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Sheet目录3887

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PIC16C5X

DS30453D-page 16

Preliminary

2002 Microchip Technology Inc.

4.3

External Crystal Oscillator Circuit

Either a prepackaged oscillator or a simple oscillator

circuit with TTL gates can be used as an external crys-

tal oscillator circuit. Prepackaged oscillators provide a

wide operating range and better stability. A well-

designed crystal oscillator will provide good perfor-

mance with TTL gates. Two types of crystal oscillator

circuits can be used: one with parallel resonance, or

one with series resonance.

Figure 4-3 shows an implementation example of a par-

allel resonant oscillator circuit. The circuit is designed

to use the fundamental frequency of the crystal. The

74AS04 inverter performs the 180-degree phase shift

that a parallel oscillator requires. The 4.7 k

resistor

provides the negative feedback for stability. The 10 k

potentiometers bias the 74AS04 in the linear region.

This circuit could be used for external oscillator

designs.

FIGURE 4-3:

EXAMPLE OF EXTERNAL

PARALLEL RESONANT

CRYSTAL OSCILLATOR

CIRCUIT (USING XT, HS

OR LP OSCILLATOR

MODE)

Figure 4-4 shows a series resonant oscillator circuit.

This circuit is also designed to use the fundamental fre-

quency of the crystal. The inverter performs a 180-

degree phase shift in a series resonant oscillator cir-

cuit. The 330 k

resistors provide the negative feed-

back to bias the inverters in their linear region.

FIGURE 4-4:

EXAMPLE OF EXTERNAL

SERIES RESONANT

CRYSTAL OSCILLATOR

CIRCUIT (USING XT, HS

OR LP OSCILLATOR

MODE)

20 pF

+5V

20 pF

10K

4.7K

10K

74AS04

XTAL

10K

74AS04

PIC16C5X

CLKIN

To Other

Devices

OSC2

Open

330K

74AS04

PIC16C5X

CLKIN

To Other

Devices

XTAL

330K

74AS04

0.1

OSC2

Open

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