PIC16F570 datasheet(40/129 Pages) MICROCHIP

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PIC16F570

DS40001684E-page 40

 2013-2014 Microchip Technology Inc.

TABLE 8-2:

CAPACITOR SELECTION FOR

CRYSTAL OSCILLATOR(2)

8.3.3

EXTERNAL CRYSTAL OSCILLATOR

CIRCUIT

Either a prepackaged oscillator or a simple oscillator

circuit with TTL gates can be used as an external

crystal oscillator circuit. Prepackaged 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 parallel

resonance, or one with series resonance.

Figure 8-3 shows implementation of a parallel resonant

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

parallel oscillator requires. The 4.7 k

 resistor provides

the negative feedback for stability. The 10 k



potentiometers bias the 74AS04 in the linear region.

This circuit could be used for external oscillator

designs.

FIGURE 8-3:

EXTERNAL PARALLEL

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

Figure 8-4 shows a series resonant oscillator circuit.

This circuit is also designed to use the fundamental

frequency of the crystal. The inverter performs a 180-

degree phase shift in a series resonant oscillator

circuit. The 330

 resistors provide the negative

feedback to bias the inverters in their linear region.

FIGURE 8-4:

EXTERNAL SERIES

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

8.3.4

EXTERNAL 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

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

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.

Osc.

Type

Resonator

Freq.

Cap. Range

32 kHz(1)

15 pF

200 kHz

1 MHz

4 MHz

47-68 pF

15 pF

47-68 pF

15 pF

20 MHz

15-47 pF

Note 1:

For VDD > 4.5V, C1 = C2

 30 pF is

recommended.

These values are for design guidance

only. Rs may be required to avoid

overdriving crystals with low drive level

specification. Since each crystal has its

own characteristics, the user should

consult the crystal manufacturer for

appropriate

values

external

components.

20 pF

+5V

20 pF

10k

4.7k

10k

74AS04

XTAL

10k

74AS04

CLKIN

To Other

Devices

PIC® Device

330

74AS04

PIC® Device

CLKIN

To Other

Devices

XTAL

330

74AS04

0.1 mF

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