PIC18F6390 Datasheet, PDF (249/412 Pages) Microchip Technology – 64/80-Pin Flash Microcontrollers with LCD Driver and nanoWatt Technology

PIC18F6390/6490/8390/8490
20.0 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 20-1.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 device VDD/VSS,
or an external voltage reference.
20.1 Configuring the Comparator
Voltage Reference
The voltage reference module is controlled through the
CVRCON register (Register 20-1). 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:
CVREF = ((CVR3:CVR0)/24) x CVRSRC
If CVRR = 0:
CVREF = (CVDD x 1/4) + (((CVR3:CVR0)/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 26-3 in Section 26.0 “Electrical
Characteristics”).
REGISTER 20-1:
CVRCON: COMPARATOR VOLTAGE REFERENCE CONTROL REGISTER
R/W-0 R/W-0 R/W-0
CVREN CVROE⁽¹⁾ CVRR
R/W-0
CVRSS
R/W-0
CVR3
R/W-0
CVR2
R/W-0
CVR1
R/W-0
CVR0
bit 7
bit 0
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 pin
0 = CVREF voltage is disconnected from the RF5/AN10/CVREF/SEG23 pin
CVRR: Comparator VREF Range Selection bit
1 = 0.00 CVRSRC to 0.75 CVRSRC, with CVRSRC/24 step size
0 = 0.25 CVRSRC to 0.75 CVRSRC, with CVRSRC/32 step size
CVRSS: Comparator VREF Source Selection bit
1 = Comparator reference source, CVRSRC = (VREF+) – (VREF-)
0 = Comparator reference source, CVRSRC = VDD – VSS
CVR3:CVR0: Comparator VREF Value Selection bits (0 ≤ (CVR3:CVR0) ≤ 15)
When CVRR = 1:
CVREF = ((CVR3:CVR0)/24) • (CVRSRC)
When CVRR = 0:
CVREF = (CVRSRC/4) + ((CVR3:CVR0)/32) • (CVRSRC)
Note 1: CVROE overrides the TRISF<5> bit setting if enabled for output; RF5 must also be
configured as an input by setting TRISF<5> to ‘1’.
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
 2004 Microchip Technology Inc.
Preliminary
DS39629B-page 247