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Sheet目录3841

136页 137页 138页 139页 [140页]141页 142页 143页 144页

PIC18F2X1X/4X1X

DS39636D-page 230

2009 Microchip Technology Inc.

FIGURE 20-1:

COMPARATOR VOLTAGE REFERENCE BLOCK DIAGRAM

20.2

Voltage Reference Accuracy/Error

The full range of voltage reference cannot be realized

due to the construction of the module. The transistors

on the top and bottom of the resistor ladder network

(Figure 20-1) keep CVREF from approaching the refer-

ence source rails. The voltage reference is derived

from the reference source; therefore, the CVREF output

changes with fluctuations in that source. The tested

absolute accuracy of the voltage reference can be

found in Section 25.0 “Electrical Characteristics”.

20.3

Operation During Sleep

When the device wakes up from Sleep through an

interrupt or a Watchdog Timer time-out, the contents of

the CVRCON register are not affected. To minimize

current consumption in Sleep mode, the voltage

reference should be disabled.

20.4

Effects of a Reset

A device Reset disables the voltage reference by

clearing bit CVREN (CVRCON<7>). This Reset also

disconnects the reference from the RA2 pin by clearing

bit CVROE (CVRCON<6>) and selects the high-voltage

range by clearing bit CVRR (CVRCON<5>). The CVR

value select bits are also cleared.

20.5

Connection Considerations

The voltage reference module operates independently

of the comparator module. The output of the reference

generator may be connected to the RA2 pin if the

CVROE bit is set. Enabling the voltage reference out-

put onto RA2 when it is configured as a digital input will

increase current consumption. Connecting RA2 as a

digital output with CVRSS enabled will also increase

current consumption.

The RA2 pin can be used as a simple D/A output with

limited drive capability. Due to the limited current drive

capability, a buffer must be used on the voltage

reference output for external connections to VREF.

Figure 20-2 shows an example buffering technique.

16-

-1

MUX

CVR3:CVR0

CVREN

CVRSS = 0

VDD

VREF+

CVRSS = 1

CVRSS = 0

VREF-

CVRSS = 1

16 Steps

CVRR

CVREF

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