16F628 datasheet(97/168 Pages) MICROCHIP

97 / 168 page

 2002 Microchip Technology Inc.

Preliminary

DS40044A-page 95

PIC16F627A/628A/648A

14.2

Oscillator Configurations

14.2.1

OSCILLATOR TYPES

The PIC16F627A/628A/648A can be operated in eight

different oscillator options. The user can program three

configuration bits (FOSC2 through FOSC0) to select

one of these eight modes:

• LP

Low Power Crystal

• XT

Crystal/Resonator

• HS

High Speed Crystal/Resonator

• RC

External Resistor/Capacitor (2 modes)

• INTOSC Internal Precision Oscillator (2 modes)

• EC

External Clock In

14.2.2

CRYSTAL OSCILLATOR / CERAMIC

RESONATORS

In XT, LP or HS modes a crystal or ceramic resonator

is connected to the OSC1 and OSC2 pins to establish

oscillation (Figure 14-1). The PIC16F627A/628A/648A

oscillator design requires the use of a parallel cut crys-

tal. Use of a series cut crystal may give a frequency out

of the crystal manufacturers specifications. When in

XT, LP or HS modes, the device can have an external

clock source to drive the OSC1 pin (Figure 14-4).

FIGURE 14-1:

CRYSTAL OPERATION

(OR CERAMIC

RESONATOR) (HS, XT OR

LP OSC

CONFIGURATION)

TABLE 14-1:

CAPACITOR SELECTION FOR

CERAMIC RESONATORS

TABLE 14-2:

CAPACITOR SELECTION FOR

CRYSTAL OSCILLATOR

14.2.3

EXTERNAL CRYSTAL OSCILLATOR

CIRCUIT

Either a prepackaged oscillator can be used or a simple

oscillator

circuit

with

TTL

gates

can

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 reso-

nant 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.

Note

A series resistor may be required for AT strip cut

crystals.

See Table 14-1 and Table 14-2 for recommended

values of C1 and C2.

XTAL

OSC2

RS(1)

OSC1

SLEEP

PIC16F627A/628A/648A

OSC

Mode

Freq

OSC1(C1)

OSC2(C2)

455 kHz

2.0 MHz

4.0 MHz

22 - 100 pF

15 - 68 pF

22 - 100 pF

15 - 68 pF

8.0 MHz

16.0 MHz

10 - 68 pF

10 - 22 pF

10 - 68 pF

10 - 22 pF

Note:

Higher capacitance increases the stability of the oscil-

lator but also increases the start-up time. These values

are for design guidance only. Since each resonator has

its own characteristics, the user should consult the res-

onator manufacturer for appropriate values of external

components.

Mode

Freq

OSC1(C1)

OSC2(C2)

32 kHz

200 kHz

15 - 30 pF

0 - 15 pF

15 - 30 pF

0 - 15 pF

100 kHz

2 MHz

4 MHz

68 - 150 pF

15 - 30 pF

150 - 200 pF

15 - 30 pF

8 MHz

10 MHz

20 MHz

15 - 30 pF

Note:

Higher capacitance increases the stability of the oscil-

lator but also increases the start-up time. These values

are for design guidance only. A series resistor (RS)

may be required in HS mode as well as XT mode to

avoid overdriving crystals with low drive level specifica-

tion. Since each crystal has its own characteristics, the

user should consult the crystal manufacturer for appro-

priate values of external components.

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