Datasheet

PIC18F2682/2685/4682/4685
DS39761C-page 266 © 2009 Microchip Technology Inc.
FIGURE 21-1: COMPARATOR VOLTAGE REFERENCE BLOCK DIAGRAM
21.2 Voltage Reference Accuracy/Error
The full range of voltage reference cannot be realized
due to the construction of the module. The transistors
on the top and bottom of the resistor ladder network
(Figure 21-1) keep CV
REF from approaching the
reference source rails. The voltage reference is derived
from the reference source; therefore, the CVREF output
changes with fluctuations in that source. The tested
absolute accuracy of the voltage reference can be
found in Section 27.0 “Electrical Characteristics”.
21.3 Operation During Sleep
When the device wakes up from Sleep through an
interrupt or a Watchdog Timer time-out, the contents of
the CVRCON register are not affected. To minimize
current consumption in Sleep mode, the voltage
reference should be disabled.
21.4 Effects of a Reset
A device Reset disables the voltage reference by
clearing bit CVREN (CVRCON<7>). This Reset also
disconnects the reference from the RA0 pin by clearing
bit, CVROE (CVRCON<6>), and selects the high-
voltage range by clearing bit, CVRR (CVRCON<5>).
The CVR value select bits are also cleared.
21.5 Connection Considerations
The voltage reference module operates independently
of the comparator module. The output of the reference
generator may be connected to the RA0 pin if the
TRISA<0> bit and the CVROE bit are both set. Enabling
the voltage reference output onto the RA0 pin, with an
input signal present, will increase current consumption.
Connecting RA0 as a digital output with CVRSS enabled
will also increase current consumption.
The RA0 pin can be used as a simple D/A output with
limited drive capability. Due to the limited current drive
capability, a buffer must be used on the voltage
reference output for external connections to V
REF.
Figure 21-2 shows an example buffering technique.
16-to-1 MUX
CVR3:CVR0
8R
R
CVREN
CVRSS = 0
V
DD
VREF+
CVRSS = 1
8R
CVRSS = 0
V
REF-
CVRSS = 1
R
R
R
R
R
R
16 Steps
CVRR
CVREF