PIC16CE623-04SLASHSS datasheet(52/108 Pages) MICROCHIP

52 / 108 page

PIC16CE62X

DS40182A-page 52

Preliminary

10.2.3

EXTERNAL CRYSTAL OSCILLATOR

CIRCUIT

Either a prepackaged oscillator can be used or a simple

oscillator

circuit

with TTL

gates

can

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 10-4 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

° 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 could be used for external oscillator designs.

FIGURE 10-4: EXTERNAL PARALLEL

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

Figure 10-5 shows a series resonant oscillator circuit.

This circuit is also designed to use the fundamental

frequency of the crystal. The inverter performs a 180

phase shift in a series resonant oscillator circuit. The

330 k

Ω resistors provide the negative feedback to bias

the inverters in their linear region.

FIGURE 10-5: EXTERNAL SERIES

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

20 pF

+5V

20 pF

10k

4.7k

10k

74AS04

XTAL

10k

74AS04

PIC16CE62X

CLKIN

To other

Devices

330

74AS04

PIC16CE62X

CLKIN

To other

Devices

XTAL

330

74AS04

0.1

µF

10.2.4

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 10-6 shows how the

R/C combination is connected to the PIC16CE62X. 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 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.

See Section 14.0 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 14.0 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.

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).

FIGURE 10-6: RC OSCILLATOR MODE

OSC2/CLKOUT

Cext

Rext

VDD

PIC16CE62X

OSC1

Fosc/4

Internal Clock

VDD

62X.bk Page 52 Tuesday, March 10, 1998 3:40 PM

PIC16CE623-04/SS Datasheet(PDF) 52 Page - Microchip Technology

PIC16CE623-04/SS Datasheet(HTML) 52 Page - Microchip Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

Datasheet Download

Link URL