Datasheet

© 2006 Microchip Technology Inc. DS41159E-page 255
PIC18FXX8
22.0 COMPARATOR VOLTAGE
REFERENCE MODULE
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 V
DD and VSS, or the external VREF+
and V
REF-, 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:
EQUATION 22-2:
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
Note: The comparator voltage reference is only
available on the PIC18F448 and
PIC18F458.
If CVRR = 1:
CV
REF = (CVR<3:0>/24) x CVRSRC
where:
CVRSS = 1, CVRSRC = (VREF+) – (VREF-)
CVRSS = 0, CV
RSRC = AVDD – AVSS
If CVRR = 0:
CV
REF = (CVRSRC x 1/4) + (CVR<3:0>/32) x CVRSRC
where:
CVRSS = 1, CV
RSRC = (VREF+) – (VREF-)
CVRSS = 0, CV
RSRC = AVDD – AVSS
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 CVREN: Comparator Voltage Reference Enable bit
1 =CV
REF circuit powered on
0 =CV
REF circuit powered down
bit 6 CVROE: Comparator VREF Output Enable bit
1 =CV
REF voltage level is also output on the RA0/AN0/CVREF pin
0 =CV
REF voltage is disconnected from the RA0/AN0/CVREF pin
bit 5 CVRR: Comparator V
REF Range Selection bit
1 =0.00 CVRSRC to 0.625 CVRSRC with CVRSRC/24 step size
0 =0.25 CV
RSRC to 0.719 CVRSRC with CVRSRC/32 step size
bit 4 CVRSS: Comparator V
REF Source Selection bit
1 = Comparator reference source, CV
RSRC = (VREF+) – (VREF-)
0 = Comparator reference source, CV
RSRC = VDD – VSS
bit 3-0 CVR<3:0>: Comparator VREF Value Selection 0 ≤ CVR3:CVR0 ≤ 15 bits
When CVRR =
1:
CV
REF = (CVR3:CVR0/24) • (CVRSRC)
When CVRR =
0:
CV
REF = 1/4 • (CVRSRC) + (CVR3:CVR0/32) • (CVRSRC)
Legend:
R = Readable bit W = Writable bit U = Unimplemented bit, read as ‘0’
-n = Value at POR ‘1’ = Bit is set ‘0’ = Bit is cleared x = Bit is unknown