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Sheet目录3875

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2010 Microchip Technology Inc.

DS41303G-page 27

PIC18F2XK20/4XK20

2.0

OSCILLATOR MODULE (WITH

FAIL-SAFE CLOCK MONITOR)

2.1

Overview

The Oscillator module has a wide variety of clock

sources and selection features that allow it to be used

in a wide range of applications while maximizing perfor-

mance and minimizing power consumption. Figure 2-1

illustrates a block diagram of the Oscillator module.

Clock sources can be configured from external

oscillators, quartz crystal resonators, ceramic resonators

and Resistor-Capacitor (RC) circuits. In addition, the

system clock source can be configured from one of two

internal oscillators, with a choice of speeds selectable via

software. Additional clock features include:

Selectable system clock source between external

or internal via software.

Two-Speed Start-up mode, which minimizes

latency between external oscillator start-up and

code execution.

Fail-Safe Clock Monitor (FSCM) designed to

detect a failure of the external clock source (LP,

XT, HS, EC or RC modes) and switch

automatically to the internal oscillator.

The Oscillator module can be configured in one of ten

primary clock modes.

Low-Power Crystal

Crystal/Resonator

High-Speed Crystal/Resonator

HSPLL

High-Speed Crystal/Resonator

with PLL enabled

External Resistor/Capacitor with

FOSC/4 output on RA6

RCIO

External Resistor/Capacitor with I/O

on RA6

INTOSC

Internal Oscillator with FOSC/4

output on RA6 and I/O on RA7

INTOSCIO Internal Oscillator with I/O on RA6

and RA7

External Clock with FOSC/4 output

10. ECIO

External Clock with I/O on RA6

Primary Clock modes are selected by the FOSC<3:0>

bits of the CONFIG1H Configuration Register. The

HFINTOSC and LFINTOSC are factory calibrated high-

frequency and low-frequency oscillators, respectively,

which are used as the internal clock sources.

FIGURE 2-1:

PIC MCU CLOCK SOURCE BLOCK DIAGRAM

4 x PLL

FOSC<3:0>

Secondary Oscillator

T1OSCEN

Enable

Oscillator

T1OSO

T1OSI

Clock Source Option

for other Modules

OSC1

OSC2

Sleep

HSPLL, HFINTOSC/PLL

LP, XT, HS, RC, EC

T1OSC

CPU

Peripherals

IDLEN

stscale

16 MHz

8 MHz

4 MHz

2 MHz

1 MHz

250 kHz

500 kHz

OSCCON<6:4>

111

110

101

100

011

010

001

000

31 kHz

Source

Internal

Oscillator

Block

WDT, PWRT, FSCM

16 MHz

Internal Oscillator

(HFINTOSC)

Clock

Control

OSCCON<1:0>

Source

16 MHz

31 kHz (LFINTOSC)

OSCTUNE<6>(1)

OSCTUNE<7>

and Two-Speed Start-up

Primary Oscillator

PIC18F2XK20/4XK20

Sleep

Main

FOSC<3:0> OSCCON<1:0>

Note

Operates only when HFINTOSC is the primary oscillator.

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