PIC16C621A Datasheet

IC型号：
PIC16C621A
PDF描述：
EPROM-Based 8-Bit CMOS Microcontrollers
文件大小：
2148.18KB
PDF页数：
128页
芯片厂家：
Microchip

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PIC16C62X

9.2.3

EXTERNAL CRYSTAL OSCILLATOR

CIRCUIT

9.2.4

RC OSCILLATOR

For timing insensitive applications the 鈥淩C鈥?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 9-6 shows how the

R/C combination is connected to the PIC16C62X. For

Rext values below 2.2 k鈩? the oscillator operation may

become unstable, or stop completely. For very high

Rext values (e.g., 1 M鈩?, the oscillator becomes

sensitive to noise, humidity and leakage. Thus, we

recommend to keep Rext between 3 k鈩?and 100 k鈩?

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.

See Section 13.0 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 13.0 for variation of oscillator frequency

due to V

for given Rext/Cext values as well as

frequency variation due to operating temperature for

given R, C, and V

values.

The oscillator frequency, divided by 4, is available on

the OSC2/CLKOUT pin, and can be used for test

purposes or to synchronize other logic (Figure 3-2 for

waveform).

Either a prepackaged oscillator can be used or a simple

oscillator circuit with TTL gates can be built.

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 series resonance, or one with parallel

resonance.

Figure 9-4 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掳 phase shift that a parallel

oscillator requires. The 4.7 k鈩?resistor provides the

negative feedback for stability. The 10 k鈩?/div>

potentiometers bias the 74AS04 in the linear region.

This could be used for external oscillator designs.

FIGURE 9-4:

EXTERNAL PARALLEL

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

To other

Devices

+5V

10k

4.7k

74AS04

PIC16C62X

CLK

10k

XTAL

10k

20 pF

Figure 9-5 shows a series resonant oscillator circuit.

This circuit is also designed to use the fundamental

frequency of the crystal. The inverter performs a 180掳

phase shift in a series resonant oscillator circuit. The

330 k鈩?resistors provide the negative feedback to bias

the inverters in their linear region.

FIGURE 9-6:

RC OSCILLATOR MODE

FIGURE 9-5:

EXTERNAL SERIES

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

To other

Devices

PIC16C62X

74AS04

CLK

PIC16C62X

Rext

OSC1

Internal Clock

Cext

Fosc/4

OSC2/CLKOUT

330 k鈩?/div>

74AS04

0.1

碌F

330 k鈩?/div>

74AS04

XTAL

DS30235G-page 48

Preliminary

漏

1998 Microchip Technology Inc.

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