to comparator input voltages and EVPOL bit settings.
Both the CMxIE bits (PIE2<6:5>) and the PEIE bit
(INTCON<6>) must be set to enable the interrupt. In
addition, the GIE bit (INTCON<7>) must also be set. If
any of these bits are clear, the interrupt is not enabled,
though the CMxIF bits will still be set if an interrupt
condition occurs. A simplified diagram of the interrupt
section is shown in Figure 23-3.
TABLE 23-2: COMPARATOR INTERRUPT GENERATION
CPOL EVPOL<1:0>
Comparator
Input Change
COUTx Transition
Interrupt
Generated
0
00
V
IN+ > VIN- Low-to-High No
V
IN+ < VIN- High-to-Low No
01
VIN+ > VIN- Low-to-High Yes
VIN+ < VIN- High-to-Low No
10
V
IN+ > VIN- Low-to-High No
VIN+ < VIN- High-to-Low Yes
11
VIN+ > VIN- Low-to-High Yes
V
IN+ < VIN- High-to-Low Yes
1
00
V
IN+ > VIN- High-to-Low No
V
IN+ < VIN- Low-to-High No
01
V
IN+ > VIN- High-to-Low No
VIN+ < VIN- Low-to-High Yes
10
V
IN+ > VIN- High-to-Low Yes
V
IN+ < VIN- Low-to-High No
11
VIN+ > VIN- High-to-Low Yes
V
IN+ < VIN- Low-to-High Yes