18F8680 datasheet(26/496 Pages) MICROCHIP

26 / 496 page

PIC18F6585/8585/6680/8680

DS30491C-page 24

 2004 Microchip Technology Inc.

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 pack-

age 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 2-3 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

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

TBD

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 (see Figure 2-1) may be required in

HS mode, as well as XT mode, to avoid

overdriving crystals with low drive level

specifications.

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

PIC18FXX80/XX85

OSC2/CLKO

CEXT

REXT

PIC18FXX80/XX85

OSC1

FOSC/4

Internal

Clock

VDD

VSS

Recommended values:

3 k

Ω ≤ REXT ≤ 100 kΩ

CEXT > 20pF

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