PIC12F508ESLASHMS datasheet(45/110 Pages) MICROCHIP

45 / 110 page

DS41236E-page 45

PIC12F508/509/16F505

TABLE 7-2:

CAPACITOR SELECTION FOR

CRYSTAL OSCILLATOR –

PIC12F508/509/16F505(2)

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 perfor-

mance 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 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-

ters 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

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

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 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

components used.

Figure 7-5 shows

how

the

R/C combination is

connected to the PIC12F508/509/16F505 devices. For

REXT values below 3.0 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 5.0 k

Ω and

100 k

Ω.

Osc

Type

Resonator

Freq.

Cap. Range

32 kHz(1)

15 pF

200 kHz

1 MHz

4 MHz

47-68 pF

15 pF

47-68 pF

15 pF

(3)

20 MHz

15-47 pF

Note 1:

For VDD > 4.5V, C1 = C2

≈ 30 pF is

recommended.

These values are for design guidance

only. Rs may be required to avoid over-

driving crystals with low drive level specifi-

cation. Since each crystal has its own

characteristics, the user should consult

the crystal manufacturer for appropriate

values of external components.

PIC16F505 only.

20 pF

+5V

20 pF

10k

4.7k

10k

74AS04

XTAL

10k

74AS04

PIC16F505

CLKIN

To Other

Devices

PIC12F508

PIC12F509

330

74AS04

PIC16F505

CLKIN

To Other

Devices

XTAL

330

74AS04

0.1 mF

PIC12F508

PIC12F509

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