PIC18F2220-ISLASHML datasheet(22/388 Pages) MICROCHIP

22 / 388 page

PIC18F2220/2320/4220/4320

DS39599C-page 20

 2003 Microchip Technology Inc.

TABLE 2-2:

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 2-2.

FIGURE 2-2:

EXTERNAL CLOCK INPUT

OPERATION (HS OSC

CONFIGURATION)

2.3

HSPLL

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

option for users who wish to use a lower frequency

crystal oscillator circuit, or to clock the device up to its

highest rated frequency from a crystal oscillator. This

may be useful for customers who are concerned with

EMI due to high-frequency crystals.

The HSPLL mode makes use of the HS mode oscillator

for frequencies up to 10 MHz. A PLL then multiplies the

oscillator output frequency by 4 to produce an internal

clock frequency up to 40 MHz.

The PLL is enabled only when the oscillator configura-

tion bits are programmed for HSPLL mode. If

programmed for any other mode, the PLL is not

enabled.

FIGURE 2-3:

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 the oscillator, but also increases the

start-up time.

2: 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.

3: Since each resonator/crystal has its own

characteristics, the user should consult

the resonator/crystal manufacturer for

appropriate

values

external

components.

4: RS may be required to avoid overdriving

crystals with low drive level specification.

5: 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

÷4

(from Configuration Register 1H)

HS Mode

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