PIC16C71-04SLASHSP datasheet(51/176 Pages) MICROCHIP

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DS30272A-page 51

PIC16C71X

8.2.3

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 8-6 shows implementation of a parallel resonant

oscillator circuit. The circuit is designed to use the fun-

damental frequency of the crystal. The 74AS04 inverter

performs the 180-degree phase shift that a parallel

oscillator requires. The 4.7 k

Ω resistor provides the

negative feedback for stability. The 10 k

Ω potentiome-

ter biases the 74AS04 in the linear region. This could

be used for external oscillator designs.

FIGURE 8-6:

EXTERNAL PARALLEL

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

Figure 8-7 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 k

Ω resistors provide the negative feed-

back to bias the inverters in their linear region.

FIGURE 8-7:

EXTERNAL SERIES

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

20 pF

+5V

20 pF

10k

4.7k

10k

74AS04

XTAL

10k

74AS04

CLKIN

To Other

Devices

PIC16CXXX

330 k

Ω

74AS04

PIC16CXXX

CLKIN

To Other

Devices

XTAL

330 k

Ω

74AS04

0.1

µF

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 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 8-8 shows how the R/C combina-

tion is connected to the PIC16CXX. For Rext values

below 2.2 k

Ω, the oscillator operation may become

unstable, or stop completely. For very high Rext values

(e.g. 1 M

Ω), the oscillator becomes sensitive to noise,

humidity and leakage. Thus, we recommend to keep

Rext between 3 k

Ω 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 3-2 for

waveform).

FIGURE 8-8:

RC OSCILLATOR MODE

OSC2/CLKOUT

Cext

VDD

Rext

VSS

PIC16CXXX

OSC1

Fosc/4

Internal

clock

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