Microchip Technology MA180025 データシート
2010 Microchip Technology Inc.
DS39933D-page 305
PIC18F87J90 FAMILY
23.0 COMPARATOR VOLTAGE
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.
A block diagram of the module is shown in Figure 23-1.
The resistor ladder is segmented to provide two ranges
of CV
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
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 the device V
The module’s supply reference can be provided from
either the device V
DD
/V
SS
or an external voltage
reference.
23.1
Configuring the Comparator
Voltage Reference
Voltage Reference
The comparator 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.
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 primary difference between the
ranges is the size of the steps selected by the CV
(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:
resolution. The equations used to calculate the output
of the comparator voltage reference are as follows:
If CVRR = 1:
CV
CV
REF
= ((CVR<3:0>)/24) x (CV
RSRC
)
If CVRR = 0:
CV
CV
REF
= (CV
RSRC
/4) + ((CVR<3:0>)/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>).
The settling time of the comparator voltage reference
must be considered when changing the CV
(CVRCON<4>).
The settling time of the comparator voltage reference
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
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
/SEG23/C1INB pin
0
= CV
REF
voltage is disconnected from the RF5/AN10/CV
REF
/SEG23/C1INB 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
= V
DD
– V
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)
(CV
RSRC
)
When CVRR = 0:
CV
CV
REF
= (CV
RSRC
/4) + ((CVR<3:0>)/32)
(CV
RSRC
)
Note 1:
CVROE overrides the TRISF<5> bit setting.