PIC16LC505T-04E/SL Ver la hoja de datos (PDF) - Microchip Technology

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PIC16LC505T-04E/SL

14-Pin, 8-Bit CMOS Microcontroller

Microchip Technology 

7.2.3 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 7-3 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 kΩ resistor

provides the negative feedback for stability. The 10 kΩ

potentiometers bias the 74AS04 in the linear region.

This circuit could be used for external oscillator

designs.

FIGURE 7-3:

EXTERNAL PARALLEL

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

+5V

10k

4.7k

74AS04

To Other

Devices

74AS04

PIC16C505

CLKIN

10k

XTAL

10k

20 pF 20 pF

Figure 7-4 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 Ω resistors provide the negative

feedback to bias the inverters in their linear region.

FIGURE 7-4:

EXTERNAL SERIES

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

330

330

To Other

Devices

74AS04

74AS04

74AS04

0.1 mF

PIC16C505

CLKIN

XTAL

PIC16C505

7.2.4 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.

Figure 7-5 shows how the R/C combination is

connected to the PIC16C505. 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

package lead frame capacitance.

The Electrical Specifications section shows RC

frequency variation from part to part due to normal

process variation. The variation is larger for larger

values of R (since leakage current variation will affect

RC frequency more for large R) and for smaller values

of C (since variation of input capacitance will affect RC

frequency more).

Also, see the Electrical Specifications section for

variation of oscillator frequency due to VDD for given

Rext/Cext values, as well as frequency variation due to

operating temperature for given R, C and VDD values.

FIGURE 7-5:

VDD

EXTERNAL RC OSCILLATOR

MODE

Rext

OSC1

Internal

clock

Cext

VSS

FOSC/4

N

PIC16C505

OSC2/CLKOUT

© 1999 Microchip Technology Inc.

DS40192C-page 29

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