Datasheet

 2002-2013 Microchip Technology Inc. DS30487D-page 127
PIC16F87/88
14.0 COMPARATOR VOLTAGE
REFERENCE MODULE
The comparator voltage reference generator is a 16-tap
resistor ladder network that provides a fixed voltage
reference when the comparators are in mode ‘010’. A
programmable register controls the function of the refer-
ence generator. Register 14-1 lists the bit functions of
the CVRCON register.
As shown in Figure 14-1, the resistor ladder is seg-
mented to provide two ranges of CV
REF values and has
a power-down function to conserve power when the
reference is not being used. The comparator reference
supply voltage (also referred to as CV
RSRC) comes
directly from V
DD. It should be noted, however, that the
voltage at the top of the ladder is CV
RSRC – VSAT,
where V
SAT is the saturation voltage of the power
switch transistor. This reference will only be as
accurate as the values of CVRSRC and VSAT.
The output of the reference generator may be
connected to the RA2/AN2/CV
REF/VREF- pin (VREF- is
available on the PIC16F88 device only). This can be
used as a simple D/A function by the user if a very high-
impedance load is used. The primary purpose of this
function is to provide a test path for testing the
reference generator function.
REGISTER 14-1: CVRCON: COMPARATOR VOLTAGE REFERENCE CONTROL REGISTER
(ADDRESS 9Dh)
R/W-0 R/W-0 R/W-0 U-0 R/W-0 R/W-0 R/W-0 R/W-0
CVREN CVROE CVRR
— CVR3 CVR2 CVR1 CVR0
bit 7 bit 0
bit 7 CVREN: Comparator Voltage Reference Enable bit
1 = CV
REF circuit powered on
0 = CV
REF circuit powered down
bit 6 CVROE: Comparator VREF Output Enable bit
1 = CVREF voltage level is output on the RA2/AN2/CVREF/VREF- pin
(1)
0 = CVREF voltage level is disconnected from the RA2/AN2/CVREF/VREF- pin
(1)
bit 5 CVRR: Comparator VREF Range Selection bit
(1)
1 = 0.00 CVRSRC to 0.625 CVRSRC with CVRSRC/24 step size
0 = 0.25 CV
RSRC to 0.72 CVRSRC with CVRSRC/32 step size
bit 4 Unimplemented: Read as ‘0’
bit 3-0 CVR<3:0>: Comparator VREF Value Selection 0  VR3:VR0  15 bits
(1)
When CVRR = 1:
CV
REF = (VR<3:0>/24)  (CVRSRC)
When CVRR = 0:
CV
REF = 1/4  (CVRSRC) + (VR3:VR0/32)  (CVRSRC)
Note 1: V
REF is available on the PIC16F88 device only.
Legend:
R = Readable bit W = Writable bit U = Unimplemented bit, read as ‘0’
-n = Value at POR ‘1’ = Bit is set ‘0’ = Bit is cleared x = Bit is unknown