PIC16F628A-ISLASHML datasheet(98/168 Pages) MICROCHIP

98 / 168 page

PIC16F627A/628A/648A

DS40044A-page 96

Preliminary

 2002 Microchip Technology Inc.

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 V

DD = 5 V and 25

°C. See

Section 17.0, Electrical Specifications, for information

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. Figure 14-4 below shows how an exter-

nal clock circuit should be configured.

FIGURE 14-4:

EXTERNAL CLOCK INPUT

OPERATION (EC, HS, XT

OR LP OSC

CONFIGURATION)

14.2.6

RC OSCILLATOR

For applications where precise timing is not a require-

ment, the RC oscillator option is available. The opera-

tion and functionality of the RC oscillator is dependent

upon a number of variables. The RC oscillator fre-

quency is a function of:

• Supply voltage

• Resistor (R

EXT) and capacitor (CEXT) values

• Operating temperature.

The oscillator frequency will vary from unit to unit due

to normal process parameter variation. The difference

in lead frame capacitance between package types will

also affect the oscillation frequency, especially for low

EXT values. The user also needs to account for the

tolerance of the external R and C components.

Figure 14-5 shows how the R/C combination is con-

nected.

FIGURE 14-5:

RC OSCILLATOR MODE

The RC Oscillator mode has two options that control

the unused OSC2 pin. The first allows it to be used as

a general purpose I/O port. The other configures the

pin as an output providing the Fosc signal (internal

clock divided by 4) for test or external synchronization

purposes.

14.2.7

CLKOUT

The PIC16F627A/628A/648A can be configured to pro-

vide a clock out signal by programming the configura-

tion word. The oscillator frequency, divided by 4 can be

used for test purposes or to synchronize other logic.

+5V

10K

4.7K

10K

74AS04

XTAL

10K

74AS04

PIC16F627A/628A/648A

CLKIN

TO OTHER

DEVICES

330 K

Ω

74AS04

PIC16F627A/

CLKIN

TO OTHER

DEVICES

XTAL

330 K

Ω

74AS04

0.1 PF

628A/648A

Clock From

ext. system

PIC16F627A/628A/648A

RA6

RA7/OSC1/CLKIN

RA6/OSC2/CLKOUT

EXT

PIC16F627A/628A/648A

OSC/4

Internal

Clock

CLKIN

RA7/OSC1/

RA6/OSC2/CLKOUT

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