PIC16C641 Datasheet

IC型号：
PIC16C641
PDF描述：
8-Bit EPROM Microcontrollers with Analog Comparators
文件大小：
661.21KB
PDF页数：
137页
芯片厂家：
MICROCHIP

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136

PIC16C64X & PIC16C66X

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 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 compo-

nents used. Figure 9-6 shows how the R/C combina-

tion is connected to the PIC16CXXX. For Rext values

below 2.2 k

鈩?/div>

, the oscillator operation may become

unstable, or stop completely. For very high Rext values

(e.g. 1 M

鈩?/div>

), the oscillator becomes sensitive to noise,

humidity and leakage. Thus, we recommend to keep

Rext between 3 k

鈩?/div>

and 100 k

鈩?/div>

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 pack-

age lead frame capacitance.

See characterization data for desired device for RC fre-

quency variation from part to part due to normal pro-

cess 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 frequency more).

See characterization data for desired device for varia-

tion of oscillator frequency due to V

for given Rext/

Cext values as well as frequency variation due to oper-

ating 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 pur-

poses or to synchronize other logic (see Figure 3-3 for

waveform).

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 9-4 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

鈩?/div>

resistor provides the

negative feedback for stability. The 10 k

鈩?/div>

potentiome-

ter biases 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

PIC16CXXX

CLKIN

10k

XTAL

10k

20 pF

Figure 9-5 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

鈩?/div>

resistors provide the negative feed-

back to bias the inverters in their linear region.

FIGURE 9-6:

Rext

RC OSCILLATOR MODE

FIGURE 9-5:

EXTERNAL SERIES

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

To Other

Devices

74AS04

PIC16CXXX

CLKIN

OSC1

330 k鈩?/div>

74AS04

0.1

碌F

XTAL

330 k鈩?/div>

74AS04

Internal

clock

PIC16CXXX

Cext

Fosc/4

OSC2/CLKOUT

DS30559A-page 58

Preliminary

漏

1996 Microchip Technology Inc.

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