Datasheet

 2010 Microchip Technology Inc. Preliminary DS39960B-page 369
PIC18F87K22 FAMILY
24.5 Comparator Control and
Configuration
Each comparator has up to eight possible combina-
tions of inputs: up to four external analog inputs and
one of two internal voltage references.
All of the comparators allow a selection of the signal
from pin, CxINA, or the voltage from the comparator
reference (CV
REF) on the non-inverting channel. This is
compared to either CxINB, CxINC, C2INB/C2IND or
the microcontroller’s fixed internal reference voltage
(V
BG, 1.024V nominal) on the inverting channel. The
comparator inputs and outputs are tied to fixed I/O pins,
defined in Table 24-1. The available comparator config-
urations and their corresponding bit settings are shown
in Figure 24-4.
TABLE 24-1: COMPARATOR INPUTS AND
OUTPUTS
24.5.1 COMPARATOR ENABLE AND
INPUT SELECTION
Setting the CON bit of the CMxCON register
(CMxCON<7>) enables the comparator for operation.
Clearing the CON bit disables the comparator, resulting
in minimum current consumption.
The CCH<1:0> bits in the CMxCON register
(CMxCON<1:0>) direct either one of three analog input
pins, or the Internal Reference Voltage (VBG), to the
comparator, V
IN-. Depending on the comparator
operating mode, either an external or internal voltage
reference may be used. For external analog pins that
are unavailable in 64-pin devices (C1INC, C2INC and
C2IND), the corresponding configurations that use
them as inputs are unavailable.
The analog signal present at V
IN- is compared to the
signal at V
IN+ and the digital output of the comparator
is adjusted accordingly.
The external reference is used when CREF =
0
(CMxCON<2>) and VIN+ is connected to the CxINA
pin. When external voltage references are used, the
comparator module can be configured to have the
reference sources externally. The reference signal
must be between V
SS and VDD and can be applied to
either pin of the comparator.
The comparator module also allows the selection of an
internally generated voltage reference (CV
REF) from
the Comparator Voltage Reference module. This
module is described in more detail in
Section 25.0
“Comparator Voltage Reference Module”
. The
reference from the comparator voltage reference
module is only available when CREF =
1. In this mode,
the internal voltage reference is applied to the
comparator’s V
IN+ pin.
24.5.2 COMPARATOR ENABLE AND
OUTPUT SELECTION
The comparator outputs are read through the CMSTAT
register. The CMSTAT<5> bit reads the Comparator 1
output, CMSTAT<6> reads Comparator 2 output and
CMSTAT<7> reads Comparator 3 output. These bits
are read-only.
The comparator outputs may also be directly output to
the RF2, RF1 and RG1 I/O pins by setting the COE bit
(CMxCON<6>). When enabled, multiplexers in the
output path of the pins switch to the output of the com-
parator. While in this mode, the TRISF<2:1> and
TRISG<1> bits still function as the digital output enable
bits for the RF2, RF1 and RG1 pins.
By default, the comparator’s output is at logic high
whenever the voltage on V
IN+ is greater than on VIN-.
The polarity of the comparator outputs can be inverted
using the CPOL bit (CMxCON<5>).
The uncertainty of each of the comparators is related to
the input offset voltage and the response time given in
the specifications, as discussed in
Section 24.2
“Comparator Operation”
.
Comparator Input or Output I/O Pin
1
C1INA (V
IN+) RF6
C1INB (V
IN-) RF5
C1INC
(1)
(VIN-) RH6
C2INB (V
IN-) RF3
C1OUT RF2
2
C2INA (V
IN+) RF4
C2INB (V
IN-) RF3
C2INC
(1)
(VIN-) RH4
C2IND
(1)
(VIN-) RH5
C2OUT RF1
3
C3INA (V
IN+) RG2
C3INB (V
IN-) RG3
C3INC (V
IN-) RG4
C2INB (V
IN-) RF3
C3OUT RG1
Note 1: C1INC, C2INC and C2IND are all
unavailable for 64-pin devices
(PIC18F6XK22).
Note: The comparator input pin selected by
CCH<1:0> must be configured as an input
by setting both the corresponding TRISF,
TRISG or TRISH bit and the corresponding
ANSELx bit in the ANCONx register.