PIC16F87 datasheet(35/230 Pages) MICROCHIP

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DS30487D-page 35

PIC16F87/88

4.0

OSCILLATOR

CONFIGURATIONS

4.1

Oscillator Types

The PIC16F87/88 can be operated in eight different

oscillator modes. The user can program three configu-

ration bits (FOSC2:FOSC0) to select one of these eight

modes

(modes

5-8

are

new

PIC16

oscillator

configurations):

Low-Power Crystal

Crystal/Resonator

High-Speed Crystal/Resonator

External Resistor/Capacitor with

FOSC/4 output on RA6

RCIO

External Resistor/Capacitor with

I/O on RA6

INTIO1

Internal Oscillator with FOSC/4

output on RA6 and I/O on RA7

INTIO2

Internal Oscillator with I/O on RA6

and RA7

ECIO

External Clock with I/O on RA6

4.2

Crystal Oscillator/Ceramic

Resonators

In XT, LP or HS modes, a crystal or ceramic resonator

is connected to the OSC1/CLKI and OSC2/CLKO pins

to establish oscillation (see Figure 4-1 and Figure 4-2).

The PIC16F87/88 oscillator design requires the use of

a parallel cut crystal. Use of a series cut crystal may

give a frequency out of the crystal manufacturer’s

specifications.

FIGURE 4-1:

CRYSTAL OPERATION

(HS, XT, OR LP

OSCILLATOR

CONFIGURATION)

TABLE 4-1:

CAPACITOR SELECTION FOR

CRYSTAL OSCILLATOR (FOR

DESIGN GUIDANCE ONLY)

Note1: See Table 4-1 for typical values of C1 and C2.

2: A series resistor (RS) may be required for AT strip

cut crystals.

3: RF varies with the crystal chosen (typically

between 2 M

 to 10 M.

C1(1)

C2(1)

XTAL

OSC2

RS(2)

OSC1

RF(3)

Sleep

To Internal

Logic

PIC16F87/88

Osc Type

Crystal

Freq

Typical Capacitor Values

Tested:

32 kHz

33 pF

200 kHz

56 pF

1 MHz

15 pF

4 MHz

15 pF

4 MHz

15 pF

8 MHz

15 pF

20 MHz

15 pF

Capacitor values are for design guidance only.

These capacitors were tested with the crystals listed

below for basic start-up and operation. These values

were not optimized.

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.

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.

2: Since each crystal has its own character-

istics, the user should consult the crystal

manufacturer for appropriate values of

external components.

3: Rs may be required in HS mode, as well

as XT mode, to avoid overdriving crystals

with low drive level specification.

4: Always verify oscillator performance over

the VDD and temperature range that is

expected for the application.

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