Microchip Technology DV164136 Data Sheet
© 2008 Microchip Technology Inc.
DS39646C-page 287
PIC18F8722 FAMILY
23.0
COMPARATOR VOLTAGE
REFERENCE MODULE
REFERENCE MODULE
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.
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 23-1.
The resistor ladder is segmented to provide two ranges
of CV
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
The module’s supply reference can be provided from
either device V
DD
/V
SS
or an external voltage reference.
23.1
Configuring the Comparator
Voltage Reference
Voltage Reference
The voltage reference module is controlled through the
CVRCON register (Register 23-1). The comparator
voltage reference provides two ranges of output
voltage, each with 16 distinct levels. The range to be
used is selected by the CVRR bit (CVRCON<5>). The
CVRCON register (Register 23-1). The comparator
voltage reference provides two ranges of output
voltage, each with 16 distinct levels. The range to be
used is selected by the CVRR bit (CVRCON<5>). The
primary difference between the ranges is the size of the
steps selected by the CV
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:
The equations used to calculate the output of the
comparator voltage reference are as follows:
If CVRR = 1:
CV
CV
REF
= ((CVR3:CVR0)/24) x (CV
RSRC
)
If CVRR = 0:
CV
CV
REF
= (CV
RSRC
/4) + ((CVR3:CVR0)/32) x
(CV
RSRC
)
The comparator reference supply voltage can come
from either V
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>).
(CVRCON<4>).
The settling time of the comparator voltage reference
must be considered when changing the CV
must be considered when changing the CV
REF
output (see Table 28-3 in Section 28.0 “Electrical
Characteristics”).
Characteristics”).
REGISTER 23-1:
CVRCON: COMPARATOR VOLTAGE REFERENCE CONTROL REGISTER
R/W-0
R/W-0
R/W-0
R/W-0
R/W-0
R/W-0
R/W-0
R/W-0
CVREN
CVROE
(1)
CVRR
CVRSS
CVR3
CVR2
CVR1
CVR0
bit 7
bit 0
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
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)
1
= CV
REF
voltage level is also output on the RF5/AN10/CV
REF
pin
0
= CV
REF
voltage is disconnected from the RF5/AN10/CV
REF
pin
bit 5
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)
bit 4
CVRSS: Comparator V
REF
Source Selection bit
1
= Comparator reference source, CV
RSRC
= (V
REF
+) – (V
REF
-)
0
= Comparator reference source, CV
RSRC
= AV
DD
– AV
SS
bit 3-0
CVR<3:0>: Comparator V
REF
Value Selection bits (0
≤ (CVR<3:0>) ≤ 15)
When CVRR = 1:
CV
CV
REF
= ((CVR<3:0>)/24) x (CV
RSRC
)
When CVRR = 0:
CV
CV
REF
= (CV
RSRC
/4) + ((CVR<3:0>)/32) x (CV
RSRC
)
Note 1:
CVROE overrides the TRISF<5> bit setting.