Microchip Technology MCP3421DM-WS Data Sheet
© 2009 Microchip Technology Inc.
DS39632E-page 281
PIC18F2455/2550/4455/4550
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 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
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
primary difference between the ranges is the size of the
steps selected by the CV
REF
Selection bits
(CVR3:CVR0), 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>).
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
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 RA2/AN2/V
REF
-/CV
REF
pin
0 = CV
REF
voltage is disconnected from the RA2/AN2/V
REF
-/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
= V
DD
– V
SS
bit 3-0
CVR3:CVR0: Comparator V
REF
Value Selection bits (0
≤ (CVR3:CVR0) ≤ 15)
When CVRR = 1:
CV
CV
REF
= ((CVR3:CVR0)/24)
• (CV
RSRC
)
When CVRR = 0:
CV
CV
REF
= (CV
RSRC
/4) + ((CVR3:CVR0)/32)
• (CV
RSRC
)
Note 1:
CVROE overrides the TRISA<2> bit setting.