PIC18F2420/2520/4420/4520
21.0
COMPARATOR VOLTAGE
REFERENCE MODULE
is selected by the CVRR bit (CVRCON<5>). The
primary difference between the ranges is the size of the
steps selected by the CV
REF
Selection bits
(CVR<3:0>), with one range offering finer resolution.
The equations used to calculate the output of the
comparator voltage reference are as follows:
If CVRR =
1:
CV
REF
= ((CVR<3:0>)/24) x CV
RSRC
If CVRR =
0:
CV
REF
= (CV
RSRC
x 1/4) + (((CVR<3:0>)/32) x
CV
RSRC
)
The comparator reference supply voltage can come
from either V
DD
and V
SS
, or the external V
REF
+ and
V
REF
- that are multiplexed with RA2 and RA3. The
voltage source is selected by the CVRSS bit
(CVRCON<4>).
The settling time of the comparator voltage reference
must be considered when changing the CV
REF
out-
put (see Table 26-3 in
The comparator voltage reference is a 16-tap resistor
ladder network that provides a selectable reference
voltage. Although its primary purpose is to provide a
reference for the analog comparators, it may also be
used independently of them.
A block diagram of the module is shown in Figure 21-1.
The resistor ladder is segmented 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 module’s supply reference can be provided from
either device V
DD
/V
SS
or an external voltage reference.
21.1
Configuring the Comparator
Voltage Reference
The voltage reference module is controlled through the
CVRCON register (Register 21-1). The comparator
voltage reference provides two ranges of output volt-
age, each with 16 distinct levels. The range to be used
REGISTER 21-1:
R/W-0
CVREN
bit 7
Legend:
R = Readable bit
-n = Value at POR
bit 7
CVRCON: COMPARATOR VOLTAGE REFERENCE CONTROL REGISTER
R/W-0
CVRR
R/W-0
CVRSS
R/W-0
CVR3
R/W-0
CVR2
R/W-0
CVR1
R/W-0
CVR0
bit 0
R/W-0
CVROE
(1)
W = Writable bit
‘1’ = Bit is set
U = Unimplemented bit, read as ‘0’
‘0’ = Bit is cleared
x = Bit is unknown
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)
1
= CV
REF
voltage level is also output on the RA2/AN2/V
REF
-/CV
REF
pin
0
= CV
REF
voltage is disconnected from the RA2/AN2/V
REF
-/CV
REF
pin
CVRR:
Comparator V
REF
Range Selection bit
1
= 0 to 0.667 CV
RSRC
, with CV
RSRC
/24 step size (low range)
0
= 0.25 CV
RSRC
to 0.75 CV
RSRC
, with CV
RSRC
/32 step size (high range)
CVRSS:
Comparator V
REF
Source Selection bit
1
= Comparator reference source, CV
RSRC
= (V
REF
+) – (V
REF
-)
0
= Comparator reference source, CV
RSRC
= V
DD
– V
SS
CVR3:CVR0:
Comparator V
REF
Value Selection bits (0
≤
(CVR<3:0>)
≤
15)
When CVRR =
1:
CV
REF
= ((CVR<3:0>)/24)
•
(CV
RSRC
)
When CVRR =
0:
CV
REF
= (CV
RSRC
/4) + ((CVR<3:0>)/32)
•
(CV
RSRC
)
CVROE overrides the TRISA<2> bit setting.
bit 6
bit 5
bit 4
bit 3-0
Note 1:
©
2008 Microchip Technology Inc.
DS39631E-page 239
MICROCHIP [ MICROCHIP TECHNOLOGY ]
