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Sheet目录3850

40页 41页 42页 43页 [44页]45页 46页 47页 48页

2007-2012 Microchip Technology Inc.

DS39778E-page 41

PIC18F87J11 FAMILY

3.4

External Oscillator Modes

3.4.1

CRYSTAL OSCILLATOR/CERAMIC

RESONATORS (HS MODES)

In HS or HSPLL Oscillator modes, a crystal or ceramic

resonator is connected to the OSC1 and OSC2 pins to

establish oscillation. Figure 3-2 shows the pin

connections.

The oscillator design requires the use of a crystal rated

for parallel resonant operation.

TABLE 3-1:

CAPACITOR SELECTION FOR

CERAMIC RESONATORS

TABLE 3-2:

CAPACITOR SELECTION FOR

CRYSTAL OSCILLATOR

FIGURE 3-2:

CRYSTAL/CERAMIC

RESONATOR OPERATION

(HS OR HSPLL

CONFIGURATION)

Note:

Use of a crystal rated for series resonant

operation may give a frequency out of the

crystal manufacturer’s specifications.

Typical Capacitor Values Used:

Mode

Freq.

OSC1

OSC2

8.0 MHz

16.0 MHz

27 pF

22 pF

27 pF

22 pF

Capacitor values are for design guidance only.

Different capacitor values may be required to produce

acceptable oscillator operation. The user should test

the performance of the oscillator over the expected

Vdd and temperature range for the application. Refer

to the following application notes for oscillator specific

information:

AN588, “PIC Microcontroller Oscillator Design

Guide”

AN826, “Crystal Oscillator Basics and Crystal

Selection for rfPIC and PIC Devices”

AN849, “Basic PIC Oscillator Design”

AN943, “Practical PIC Oscillator Analysis and

Design”

AN949, “Making Your Oscillator Work”

See the notes following Table 3-2 for additional

information.

Osc Type

Crystal

Freq.

Typical Capacitor Values

Tested:

4 MHz

27 pF

8 MHz

22 pF

20 MHz

15 pF

Capacitor values are for design guidance only.

Different capacitor values may be required to produce

acceptable oscillator operation. The user should test

the performance of the oscillator over the expected

Vdd and temperature range for the application.

Refer to the Microchip application notes cited in

Table 3-1 for oscillator specific information. Also see

the notes following this table for additional

information.

Note 1:

Higher capacitance increases the stability

of oscillator but also increases the start-up

time.

Since each resonator/crystal has its own

characteristics, the user should consult

the resonator/crystal manufacturer for

appropriate

values

external

components.

Rs may be required to avoid overdriving

crystals with low drive level specification.

Always verify oscillator performance over

the VDD and temperature range that is

expected for the application.

Note 1:

See Table 3-1 and Table 3-2 for initial values of

C1 and C2.

A series resistor (RS) may be required for AT

strip cut crystals.

RF varies with the oscillator mode chosen.

C1(1)

C2(1)

XTAL

OSC2

OSC1

RF(3)

Sleep

Logic

PIC18F87J11

RS(2)

Internal

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