PIC18F4550EMLQTP datasheet(27/430 Pages) MICROCHIP

27 / 430 page

Preliminary

DS39632D-page 25

PIC18F2455/2550/4455/4550

2.2.2

CRYSTAL OSCILLATOR/CERAMIC

RESONATORS

In HS, HSPLL, XT and XTPLL Oscillator modes, a

crystal or ceramic resonator is connected to the OSC1

and OSC2 pins to establish oscillation. Figure 2-2

shows the pin connections.

The oscillator design requires the use of a parallel cut

crystal.

FIGURE 2-2:

CRYSTAL/CERAMIC

RESONATOR OPERATION

(XT, HS OR HSPLL

CONFIGURATION)

TABLE 2-1:

CAPACITOR SELECTION FOR

CERAMIC RESONATORS

TABLE 2-2:

CAPACITOR SELECTION FOR

CRYSTAL OSCILLATOR

An internal postscaler allows users to select a clock

frequency other than that of the crystal or resonator.

Frequency division is determined by the CPUDIV

Configuration bits. Users may select a clock frequency

of the oscillator frequency, or 1/2, 1/3 or 1/4 of the

frequency.

An external clock may also be used when the micro-

controller is in HS Oscillator mode. In this case, the

OSC2/CLKO pin is left open (Figure 2-3).

Note:

Use of a series cut crystal may give a fre-

quency out of the crystal manufacturer’s

specifications.

Typical Capacitor Values Used:

Mode

Freq

OSC1

OSC2

4.0 MHz

33 pF

8.0 MHz

16.0 MHz

27 pF

22 pF

27 pF

22 pF

Capacitor values are for design guidance only.

These capacitors were tested with the resonators

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 Table 2-2 for additional

information.

Resonators Used:

4.0 MHz

8.0 MHz

16.0 MHz

Note 1:

See Table 2-1 and Table 2-2 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

(3)

Sleep

Logic

PIC18FXXXX

RS(2)

Internal

Osc Type

Crystal

Freq

Typical Capacitor Values

Tested:

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:

4 MHz

8 MHz

20 MHz

Note 1: Higher capacitance increases the stability

of 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.

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