Information

© 2008 Microchip Technology Inc. DS39646C-page 287
PIC18F8722 FAMILY
23.0 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 23-1.
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.
23.1 Configuring the Comparator
Voltage Reference
The voltage reference module is controlled through the
CVRCON register (Register 23-1). 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 = ((CVR3:CVR0)/24) x (CVRSRC)
If CVRR =
0:
CV
REF =(CVRSRC/4) + ((CVR3:CVR0)/32) x
(CV
RSRC)
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 28-3 in Section 28.0 “Electrical
Characteristics”).
REGISTER 23-1: 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 V
REF Output Enable bit
(1)
1 =CVREF voltage level is also output on the RF5/AN10/CVREF pin
0 =CV
REF voltage is disconnected from the RF5/AN10/CVREF pin
bit 5 CVRR: Comparator VREF Range Selection bit
1 = 0 to 0.667 CVRSRC, 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 V
REF Source Selection bit
1 = Comparator reference source, CV
RSRC = (VREF+) – (VREF-)
0 = Comparator reference source, CV
RSRC = AVDD – AVSS
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) x (CVRSRC)
When CVRR =
0:
CV
REF = (CVRSRC/4) + ((CVR<3:0>)/32) x (CVRSRC)
Note 1: CVROE overrides the TRISF<5> bit setting.