PIC16C66T-04 データシートの表示（PDF） - Microchip Technology

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PIC16C66T-04

8-Bit CMOS Microcontrollers

Microchip Technology 

13.2.3 EXTERNAL CRYSTAL OSCILLATOR

CIRCUIT

Either a prepackaged oscillator can be used or a simple

oscillator circuit with TTL gates can be built. Prepack-

aged oscillators provide a wide operating range and

better stability. A well-designed crystal oscillator will

provide good performance with TTL gates. Two types

of crystal oscillator circuits can be used; one with series

resonance, or one with parallel resonance.

Figure 13-6 shows implementation of a parallel reso-

nant oscillator circuit. The circuit is designed to use the

fundamental frequency of the crystal. The 74AS04

inverter performs the 180-degree phase shift that a par-

allel oscillator requires. The 4.7 kΩ resistor provides

the negative feedback for stability. The 10 kΩ potenti-

ometer biases the 74AS04 in the linear region. This

could be used for external oscillator designs.

FIGURE 13-6: EXTERNAL PARALLEL

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

+5V

10k

4.7k

74AS04

To Other

Devices

74AS04

PIC16CXX

CLKIN

10k

XTAL

10k

20 pF 20 pF

Figure 13-7 shows a series resonant oscillator circuit.

This circuit is also designed to use the fundamental fre-

quency of the crystal. The inverter performs a 180-

degree phase shift in a series resonant oscillator cir-

cuit. The 330 kΩ resistors provide the negative feed-

back to bias the inverters in their linear region.

FIGURE 13-7: EXTERNAL SERIES

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

330 kΩ

74AS04

330 kΩ

74AS04

0.1 µF

XTAL

To Other

Devices

74AS04

PIC16CXX

CLKIN

PIC16C6X

13.2.4 RC OSCILLATOR

For timing insensitive applications the RC device option

offers additional cost savings. The RC oscillator fre-

quency 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 param-

eter 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 components used.

Figure 13-8 shows how the RC combination is con-

nected to the PIC16CXX. For Rext values below

2.2 kΩ, the oscillator operation may become unstable

or stop completely. For very high Rext values (e.g.

1 MΩ), the oscillator becomes sensitive to noise,

humidity and leakage. Thus, we recommend keeping

Rext between 3 kΩ and 100 kΩ.

Although the oscillator will operate with no external

capacitor (Cext = 0 pF), we recommend using values

above 20 pF for noise and stability reasons. With no or

small external capacitance, the oscillation frequency

can vary dramatically due to changes in external

capacitances, such as PCB trace capacitance or pack-

age lead frame capacitance.

See characterization data for desired device for RC fre-

quency variation from part to part due to normal pro-

cess variation. The variation is larger for larger R (since

leakage current variation will affect RC frequency more

for large R) and for smaller C (since variation of input

capacitance will affect RC frequency more).

See characterization data for desired device for varia-

tion of oscillator frequency due to VDD for given Rext/

Cext values as well as frequency variation due to oper-

ating temperature for given R, C, and VDD values.

The oscillator frequency, divided by 4, is available on

the OSC2/CLKOUT pin, and can be used for test pur-

poses or to synchronize other logic (see Figure 3-5 for

waveform).

FIGURE 13-8: RC OSCILLATOR MODE

V DD

Rext

Cext

VSS

OSC1

Internal

clock

PIC16CXX

OSC2/CLKOUT

Fosc/4

© 1997 Microchip Technology Inc.

DS30234D-page 127

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