Datasheet

© 2007 Microchip Technology Inc. Preliminary DS39625C-page 263
PIC18F2585/2680/4585/4680
21.0 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 21-1.
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.
21.1 Configuring the Comparator
Voltage Reference
The voltage reference module is controlled through the
CVRCON register (Register 21-1). 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 CVREF Selection bits
(CVR3:CVR0), 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 = ((CVR3:CVR0)/24) x CVRSRC
If CVRR = 0:
CV
REF = (CVDD x 1/4) + (((CVR3:CVR0)/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 27-3 in Section 27.0 “Electrical
Characteristics”).
REGISTER 21-1: 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
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
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’.
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 V
REF Source Selection bit
1 = Comparator reference source, CV
RSRC = (VREF+) – (VREF-)
0 = Comparator reference source, CV
RSRC = VDD – VSS
bit 3-0 CVR3:CVR0: Comparator VREF Value Selection bits (0 ≤ (CVR3:CVR0) ≤ 15)
When CVRR = 1:
CV
REF = ((CVR3:CVR0)/24) • (CVRSRC)
When CVRR = 0:
CV
REF = (CVRSRC/4) + ((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