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PIC18F2331/2431/4331/4431

DS39616D-page 30

2010 Microchip Technology Inc.

TABLE 3-2:

CAPACITOR SELECTION FOR

CRYSTAL OSCILLATOR

An external clock source may also be connected to the

OSC1 pin in the HS mode, as shown in Figure 3-2.

FIGURE 3-2:

EXTERNAL CLOCK INPUT

OPERATION (HS OSC

CONFIGURATION)

3.3

PLL Frequency Multiplier

A Phase Locked Loop (PLL) circuit is provided as an

option for users who wish to use a lower frequency

oscillator circuit or to clock the device up to its highest

rated frequency from a crystal oscillator. This may be

useful for those concerned with EMI from high-

frequency crystals or users requiring higher clock

speeds from an internal oscillator.

3.3.1

HSPLL OSCILLATOR MODE

The HSPLL mode uses the HS Oscillator mode for

frequencies up to 10 MHz. A PLL circuit then multiplies

the oscillator output frequency by four to produce an

internal clock frequency up to 40 MHz. The PLLEN bit

is not available in this oscillator mode.

The PLL is only available to the crystal oscillator when

the FOSC<3:0> Configuration bits are programmed for

HSPLL mode (‘0110’).

FIGURE 3-3:

PLL BLOCK DIAGRAM

Osc Type

Crystal

Freq

Typical Capacitor Values

Tested:

32 kHz

33 pF

200 kHz

15 pF

1 MHz

33 pF

4 MHz

27 pF

4 MHz

27 pF

8 MHz

22 pF

20 MHz

15 pF

Capacitor values are for design guidance only.

These capacitors were tested with the crystals listed

below for basic start-up and operation. These values

are not optimized.

Different capacitor values may be required to produce

acceptable oscillator operation. The user should test

the performance of the oscillator over the expected

VDD and temperature range for the application.

See the notes following this table for additional

information.

Crystals Used:

32 kHz

4 MHz

200 kHz

8 MHz

1 MHz

20 MHz

Note 1:

Higher

capacitance

increases

the

stability of oscillator, but also increases

the start-up time.

When operating below 3V VDD, or when

using certain ceramic resonators at any

voltage, it may be necessary to use the

HS mode or switch to a crystal oscillator.

Since each resonator/crystal has its own

characteristics, the user should consult

the resonator/crystal manufacturer for

appropriate

values

external

components.

Rs may be required to avoid overdriving

crystals with low drive level specification.

Always verify oscillator performance over

the VDD and temperature range that is

expected for the application.

OSC1

OSC2

Open

Clock from

Ext. System

PIC18FXXXX

(HS Mode)

VCO

Loop

Filter

Crystal

Osc

OSC2

OSC1

PLL Enable

FIN

FOUT

SYSCLK

Phase

Comparator

HS Osc Enable

(from Configuration Register 1H)

HS Mode

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