PIC18F258-ESLASHLSQTP datasheet(20/402 Pages) MICROCHIP

20 / 402 page

PIC18FXX8

DS41159E-page 18

TABLE 2-2:

CAPACITOR SELECTION FOR

CRYSTAL OSCILLATOR

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) values 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 differ-

ence in lead frame capacitance between package types

will also affect the oscillation frequency, especially for

low CEXT values. The user also needs to take into

account variation due to tolerance of external R and C

components used. Figure 2-2 shows how the RC

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 2-2:

RC OSCILLATOR MODE

The RCIO Oscillator mode functions like the RC mode,

except that the OSC2 pin becomes an additional

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

PORTA (RA6).

Osc Type

Crystal

Freq

Cap. Range

32.0 kHz

33 pF

200 kHz

15 pF

200 kHz

47-68 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 on this page.

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: Recommended values of C1 and C2 are

identical to the ranges tested (Table 2-1).

2: Higher capacitance increases the stability

of the oscillator but also increases the

start-up time.

3: Since each resonator/crystal has its own

characteristics, the user should consult

the resonator/crystal manufacturer for

appropriate

values

external

components.

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

as XT mode, to avoid overdriving crystals

with low drive level specification.

Note:

If the oscillator frequency divided by 4

signal is not required in the application, it

is recommended to use RCIO mode to

save current.

OSC2/CLKO

CEXT

REXT

PIC18FXX8

OSC1

FOSC/4

Internal

Clock

VDD

VSS

Recommended values:

3 k

Ω ≤ REXT ≤ 100 kΩ

CEXT > 20 pF

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