Datasheet

 2003-2013 Microchip Technology Inc. DS39609C-page 229
PIC18F6520/8520/6620/8620/6720/8720
21.0 COMPARATOR VOLTAGE
REFERENCE MODULE
The comparator voltage reference is a 16-tap resistor
ladder network that provides a selectable voltage refer-
ence. 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 CVRCON register controls the operation of the
reference as shown in Register 21-1. The block diagram
is given in Figure 21-1.
The comparator reference supply voltage can come
from either V
DD or VSS, or the external VREF+ and
V
REF- that are multiplexed with RA3 and RA2. The
comparator reference supply voltage is controlled by
the CVRSS bit.
21.1 Configuring the Comparator
Voltage Reference
The comparator voltage reference can output 16 distinct
voltage levels for each range. 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 x1/4)+(CVR<3:0>/32)xCVRSRC
The settling time of the comparator voltage reference
must be considered when changing the CV
REF output
(Section 26.0 “Electrical Characteristics”).
REGISTER 21-1: CVRCON REGISTER
Note: In order to select external VREF+ and VREF-
supply voltages, the Voltage Reference Con-
figuration bits (VCFG1:VCFG0) of the
ADCON1 register must be set appropriately.
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 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 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 = 0.00 CVRSRC 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 VREF Source Selection bit
(2)
1 = Comparator reference source CVRSRC = VREF+ – VREF-
0 = Comparator reference source CV
RSRC = VDD – VSS
bit 3-0 CVR3:CVR0: Comparator VREF Value Selection bits (0  VR3:VR0  15)
When CVRR =
1:
CV
REF = (CVR<3:0>/24)  (CVRSRC)
When CVRR =
0:
CV
REF = 1/4  (CVRSRC) + (CVR3:CVR0/32)  (CVRSRC)
Note 1: If enabled for output, RF5 must also be configured as an input by setting TRISF<5>
to ‘1’.
2: In order to select external V
REF+ and VREF- supply voltages, the Voltage
Reference Configuration bits (VCFG1:VCFG0) of the ADCON1 register must be
set appropriately.
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