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2009 Microchip Technology Inc.

DS39689F-page 29

PIC18F2221/2321/4221/4321 FAMILY

3.0

OSCILLATOR

CONFIGURATIONS

3.1

Oscillator Types

The PIC18F2221/2321/4221/4321 family of devices

can be operated in ten different oscillator modes. The

user can program the Configuration bits, FOSC<3:0>,

in Configuration Register 1H to select one of these ten

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

INTIO1 Internal Oscillator with FOSC/4 output

on RA6 and I/O on RA7

INTIO2 Internal Oscillator with I/O on RA6

and RA7

External Clock with FOSC/4 output

10. ECIO

External Clock with I/O on RA6

3.2

Crystal Oscillator/Ceramic

Resonators

In XT, LP, HS or HSPLL Oscillator modes, a crystal or

ceramic resonator is connected to the OSC1 and

OSC2 pins to establish oscillation. Figure 3-1 shows

the pin connections.

The oscillator design requires the use of a parallel cut

crystal.

FIGURE 3-1:

CRYSTAL/CERAMIC

RESONATOR OPERATION

(XT, LP, HS OR HSPLL

CONFIGURATION)

TABLE 3-1:

CAPACITOR SELECTION FOR

CERAMIC RESONATORS

Note:

Use of a series cut crystal may give a

frequency out of the crystal manufacturer’s

specifications.

Typical Capacitor Values Used:

Mode

Freq

OSC1

OSC2

3.58 MHz

22 pF

Capacitor values are for design guidance only.

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. Refer

to the following application notes for oscillator specific

information:

AN588, “PIC Microcontroller Oscillator Design

Guide”

AN826, “Crystal Oscillator Basics and Crystal

Selection for rfPIC and PIC Devices”

AN849, “Basic PIC Oscillator Design”

AN943, “Practical PIC Oscillator Analysis and

Design”

AN949, “Making Your Oscillator Work”

See the notes following Table 3-2 for additional

information.

Note:

When using resonators with frequencies

above 3.5 MHz, the use of HS mode,

rather than XT mode, is recommended.

HS mode may be used at any VDD for

which the controller is rated. If HS is

selected, it is possible that the gain of the

oscillator will overdrive the resonator.

Therefore, a series resistor may be placed

between the OSC2 pin and the resonator.

good

starting

point,

the

recommended value of RS is 330

Ω.

Note 1: See Table 3-1 and Table 3-2 for initial values of

C1 and C2.

2: A series resistor (RS) may be required for AT

strip cut crystals.

3: RF varies with the oscillator mode chosen.

C1(1)

C2(1)

XTAL

OSC2

OSC1

RF(3)

Sleep

Logic

PIC18FXXXX

RS(2)

Internal

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