Datasheet

© 2009 Microchip Technology Inc. DS39637D-page 269
PIC18F2480/2580/4480/4580
22.0 COMPARATOR VOLTAGE
REFERENCE MODULE
The comparator voltage reference is a 16-tap resistor
ladder network that provides a selectable reference
voltage. Although its primary purpose is to provide a
reference for the analog comparators, it may also be
used independently of them.
A block diagram is of the module shown in
Figure 22-1.The resistor ladder is segmented to
provide two ranges of CV
REF values and has a
power-down function to conserve power when the
reference is not being used. The module’s supply
reference can be provided from either device V
DD/VSS
or an external voltage reference.
22.1 Configuring the Comparator
Voltage Reference
The voltage reference module is controlled through the
CVRCON register (Register 22-1). The comparator
voltage reference provides two ranges of output voltage,
each with 16 distinct levels. The range to be used is
selected by the CVRR bit (CVRCON<5>). The primary
difference between the ranges is the size of the steps
selected by the CV
REF Selection bits (CVR<3:0>), with
one range offering finer resolution. The equations used
to calculate the output of the comparator voltage
reference are as follows:
If CVRR =
1:
CV
REF = ((CVR<3:0>)/24) x CVRSRC
If CVRR = 0:
CV
REF = (CVDD x 1/4) + (((CVR<3:0>)/32) x
CV
RSRC)
The comparator reference supply voltage can come
from either V
DD and VSS, or the external VREF+ and
V
REF- that are multiplexed with RA2 and RA3. The
voltage source is selected by the CVRSS bit
(CVRCON<4>).
The settling time of the comparator voltage reference
must be considered when changing the CV
REF output
(see Table 28-3 in Section 28.0 “Electrical
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
(1)
CVRR CVRSS CVR3 CVR2 CVR1 CVR0
bit 7 bit 0
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
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)
1 =CVREF 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 CV
RSRC to 0.75 CVRSRC, with CVRSRC/24 step size
0 = 0.25 CV
RSRC to 0.75 CVRSRC, with CVRSRC/32 step size
bit 4 CVRSS: Comparator VREF Source Selection bit
1 = Comparator reference source, CVRSRC = (VREF+) – (VREF-)
0 = Comparator reference source, CV
RSRC = VDD – VSS
bit 3-0 CVR<3:0>: Comparator VREF Value Selection bits (0 ≤ (CVR<3:0>) ≤ 15)
When CVRR =
1:
CV
REF = ((CVR<3:0>)/24) • (CVRSRC)
When CVRR =
0:
CV
REF = (CVRSRC/4) + ((CVR<3:0>)/32) • (CVRSRC)
Note 1: CVROE overrides the TRISA<0> bit setting. If enabled for output, RA2 must also be configured as an
input by setting TRISA<2> to ‘1’.