PIC18FXX8 Datasheet, PDF (257/402 Pages) Microchip Technology – 28/40-Pin High-Performance, Enhanced Flash Microcontrollers with CAN Module

PIC18FXX8
22.0 COMPARATOR VOLTAGE
REFERENCE MODULE
Note:
The comparator voltage reference is only
available on the PIC18F448 and
PIC18F458.
This module is a 16-tap resistor ladder network that
provides a selectable voltage reference. The resistor
ladder is segmented to provide two ranges of CVREF
values and has a power-down function to conserve
power when the reference is not being used. The
CVRCON register controls the operation of the
reference, as shown in Register 22-1. The block
diagram is shown in Figure 22-1.
The comparator and reference supply voltage can
come from either VDD and VSS, or the external VREF+
and VREF-, that are multiplexed with RA3 and RA2. The
comparator reference supply voltage is controlled by
the CVRSS bit.
22.1 Configuring the Comparator
Voltage Reference
The comparator voltage reference can output 16 distinct
voltage levels for each range. The equations used to
calculate the output of the comparator voltage reference
are as follows.
EQUATION 22-1:
If CVRR = 1:
CVREF = (CVR<3:0>/24) x CVRSRC
where:
CVRSS = 1, CVRSRC = (VREF+) – (VREF-)
CVRSS = 0, CVRSRC = AVDD – AVSS
EQUATION 22-2:
If CVRR = 0:
CVREF = (CVRSRC x 1/4) + (CVR<3:0>/32) x CVRSRC
where:
CVRSS = 1, CVRSRC = (VREF+) – (VREF-)
CVRSS = 0, CVRSRC = AVDD – AVSS
The settling time of the Comparator Voltage Reference
must be considered when changing the RA0/AN0/
CVREF output (see Table 27-4 in Section 27.2 “DC
Characteristics”).
REGISTER 22-1:
CVRCON: COMPARATOR VOLTAGE REFERENCE CONTROL REGISTER
R/W-0 R/W-0 R/W-0 R/W-0 R/W-0 R/W-0 R/W-0 R/W-0
CVREN CVROE CVRR CVRSS CVR3 CVR2 CVR1 CVR0
bit 7
bit 0
bit 7
bit 6
bit 5
bit 4
bit 3-0
CVREN: Comparator Voltage Reference Enable bit
1 = CVREF circuit powered on
0 = CVREF circuit powered down
CVROE: Comparator VREF Output Enable bit
1 = CVREF voltage level is also output on the RA0/AN0/CVREF pin
0 = CVREF voltage is disconnected from the RA0/AN0/CVREF pin
CVRR: Comparator VREF Range Selection bit
1 = 0.00 CVRSRC to 0.625 CVRSRC with CVRSRC/24 step size
0 = 0.25 CVRSRC to 0.719 CVRSRC with CVRSRC/32 step size
CVRSS: Comparator VREF Source Selection bit
1 = Comparator reference source, CVRSRC = (VREF+) – (VREF-)
0 = Comparator reference source, CVRSRC = VDD – VSS
CVR<3:0>: Comparator VREF Value Selection 0 ≤ CVR3:CVR0 ≤ 15 bits
When CVRR = 1:
CVREF = (CVR3:CVR0/24) • (CVRSRC)
When CVRR = 0:
CVREF = 1/4 • (CVRSRC) + (CVR3:CVR0/32) • (CVRSRC)
Legend:
R = Readable bit
-n = Value at POR
W = Writable bit
‘1’ = Bit is set
U = Unimplemented bit, read as ‘0’
‘0’ = Bit is cleared x = Bit is unknown
 2004 Microchip Technology Inc.
DS41159D-page 255