PIC18F8585T-I/PT【PDF 187/424 页】IC PIC MCU FLASH 24KX16 80TQFP

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Sheet目录3881

183页 184页 185页 186页 [187页]188页 189页 190页 191页

2004 Microchip Technology Inc.

DS30491C-page 265

PIC18F6585/8585/6680/8680

21.0

COMPARATOR VOLTAGE

REFERENCE MODULE

The comparator voltage reference 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 21-1.

The block diagram is given in Figure 21-1.

The comparator reference supply voltage can come

from either VDD or 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.

21.1

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:

If CVRR = 1:

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

If CVRR = 0:

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

The settling time of the comparator voltage reference

must be considered when changing the CVREF output

(Section 27.0 “Electrical Characteristics”).

REGISTER 21-1:

CVRCON REGISTER

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(1)

=CVREF voltage level is also output on the RF5/AN10/C1IN+/CVREF pin

=CVREF voltage is disconnected from the RF5/AN10/C1IN+/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 0.71875 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

Note:

To select (VREF+ – VREF-) as the comparator voltage reference source, the voltage

reference configuration bits in the ADCON1 register (ADCON1<5:4>) must also be

set to ‘11’.

bit 3-0

CVR3:CVR0: Comparator VREF Value Selection bits (0

≤ VR3:VR0 ≤ 15)

When CVRR = 1:

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

(CVRSRC)

When CVRR = 0:

CVREF = 1/4

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

Note 1: If enabled for output, RF5 must also be configured as an input by setting TRISF<5>

to ‘1’.

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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