PIC18F258-ESLASHLSQTP datasheet(257/402 Pages) MICROCHIP

257 / 402 page

DS41159E-page 255

PIC18FXX8

22.0

COMPARATOR VOLTAGE

REFERENCE MODULE

This module is a 16-tap resistor ladder network that

provides a selectable voltage reference. The resistor

ladder is segmented to provide two ranges of CVREF

values and has a power-down function to conserve

power when the reference is not being used. The

CVRCON register controls the operation of the

reference, as shown in Register 22-1. The block

diagram is shown in Figure 22-1.

The comparator and reference supply voltage can

come from either VDD and VSS, or the external VREF+

and VREF-, that are multiplexed with RA3 and RA2. The

comparator reference supply voltage is controlled by

the CVRSS bit.

22.1

Configuring the Comparator

Voltage Reference

The comparator voltage reference can output 16 distinct

voltage levels for each range. The equations used to

calculate the output of the comparator voltage reference

are as follows.

EQUATION 22-1:

EQUATION 22-2:

The settling time of the Comparator Voltage Reference

must be considered when changing the RA0/AN0/

CVREF output (see Table 27-4 in Section 27.2 “DC

Characteristics”).

CVRCON: COMPARATOR VOLTAGE REFERENCE CONTROL REGISTER

Note:

The comparator voltage reference is only

available

the

PIC18F448

and

PIC18F458.

If CVRR = 1:

CVREF = (CVR<3:0>/24) x CVRSRC

where:

CVRSS = 1, CVRSRC = (VREF+) – (VREF-)

CVRSS = 0, CVRSRC = AVDD – AVSS

If CVRR = 0:

CVREF = (CVRSRC x 1/4) + (CVR<3:0>/32) x CVRSRC

where:

CVRSS = 1, CVRSRC = (VREF+) – (VREF-)

CVRSS = 0, CVRSRC = AVDD – AVSS

R/W-0

CVREN

CVROE

CVRR

CVRSS

CVR3

CVR2

CVR1

CVR0

bit 7

bit 0

bit 7

CVREN: Comparator Voltage Reference Enable bit

=CVREF circuit powered on

=CVREF circuit powered down

bit 6

CVROE: Comparator VREF Output Enable bit

=CVREF voltage level is also output on the RA0/AN0/CVREF pin

=CVREF voltage is disconnected from the RA0/AN0/CVREF pin

bit 5

CVRR: Comparator VREF Range Selection bit

=0.00 CVRSRC to 0.625 CVRSRC with CVRSRC/24 step size

=0.25 CVRSRC to 0.719 CVRSRC with CVRSRC/32 step size

bit 4

CVRSS: Comparator VREF Source Selection bit

= Comparator reference source, CVRSRC = (VREF+) – (VREF-)

= Comparator reference source, CVRSRC = VDD – VSS

bit 3-0

CVR<3:0>: Comparator VREF Value Selection 0

≤ CVR3:CVR0 ≤ 15 bits

When CVRR = 1:

CVREF = (CVR3:CVR0/24)

• (CVRSRC)

When CVRR = 0:

CVREF = 1/4

• (CVRSRC) + (CVR3:CVR0/32) • (CVRSRC)

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

PIC18F258-E/LSQTP Datasheet(PDF) 257 Page - Microchip Technology

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