PIC16F5X datasheet(24/88 Pages) MICROCHIP

24 / 88 page

PIC16F5X

DS41213D-page 22

4.3

External Crystal Oscillator Circuit

Either a pre-packaged oscillator or a simple oscillator

circuit with TTL gates can be used as an external

crystal oscillator circuit. Pre-packaged oscillators

provide a wide operating range and better stability. A

well designed crystal oscillator will provide good perfor-

mance with TTL gates. Two types of crystal oscillator

circuits can be used: one with parallel resonance or one

with series resonance.

Figure 4-3 shows an implementation example of a

parallel resonant oscillator circuit. The circuit is

designed to use the fundamental frequency of the

crystal. The 74AS04 inverter performs the 180

° phase

shift that a parallel oscillator requires. The 4.7 k

resistor provides the negative feedback for stability.

The 10 k

Ω potentiometers bias the 74AS04 in the

linear region. This circuit could be used for external

oscillator designs.

FIGURE 4-3:

EXTERNAL PARALLEL

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

(USING XT, HS OR LP

OSCILLATOR MODE)

Figure 4-4 shows a series resonant oscillator circuit.

This circuit is also designed to use the fundamental

frequency of the crystal. The inverters perform a 360

phase shift in a series resonant oscillator circuit. The

330 k

Ω resistors provide the negative feedback to bias

the inverters in their linear region.

FIGURE 4-4:

EXTERNAL SERIES

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

(USING XT, HS OR LP

OSCILLATOR MODE)

4.4

RC Oscillator

For applications where precise timing is not a require-

ment, the RC oscillator option is available. The

operation and functionality of the RC oscillator is

dependent upon a number of variables. The RC

oscillator frequency is a function of:

• Supply voltage

• Resistor (REXT) and capacitor (CEXT) values

• Operating temperature.

The oscillator frequency will vary from unit to unit due

to normal process parameter variation. 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 account for the

tolerance of the external R and C components.

Figure 4-5 shows

how

the

R/C combination is

connected.

The oscillator frequency, divided by 4, is available on

the OSC2/CLKOUT pin and can be used for test

purposes or to synchronize other logic.

FIGURE 4-5:

RC OSCILLATOR MODE

20 pF

+5V

20 pF

10k

4.7k

10k

74AS04

XTAL

10k

74AS04

PIC16F5X

CLKIN

To Other

Devices

OSC2

Open

330

74AS04

PIC16F5X

CLKIN

To Other

Devices

XTAL

330

74AS04

0.1

μF

OSC2

Open

VDD

REXT

CEXT

VSS

OSC1

Internal

clock

OSC2/CLKOUT

FOSC/4

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