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Sheet目录3874

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PIC16F627A/628A/648A

DS40044G-page 100

2009 Microchip Technology Inc.

14.2.3

EXTERNAL CRYSTAL OSCILLATOR

CIRCUIT

Either a prepackaged oscillator can be used or a simple

oscillator circuit with TTL gates can be built.

Prepackaged oscillators provide a wide operating

range and better stability. A well-designed crystal

oscillator will provide good performance with TTL

gates. Two types of crystal oscillator circuits can be

used; one with series resonance, or one with parallel

resonance.

Figure 14-2 shows implementation of a parallel

resonant oscillator circuit. The circuit is designed to

use the fundamental frequency of the crystal. The

74AS04 inverter performs the 180

° phase shift that a

parallel oscillator requires. The 4.7 k

Ω resistor

provides the negative feedback for stability. The

10 k

Ω potentiometers bias the 74AS04 in the linear

region. This could be used for external oscillator

designs.

FIGURE 14-2:

EXTERNAL PARALLEL

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

Figure 14-3 shows a series resonant oscillator circuit.

This circuit is also designed to use the fundamental

frequency of the crystal. The inverter performs a 180

phase shift in a series resonant oscillator circuit. The

330 k

Ω resistors provide the negative feedback to bias

the inverters in their linear region.

FIGURE 14-3:

EXTERNAL SERIES

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

14.2.4

PRECISION INTERNAL 4 MHZ

OSCILLATOR

The internal precision oscillator provides a fixed 4 MHz

(nominal) system clock at VDD = 5V and 25

°C. See

Section 17.0 “Electrical Specifications”

, for informa-

tion on variation over voltage and temperature.

14.2.5

EXTERNAL CLOCK IN

For applications where a clock is already available

elsewhere, users may directly drive the PIC16F627A/

628A/648A provided that this external clock source meets

the AC/DC timing requirements listed in Section 17.6

“Timing Diagrams and Specifications”

. Figure 14-4

below shows how an external clock circuit should be

configured.

FIGURE 14-4:

EXTERNAL CLOCK INPUT

OPERATION (EC, HS, XT

OR LP OSC

CONFIGURATION)

+5V

10K

4.7K

10K

74AS04

XTAL

10K

74AS04

PIC16F627A/628A/648A

CLKIN

To other

Devices

330 K

74AS04

PIC16F627A/

CLKIN

To other

Devices

XTAL

330 K

74AS04

0.1 pF

628A/648A

Clock from

ext. system

PIC16F627A/628A/648A

RA6

RA7/OSC1/CLKIN

RA6/OSC2/CLKOUT

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