16C84 datasheet(37/110 Pages) MICROCHIP

37 / 110 page

PIC16C84

 1996-2013 Microchip Technology Inc.

DS30445D-page 37

8.2.3

EXTERNAL CRYSTAL OSCILLATOR

CIRCUIT

Either a prepackaged oscillator can be used or a simple

oscillator

circuit

with

TTL

gates

can

built.

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 are

available; one with series resonance, and one with

parallel resonance.

Figure 8-4 shows 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 negative

feedback for stability. The 10 k

 potentiometer biases

the 74AS04 in the linear region. This could be used for

external oscillator designs.

FIGURE 8-4:

EXTERNAL PARALLEL

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

Figure 8-5 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. The 330 k

 resistors provide the

negative feedback to bias the inverters in their linear

region.

FIGURE 8-5:

EXTERNAL SERIES

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

8.2.4

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) values, 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 also affects the oscillation frequency, especially

for low Cext values. The user needs to take into

account variation due to tolerance of the external

R and C components. Figure 8-6 shows how an R/C

combination is connected to the PIC16C84. For Rext

values below 2.2 k

, the oscillator operation may

become unstable, or stop completely. For very high

Rext values (e.g., 1 M

), the oscillator becomes

sensitive to noise, humidity and leakage. Thus, we

recommend keeping Rext between 3 k

 and 100 k.

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 little

or no external capacitance, the oscillation frequency

can vary dramatically due to changes in external

capacitances, such as PCB trace capacitance or

package lead frame capacitance.

See

the

electrical

specification

section

for

frequency variation from part to part due to normal

process variation. The variation is larger for larger R

(since

leakage

current

variation

will

affect

frequency more for large R) and for smaller C (since

variation of input capacitance has a greater affect on

RC frequency).

See the electrical specification section for variation of

oscillator frequency due to VDD for given Rext/Cext

values

well

frequency

variation

due

operating temperature.

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 (see Figure 3-2

for waveform).

FIGURE 8-6:

RC OSCILLATOR MODE

20 pF

+5V

20 pF

10k

4.7k

10k

74AS04

XTAL

10k

74AS04

PIC16CXX

CLKIN

To Other

Devices

330 k



74AS04

PIC16CXX

CLKIN

To Other

Devices

XTAL

330 k



74AS04

0.1

F

Note:

When the device oscillator is in RC mode,

do not drive the OSC1 pin with an external

clock or you may damage the device.

OSC2/CLKOUT

Cext

Rext

PIC16CXX

OSC1

Fosc/4

Internal

clock

VDD

VSS

Recommended values:

3 k

  Rext  100 k

Cext > 20pF

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