PIC18F2220/2320/4220/4320
21.0
COMPARATOR VOLTAGE
REFERENCE MODULE
21.1
Configuring the Comparator
Voltage Reference
The comparator voltage reference is a 16-tap resistor
ladder network that provides a selectable voltage refer-
ence. The resistor ladder is segmented to provide two
ranges of CV
REF
values and has a power-down func-
tion to conserve power when the reference is not being
used. The CVRCON register controls the operation of
the reference as shown in Register 21-1. The block
diagram is given in Figure 21-1.
The comparator reference supply voltage comes from
V
DD
and V
SS
.
The comparator voltage reference can output 16 distinct
voltage levels for each range. The equations used to cal-
culate the output of the comparator voltage reference
are as follows:
EQUATION 21-1:
If CVRR =
1:
V
DD
CV
REF
= (CVR<3:0>)
•
24
If CVRR =
0:
V
DD
CV
REF
= (CVR<3:0> + 8)
•
32
The settling time of the comparator voltage reference
must be considered when changing the CV
REF
output (see Table 26-2 in
REGISTER 21-1:
CVRCON REGISTER
R/W-0
CVREN
bit 7
R/W-0
CVROE
R/W-0
CVRR
U-0
—
R/W-0
CVR3
R/W-0
CVR2
R/W-0
CVR1
R/W-0
CVR0
bit 0
bit 7
CVREN:
Comparator Voltage Reference Enable bit
1
= CV
REF
circuit powered on
0
= CV
REF
circuit powered down
CVROE:
Comparator V
REF
Output Enable bit
1
= CV
REF
voltage level is also output on the RA2/AN2/V
REF
-/CV
REF
(1)
pin
0
= CV
REF
voltage is disconnected from the RA2/AN2/V
REF
-/CV
REF
pin
Note 1:
CVROE overrides the TRISA<2> bit setting.
bit 6
bit 5
CVRR:
Comparator V
REF
Range Selection bit
1
= 0.00 V
DD
to 0.75 V
DD
, with V
DD
/24 step size
0
= 0.25 V
DD
to 0.75 V
DD
, with V
DD
/32 step size
Unimplemented:
Read as ‘0’
CVR3:CVR0:
Comparator V
REF
Value Selection 0
≤
VR3:VR0
≤
15 bits
When CVRR =
1:
V
DD
CV
REF
= (CVR<3:0>)
•
24
When CVRR =
0:
V
DD
CV
REF
= 1/4
•
(CV
RSRC
) + (CVR<3:0> + 8)
•
32
bit 4
bit 3-0
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
2003 Microchip Technology Inc.
DS39599C-page 227
MICROCHIP [ MICROCHIP TECHNOLOGY ]
