PIC17C44-08SLASHP datasheet(102/240 Pages) MICROCHIP

102 / 240 page

PIC17C4X

DS30412C-page 102

14.2.4

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 14-5 shows implementation of a parallel reso-

nant 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 par-

allel oscillator requires. The 4.7 k

Ω resistor provides the

negative feedback for stability. The 10 k

Ω potentiometer

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

used for external oscillator designs.

FIGURE 14-5: EXTERNAL PARALLEL

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

Figure 14-6 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

circuit. The 330 k

Ω resistors provide the negative feed-

back to bias the inverters in their linear region.

FIGURE 14-6: EXTERNAL SERIES

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

20 pF

+5V

20 pF

10k

4.7k

10k

74AS04

XTAL

10k

74AS04

PIC17CXX

OSC1

To Other

Devices

330 k

Ω

74AS04

PIC17CXX

OSC1

To Other

Devices

XTAL

330 k

Ω

74AS04

0.1

µF

14.2.5

RC OSCILLATOR

For timing insensitive applications, the RC device

option offers additional cost savings. RC oscillator fre-

quency is a function of the supply voltage, the resistor

(Rext) and capacitor (Cext) values, and the operating

temperature. In addition to this, 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

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 14-6 shows how the R/C combination is con-

nected to the PIC17CXX. 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

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 little

or no external capacitance, oscillation frequency can

vary dramatically due to changes in external capaci-

tances, such as PCB trace capacitance or package

lead frame capacitance.

See Section 18.0 for 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 RC frequency more for large R) and for smaller C

(since variation of input capacitance will affect RC fre-

quency more).

See Section 18.0 for variation of oscillator frequency

due to VDD for given Rext/Cext values as well as fre-

quency variation due to operating 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 14-7: RC OSCILLATOR MODE

VDD

Rext

Cext

VSS

OSC1

Internal

clock

OSC2/CLKOUT

Fosc/4

PIC17CXX

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