PIC18F4520ESLASHMLQTP datasheet(241/390 Pages) MICROCHIP

241 / 390 page

 2004 Microchip Technology Inc.

Preliminary

DS39631A-page 239

PIC18F2420/2520/4420/4520

21.0

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 21-1.

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 module’s supply reference can be provided from

either device VDD/VSS or an external voltage reference.

21.1

Configuring the Comparator

Voltage Reference

The voltage reference module is controlled through the

CVRCON register (Register 21-1). The comparator

voltage reference provides two ranges of output volt-

age, each with 16 distinct levels. The range to be used

is selected by the CVRR bit (CVRCON<5>). The pri-

mary difference between the ranges is the size of the

steps

selected

the

CVREF

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:

If CVRR = 1:

CVREF = ((CVR3:CVR0)/24) x CVRSRC

If CVRR = 0:

CVREF = (CVRSRC x 1/4) + (((CVR3:CVR0)/32) x

CVRSRC)

The comparator reference supply voltage can come

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

VREF- 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 CVREF out-

put (see Table 26-3 in Section 26.0 “Electrical

Characteristics”).

CVRCON REGISTER

R/W-0

CVREN

CVROE(1)

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(1)

=CVREF voltage level is also output on the RA2/AN2/VREF-/CVREF pin

=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

= 0 to 0.667 CVRSRC, with CVRSRC/24 step size (low range)

= 0.25 CVRSRC to 0.75 CVRSRC, with CVRSRC/32 step size (high range)

bit 4

CVRSS: Comparator VREF Source Selection bit

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

= Comparator reference source, CVRSRC = VDD – VSS

bit 3-0

CVR3:CVR0: Comparator VREF Value Selection bits (0

≤ (CVR3:CVR0) ≤ 15)

When CVRR = 1:

CVREF = ((CVR3:CVR0)/24)

• (CVRSRC)

When CVRR = 0:

CVREF = (CVRSRC/4) + ((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

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