2003 Microchip Technology Inc.
DS39582B-page 141
PIC16F87XA
13.0
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
‘
110
’
. A programmable register controls the function of
the reference generator. Register 13-1 lists the bit
functions of the CVRCON register.
As shown in Figure 13-1, 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
– V
SAT
,
where V
SAT
is the saturation voltage of the power
switch transistor. This reference will only be as
accurate as the values of CV
RSRC
and V
SAT
.
The output of the reference generator may be con-
nected to the RA2/AN2/V
REF
-/CV
REF
pin. 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 13-1:
CVRCON 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 V
REF
Output Enable bit
1
= CV
REF
voltage level is output on RA2/AN2/V
REF
-/CV
REF
pin
0
= CV
REF
voltage level is disconnected from RA2/AN2/V
REF
-/CV
REF
pin
bit 5
CVRR: Comparator V
REF
Range Selection bit
1
= 0 to 0.75 CV
RSRC
, with CV
RSRC
/24 step size
0
= 0.25 CV
RSRC
to 0.75 CV
RSRC
, with CV
RSRC
/32 step size
bit 4
Unimplemented: Read as ‘
0
’
bit 3-0
CVR3:CVR0: Comparator V
REF
Value Selection bits 0
≤
VR3:VR0
≤
15
When CVRR =
1
:
CV
REF
= (VR<3:0>/ 24)
•
(CV
RSRC
)
When CVRR =
0
:
CV
REF
= 1/4
•
(CV
RSRC
) + (VR3:VR0/ 32)
•
(CV
RSRC
)
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
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