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PIC16F87X

DS30292C-page 122

2001 Microchip Technology Inc.

TABLE 12-2:

CAPACITOR SELECTION FOR

CRYSTAL OSCILLATOR

12.2.3

RC OSCILLATOR

For timing insensitive applications, the “RC” device

option offers additional cost savings. The RC oscillator

frequency is a function of the supply voltage, the resis-

tor (REXT) and capacitor (CEXT) values, and the operat-

ing temperature. In addition to this, the oscillator

frequency will vary from unit to unit due to normal pro-

cess parameter variation. Furthermore, the difference

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 compo-

nents used. Figure 12-3 shows how the R/C combina-

tion is connected to the PIC16F87X.

FIGURE 12-3:

RC OSCILLATOR MODE

Osc Type

Crystal

Freq.

Cap. Range

32 kHz

33 pF

200 kHz

15 pF

200 kHz

47-68 pF

1 MHz

15 pF

4 MHz

15 pF

4 MHz

15 pF

8 MHz

15-33 pF

20 MHz

15-33 pF

These values are for design guidance only.

See notes following this table.

Crystals Used

32 kHz

Epson C-001R32.768K-A

± 20 PPM

200 kHz

STD XTL 200.000KHz

± 20 PPM

1 MHz

ECS ECS-10-13-1

± 50 PPM

4 MHz

ECS ECS-40-20-1

± 50 PPM

8 MHz

EPSON CA-301 8.000M-C

± 30 PPM

20 MHz

EPSON CA-301 20.000M-

± 30 PPM

Note 1: Higher capacitance increases the stability

of oscillator, but also increases the start-

up time.

2: Since each resonator/crystal has its own

characteristics, the user should consult the

resonator/crystal manufacturer for appro-

priate values of external components.

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

as XT mode, to avoid overdriving crystals

with low drive level specification.

4: When migrating from other PICmicro

devices, oscillator performance should be

verified.

OSC2/CLKOUT

CEXT

REXT

PIC16F87X

OSC1

FOSC/4

Internal

Clock

VDD

VSS

Recommended values:

3 k

≤ REXT ≤ 100 k

CEXT > 20pF

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