PIC18F45J10 FAMILY
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:
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.
If CVRR = 1:
CVREF = ((CVR<3:0>)/24) x CVRSRC
A block diagram of the module is 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.
If CVRR = 0:
CVREF = (CVRSRC x 1/4) + (((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>).
20.1 Configuring the Comparator
Voltage Reference
The settling time of the comparator voltage reference
must be considered when changing the CVREF
output (see Table 24-3 in Section 24.0 “Electrical
Characteristics”).
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
REGISTER 20-1: CVRCON: COMPARATOR VOLTAGE REFERENCE CONTROL REGISTER
R/W-0
R/W-0
CVROE(1)
R/W-0
CVRR
R/W-0
R/W-0
CVR3
R/W-0
CVR2
R/W-0
CVR1
R/W-0
CVR0
CVREN
CVRSS
bit 7
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 RA2/AN2/VREF-/CVREF pin
0= CVREF voltage is disconnected from the RA2/AN2/VREF-/CVREF 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 TRISA<2> bit setting.
© 2009 Microchip Technology Inc.
DS39682E-page 231
MICROCHIP [ MICROCHIP ]
