PIC18F23K20_10 Datasheet, PDF (289/456 Pages) Microchip Technology – 28/40/44-Pin Flash Microcontrollers with nanoWatt XLP Technology

21.0 VOLTAGE REFERENCES
There are two independent voltage references
available:
• Programmable Comparator Voltage Reference
• 1.2V Fixed Voltage Reference
21.1 Comparator Voltage Reference
The Comparator Voltage Reference module provides
an internally generated voltage reference for the com-
parators. The following features are available:
• Independent from Comparator operation
• Two 16-level voltage ranges
• Output clamped to VSS
• Ratiometric with VDD
• 1.2 Fixed Reference Voltage (FVR)
The CVRCON register (Register 21-1) controls the
Voltage Reference module shown in Figure 21-1.
21.1.1 INDEPENDENT OPERATION
The comparator voltage reference is independent of
the comparator configuration. Setting the CVREN bit of
the CVRCON register will enable the voltage reference
by allowing current to flow in the CVREF voltage divider.
When both the CVREN bit is cleared, current flow in the
CVREF voltage divider is disabled minimizing the power
drain of the voltage reference peripheral.
21.1.2 OUTPUT VOLTAGE SELECTION
The CVREF voltage reference has 2 ranges with 16
voltage levels in each range. Range selection is
controlled by the CVRR bit of the CVRCON register.
The 16 levels are set with the CVR<3:0> bits of the
CVRCON register.
The CVREF output voltage is determined by the
following equations:
EQUATION 21-1: CVREF OUTPUT VOLTAGE
CVRR = 1 (low range):
CVREF = (CVRSRC/24) X CVR<3:0> + VREF-
CVRR = 0 (high range):
CVREF = (CVRSRC/32) X (8 + CVR<3:0>) + VREF-
CVRSRC = VDD or [(VREF+) - (VREF-)]
Note: VREF- is 0 when CVRSS = 0
The full range of VSS to VDD cannot be realized due to
the construction of the module. See Figure 21-1.
PIC18F2XK20/4XK20
21.1.3 OUTPUT CLAMPED TO VSS
The CVREF output voltage can be set to Vss with no
power consumption by configuring CVRCON as
follows:
• CVREN = 0
• CVRR = 1
• CVR<3:0> = 0000
This allows the comparator to detect a zero-crossing
while not consuming additional CVREF module current.
21.1.4 OUTPUT RATIOMETRIC TO VDD
The comparator voltage reference is VDD derived and
therefore, the CVREF output changes with fluctuations in
VDD. The tested absolute accuracy of the Comparator
Voltage Reference can be found in Section 26.0
“Electrical Characteristics”.
21.1.5 VOLTAGE REFERENCE OUTPUT
The CVREF voltage reference can be output to the
device CVREF pin by setting the CVROE bit of the CVR-
CON register to ‘1’. Selecting the reference voltage for
output on the CVREF pin automatically overrides the
digital output buffer and digital input threshold detector
functions of that pin. Reading the CVREF pin when it
has been configured for reference voltage output will
always return a ‘0’.
Due to the limited current drive capability, a buffer must
be used on the voltage reference output for external
connections to CVREF. Figure 21-2 shows an example
buffering technique.
21.1.6 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.
21.1.7 EFFECTS OF A RESET
A device Reset affects the following:
• Comparator voltage reference is disabled
• Fixed voltage reference is disabled
• CVREF is removed from the CVREF pin
• The high-voltage range is selected
• The CVR<3:0> range select bits are cleared
 2010 Microchip Technology Inc.
DS41303G-page 289