PIC18F87J90 Datasheet, PDF (305/450 Pages) Microchip Technology – 64/80-Pin, High-Performance Microcontrollers with LCD Driver and nanoWatt Technology

PIC18F87J90 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 CVREF 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 the device VDD/VSS or an external voltage
reference.
23.1 Configuring the Comparator
Voltage Reference
The comparator 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 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:
CVREF = ((CVR<3:0>)/24) x (CVRSRC)
If CVRR = 0:
CVREF = (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
VREF- 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 CVREF
output (see Table 28-3 in Section 28.0 “Electrical
Characteristics”).
REGISTER 23-1: CVRCON: COMPARATOR VOLTAGE REFERENCE CONTROL REGISTER
R/W-0
CVREN
bit 7
R/W-0
CVROE⁽¹⁾
R/W-0
CVRR
R/W-0
CVRSS
R/W-0
CVR3
R/W-0
CVR2
R/W-0
CVR1
R/W-0
CVR0
bit 0
Legend:
R = Readable bit
-n = Value at POR
W = Writable bit
‘1’ = Bit is set
U = Unimplemented bit, read as ‘0’
‘0’ = Bit is cleared
x = Bit is unknown
bit 7
bit 6
bit 5
bit 4
bit 3-0
CVREN: Comparator Voltage Reference Enable bit
1 = CVREF circuit powered on
0 = CVREF circuit powered down
CVROE: Comparator VREF Output Enable bit⁽¹⁾
1 = CVREF voltage level is also output on the RF5/AN10/CVREF/SEG23/C1INB pin
0 = CVREF voltage is disconnected from the RF5/AN10/CVREF/SEG23/C1INB pin
CVRR: Comparator VREF Range Selection bit
1 = 0 to 0.667 CVRSRC, with CVRSRC/24 step size (low range)
0 = 0.25 CVRSRC to 0.75 CVRSRC, with CVRSRC/32 step size (high range)
CVRSS: Comparator VREF Source Selection bit
1 = Comparator reference source, CVRSRC = (VREF+) – (VREF-)
0 = Comparator reference source, CVRSRC = VDD – VSS
CVR<3:0>: Comparator VREF Value Selection bits (0  (CVR<3:0>)  15)
When CVRR = 1:
CVREF = ((CVR<3:0>)/24)  (CVRSRC)
When CVRR = 0:
CVREF = (CVRSRC/4) + ((CVR<3:0>)/32)  (CVRSRC)
Note 1: CVROE overrides the TRISF<5> bit setting.
 2010 Microchip Technology Inc.
DS39933D-page 305