Datasheet

 2009-2011 Microchip Technology Inc. DS39960D-page 371
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 Reference Voltages.
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 Refer-
ence Voltage 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 reference voltages 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 reference voltage from the Com-
parator Voltage Reference (CV
REF) module. This
module is described in more detail in
Section 25.0
“Comparator Voltage Reference Module”
. The
reference from the comparator reference voltage
module is only available when CREF =
1. In this mode,
the Internal Reference Voltage 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.