P12C508 datasheet(27/84 Pages) MICROCHIP

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Advance Information

DS40139A-page 27

PIC12C5XX

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 7-4

shows

implementation

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

Ω 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 7-4:

EXTERNAL PARALLEL

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

Figure 7-5 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 in a series resonant oscillator

circuit. The 330

Ω resistors provide the negative

feedback to bias the inverters in their linear region.

FIGURE 7-5:

EXTERNAL SERIES

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

20 pF

+5V

20 pF

10k

4.7k

10k

74AS04

XTAL

10k

74AS04

PIC12C5XX

CLKIN

To Other

Devices

330

74AS04

PIC12C5XX

CLKIN

To Other

Devices

XTAL

330

74AS04

0.1

µF

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 7-6

shows

how

the

R/C

combination

connected to the PIC12C5XX. 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 keeping

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

package lead frame capacitance.

The Electrical Speciﬁcations sections show RC

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 will affect RC frequency

more).

Also, see the Electrical Speciﬁcations sections 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.

P12C508 Datasheet(PDF) 27 Page - Microchip Technology

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