PIC18F4550ESPQTP datasheet(28/438 Pages) MICROCHIP

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PIC18F2455/2550/4455/4550

DS39632E-page 26

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).

FIGURE 2-3:

EXTERNAL CLOCK INPUT

OPERATION (HS OSC

CONFIGURATION)

2.2.3

EXTERNAL CLOCK INPUT

The EC, ECIO, ECPLL and ECPIO Oscillator modes

require an external clock source to be connected to the

OSC1 pin. There is no oscillator start-up time required

after a Power-on Reset or after an exit from Sleep

mode.

In the EC and ECPLL Oscillator modes, the oscillator

frequency divided by 4 is available on the OSC2 pin.

This signal may be used for test purposes or to

synchronize other logic. Figure 2-4 shows the pin

connections for the EC Oscillator mode.

FIGURE 2-4:

EXTERNAL CLOCK

INPUT OPERATION

(EC AND ECPLL

CONFIGURATION)

The ECIO and ECPIO Oscillator modes function like the

EC and ECPLL modes, except that the OSC2 pin

becomes an additional general purpose I/O pin. The I/O

pin becomes bit 6 of PORTA (RA6). Figure 2-5 shows

the pin connections for the ECIO Oscillator mode.

FIGURE 2-5:

EXTERNAL CLOCK

INPUT OPERATION

(ECIO AND ECPIO

CONFIGURATION)

The internal postscaler for reducing clock frequency in

XT and HS modes is also available in EC and ECIO

modes.

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.

OSC1

OSC2

Open

Clock from

Ext. System

PIC18FXXXX

(HS Mode)

OSC1/CLKI

OSC2/CLKO

FOSC/4

Clock from

Ext. System

PIC18FXXXX

OSC1/CLKI

I/O (OSC2)

RA6

Clock from

Ext. System

PIC18FXXXX

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