PIC18F4550EMLQTP datasheet(27/438 Pages) MICROCHIP

27 / 438 page

DS39632E-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

Note:

Use of a series cut crystal may give a fre-

quency out of the crystal manufacturer’s

specifications.

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

RF(3)

Sleep

Logic

PIC18FXXXX

RS(2)

Internal

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

When using ceramic resonators with frequencies

above 3.5 MHz, HS mode is recommended over XT

mode. HS mode may be used at any VDD for which

the controller is rated. If HS is selected, the gain of the

oscillator may overdrive the resonator. Therefore, a

series resistor should be placed between the OSC2

pin and the resonator. As a good starting point, the

recommended value of RS is 330

Ω.

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