PIC16F628AESS datasheet(99/180 Pages) MICROCHIP

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DS40044D-page 97

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

crystal. 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

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.

C1(2)

C2(2)

XTAL

OSC2

RS(1)

OSC1

Sleep

PIC16F627A/628A/648A

FOSC

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 oscillator, 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 resonator manufacturer for

appropriate values of external compo-

nents.

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 oscillator, 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 specification. Since each

crystal has its own characteristics, the

user should consult the crystal manufac-

turer for appropriate values of external

components.

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