PIC18F2682 Datasheet, PDF (265/484 Pages) Microchip Technology – 28/40/44-Pin Enhanced Flash Microcontrollers with ECAN Technology, 10-Bit A/D and nanoWatt Technology

PIC18F2682/2685/4682/4685
21.0 COMPARATOR VOLTAGE
REFERENCE MODULE
Note: Comparators are only available in 40/44-pin
devices (PIC18F4682/4685).
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 21-1.
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.
21.1 Configuring the Comparator
Voltage Reference
The voltage reference module is controlled through the
CVRCON register (Register 21-1). 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 27-3 in Section 27.0 “Electrical
Characteristics”).
REGISTER 21-1: 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 RA0/AN0/CVREF pin
0 = CVREF voltage is disconnected from the RA0/AN0/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
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 TRISA<0> bit setting. If enabled for output, RA2 must also be configured as an
input by setting TRISA<2> to ‘1’.
© 2007 Microchip Technology Inc.
Preliminary
DS39761B-page 263