PIC18FXX20 Datasheet, PDF (231/366 Pages) Microchip Technology – 64/80-Pin High Performance 1 Mbit Enhanced FLASH Microcontrollers with A/D

PIC18FXX20
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 CVREF values and has a power-down func-
tion 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 VDD or VSS, or the external VREF+ and
VREF- that are multiplexed with RA3 and RA2. The
comparator reference supply voltage is controlled by
the CVRSS bit.
Note:
In order to select external VREF+ and VREF-
supply voltages, the Voltage Reference
Configuration bits (VCFG1:VCFG0) of the
ADCON1 register must be set appropriately.
21.1 Configuring the Comparator
Voltage Reference
The Comparator Voltage Reference can output 16 dis-
tinct voltage levels for each range. 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 = (CVDD x 1/4) + (CVR<3:0>/32) x CVRSRC
The settling time of the Comparator Voltage Reference
must be considered when changing the CVREF output
(Section 26.0).
REGISTER 21-1:
CVRCON REGISTER
R/W-0
CVREN
bit 7
R/W-0
CVROE
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
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 pin
0 = CVREF voltage is disconnected from the RF5/AN10/CVREF 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⁽²⁾
1 = Comparator reference source CVRSRC = VREF+ – VREF-
0 = Comparator reference source CVRSRC = VDD – VSS
CVR3:CVR0: Comparator VREF Value Selection bits (0 ≤ VR3:VR0 ≤ 15)
When CVRR = 1:
CVREF = (CVR<3:0>/ 24) • (CVRSRC)
When CVRR = 0:
CVREF = 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 VREF+ and VREF- supply voltages, the Voltage Refer-
ence Configuration bits (VCFG1:VCFG0) of the ADCON1 register must be set
appropriately.
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
 2003 Microchip Technology Inc.
Advance Information
DS39609A-page 229