Datasheet

 2010 Microchip Technology Inc. Preliminary DS39979A-page 299
PIC18F87J72 FAMILY
24.0 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 of the module is shown in Figure 24-1.
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.
24.1 Configuring the Comparator
Voltage Reference
The comparator voltage reference module is controlled
through the CVRCON register (Register 24-1). 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 (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 = (CVRSRC/4) + ((CVR<3:0>)/32) x (CVRSRC)
The comparator reference supply voltage can come
from either VDD 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 29-3 in Section 29.0 “Electrical
Characteristics”).
REGISTER 24-1: 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 RF5/AN10/CVREF/SEG23/C1INB pin
0 =CV
REF voltage is disconnected from the RF5/AN10/CVREF/SEG23/C1INB pin
bit 5 CVRR: Comparator V
REF Range Selection bit
1 = 0 to 0.667 CV
RSRC, with CVRSRC/24 step size (low range)
0 = 0.25 CV
RSRC to 0.75 CVRSRC, with CVRSRC/32 step size (high range)
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 TRISF<5> bit setting.