PIC18F258 datasheet(21/402 Pages) MICROCHIP

21 / 402 page

 2004 Microchip Technology Inc.

DS41159D-page 19

PIC18FXX8

2.4

External Clock Input

The EC and ECIO Oscillator modes require an external

clock source to be connected to the OSC1 pin. The

feedback device between OSC1 and OSC2 is turned

off in these modes to save current. There is no oscilla-

tor start-up time required after a Power-on Reset or

after a recovery from Sleep mode.

In the EC Oscillator mode, 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 2-3 shows the pin connections for the EC

Oscillator mode.

FIGURE 2-3:

EXTERNAL CLOCK INPUT

OPERATION (EC OSC

CONFIGURATION)

The ECIO Oscillator mode functions like the EC mode,

except that the OSC2 pin becomes an additional

general purpose I/O pin. Figure 2-4 shows the pin

connections for the ECIO Oscillator mode.

FIGURE 2-4:

EXTERNAL CLOCK INPUT

OPERATION (ECIO

CONFIGURATION)

2.5

HS4 (PLL)

A Phase Locked Loop circuit is provided as a program-

mable option for users that want to multiply the

frequency of the incoming crystal oscillator signal by 4.

For an input clock frequency of 10 MHz, the internal

clock frequency will be multiplied to 40 MHz. This is

useful for customers who are concerned with EMI due

to high-frequency crystals.

The PLL can only be enabled when the oscillator

configuration bits are programmed for HS mode. If they

are programmed for any other mode, the PLL is not

enabled and the system clock will come directly from

OSC1.

The PLL is one of the modes of the FOSC2:FOSC0

configuration bits. The oscillator mode is specified

during device programming.

A PLL lock timer is used to ensure that the PLL has

locked before device execution starts. The PLL lock

timer has a time-out referred to as TPLL.

FIGURE 2-5:

PLL BLOCK DIAGRAM

OSC1

OSC2

FOSC/4

Clock from

Ext. System

PIC18FXX8

OSC1

I/O (OSC2)

Clock from

Ext. System

PIC18FXX8

VCO

Loop

Filter

Divide by 4

Crystal

Osc

OSC2

OSC1

FIN

FOUT

SYSCLK

Phase

Comparator

FOSC2:FOSC0 = 110

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