PIC18F242 datasheet(20/332 Pages) MICROCHIP

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PIC18FXX2

DS39564C-page 18

TABLE 2-2:

CAPACITOR SELECTION FOR

CRYSTAL OSCILLATOR

An external clock source may also be connected to the

OSC1 pin in the HS, XT and LP modes, as shown in

Figure 2-2.

FIGURE 2-2:

EXTERNAL CLOCK INPUT

OPERATION (HS, XT OR LP

OSC CONFIGURATION)

2.3

RC Oscillator

For timing-insensitive applications, the “RC” and

“RCIO” device options offer additional cost savings.

The RC oscillator frequency is a function of the supply

voltage, the resistor (REXT) and capacitor (CEXT) val-

ues and the operating temperature. In addition to this,

the oscillator frequency will vary from unit to unit due to

normal process parameter variation. Furthermore, the

difference in lead frame capacitance between package

types will also affect the oscillation frequency, espe-

cially for low CEXT values. The user also needs to take

into account variation due to tolerance of external R

and C components used. Figure 2-3 shows how the

R/C combination is connected.

In the RC Oscillator mode, the oscillator frequency

divided by 4 is available on the OSC2 pin. This signal

may be used for test purposes or to synchronize other

logic.

FIGURE 2-3:

RC OSCILLATOR MODE

The RCIO Oscillator mode functions like the RC mode,

except that the OSC2 pin becomes an additional gen-

eral purpose I/O pin. The I/O pin becomes bit 6 of

PORTA (RA6).

Ranges Tested:

Mode

Freq

32.0 kHz

33 pF

200 kHz

15 pF

200 kHz

22-68 pF

1.0 MHz

15 pF

4.0 MHz

15 pF

4.0 MHz

15 pF

8.0 MHz

15-33 pF

20.0 MHz

15-33 pF

25.0 MHz

15-33 pF

These values are for design guidance only.

See notes following this table.

Crystals Used

32.0 kHz

Epson C-001R32.768K-A

± 20 PPM

200 kHz

STD XTL 200.000KHz

± 20 PPM

1.0 MHz

ECS ECS-10-13-1

± 50 PPM

4.0 MHz

ECS ECS-40-20-1

± 50 PPM

8.0 MHz

Epson CA-301 8.000M-C

± 30 PPM

20.0 MHz

Epson CA-301 20.000M-C

± 30 PPM

Note 1: Higher capacitance increases the stability

of the oscillator, but also increases the

start-up time.

2: Rs may be required in HS mode, as well

as XT mode, to avoid overdriving crystals

with low drive level specification.

3: Since each resonator/crystal has its own

characteristics, the user should consult the

resonator/crystal manufacturer for appro-

priate values of external components., or

verify oscillator performance.

OSC1

OSC2

Open

Clock from

Ext. System

PIC18FXXX

Note:

If the oscillator frequency divided by 4 sig-

nal is not required in the application, it is

recommended to use RCIO mode to save

current.

OSC2/CLKO

CEXT

REXT

PIC18FXXX

OSC1

FOSC/4

Internal

Clock

VDD

VSS

Recommended values:3 k

Ω ≤ REXT ≤ 100 kΩ

CEXT > 20pF

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