18F4539 datasheet(21/322 Pages) MICROCHIP

21 / 322 page

 2002 Microchip Technology Inc.

Preliminary

DS30485A-page 19

PIC18FXX39

2.0

OSCILLATOR

CONFIGURATIONS

2.1

Oscillator Types

The PIC18FXX39 can be operated in four different

Oscillator modes at a frequency of 20 MHz. The user

can program three configuration bits (FOSC2, FOSC1,

and FOSC0) to select one of these four modes:

High Speed Crystal/Resonator

HS + PLL

High Speed Crystal/Resonator

with PLL enabled using 5 MHz

crystal

External Clock

ECIO

External Clock with I/O pin

enabled

2.2

Crystal Oscillator/Ceramic

Resonators

In HS or HS+PLL Oscillator modes, a crystal or ceramic

resonator is connected to the OSC1 and OSC2 pins to

establish oscillation. Figure 2-1 shows the pin

connections.

The PIC18FXX39 oscillator design requires the use of

a parallel cut crystal.

FIGURE 2-1:

CRYSTAL/CERAMIC

RESONATOR OPERATION

(HS CONFIGURATION)

An external clock source may also be connected to the

OSC1 pin in the HS mode, as shown in Figure 2-2.

FIGURE 2-2:

EXTERNAL CLOCK INPUT

OPERATION (HS OSC

CONFIGURATION)

Note:

The operation of the Motor Control kernel

and its APIs (Section 14.0) is based on an

assumed clock frequency of 20 MHz.

Changing the oscillator frequency will

change the timing used in the Motor Con-

trol kernel accordingly. To achieve the best

results in motor control applications, a

clock frequency of 20 MHz is highly

recommended.

Note:

Use of a series cut crystal may give a fre-

quency out of the crystal manufacturers

specifications.

Note 1: Higher capacitance increases the stability

of the oscillator, but also increases the

start-up time.

2: Since each resonator/crystal has its own

characteristics, the user should consult the

resonator/crystal manufacturer for appro-

priate values of external components, or

verify oscillator performance.

Note 1: See Table 2-1 for recommended 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

PIC18FXX39

RS(2)

Internal

OSC1

OSC2

Open

Clock from

Ext. System

PIC18FXX39

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