PIC18F2220/2320/4220/4320
21.1 Configuring the Comparator
Voltage Reference
21.0 COMPARATOR VOLTAGE
REFERENCE MODULE
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:
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 CVREF 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.
EQUATION 21-1:
If CVRR = 1:
CVREF = (CVR<3:0>) •
VDD
24
The comparator reference supply voltage comes from
VDD and VSS.
If CVRR = 0:
CVREF = (CVR<3:0> + 8) •
VDD
32
The settling time of the comparator voltage reference
must be considered when changing the CVREF
output (see Table 26-2 in Section 26.0 “Electrical
Characteristics”).
REGISTER 21-1:
CVRCON REGISTER
R/W-0
R/W-0
R/W-0
CVRR
U-0
—
R/W-0
CVR3
R/W-0
CVR2
R/W-0
CVR1
R/W-0
CVR0
CVREN
CVROE
bit 7
bit 0
bit 7
bit 6
CVREN: Comparator Voltage Reference Enable bit
1= CVREF circuit powered on
0= CVREF circuit powered down
CVROE: Comparator VREF Output Enable bit
1= CVREF voltage level is also output on the RA2/AN2/VREF-/CVREF(1) pin
0= CVREF voltage is disconnected from the RA2/AN2/VREF-/CVREF pin
Note 1: CVROE overrides the TRISA<2> bit setting.
bit 5
CVRR: Comparator VREF Range Selection bit
1= 0.00 VDD to 0.75 VDD, with VDD/24 step size
0= 0.25 VDD to 0.75 VDD, with VDD/32 step size
bit 4
Unimplemented: Read as ‘0’
bit 3-0
CVR3:CVR0: Comparator VREF Value Selection 0 ≤ VR3:VR0 ≤ 15 bits
When CVRR = 1:
VDD
CVREF = (CVR<3:0>) •
24
When CVRR = 0:
CVREF = 1/4 • (CVRSRC) + (CVR<3:0> + 8) •
VDD
32
Legend:
R = Readable bit
W = Writable bit
‘1’ = Bit is set
U = Unimplemented bit, read as ‘0’
‘0’ = Bit is cleared x = Bit is unknown
- n = Value at POR
2003 Microchip Technology Inc.
DS39599C-page 227
MICROCHIP [ MICROCHIP ]
