PIC16F87-ISLASHML datasheet(39/214 Pages) MICROCHIP

39 / 214 page

 2003 Microchip Technology Inc.

Preliminary

DS30487B-page 37

PIC16F87/88

4.4

RC Oscillator

For timing insensitive applications, the “RC” and

“RCIO” device options offer additional cost savings.

The RC oscillator frequency is a function of the supply

voltage, the resistor (REXT) and capacitor (CEXT) val-

ues, and the operating temperature. In addition to this,

the oscillator frequency will vary from unit to unit due to

normal manufacturing variation. Furthermore, the dif-

ference 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 4-4 shows how the

R/C combination is connected.

In the RC Oscillator mode, the oscillator frequency

divided by 4 is available on the OSC2 pin. This signal may

be used for test purposes or to synchronize other logic.

FIGURE 4-4:

RC OSCILLATOR MODE

The RCIO Oscillator mode (Figure 4-5) functions like

the RC mode, except that the OSC2 pin becomes an

additional general purpose I/O pin. The I/O pin

becomes bit 6 of PORTA (RA6).

FIGURE 4-5:

RCIO OSCILLATOR MODE

4.5

Internal Oscillator Block

The PIC16F87/88 devices include an internal oscillator

block, which generates two different clock signals;

either can be used as the system’s clock source. This

can eliminate the need for external oscillator circuits on

the OSC1 and/or OSC2 pins.

The main output (INTOSC) is an 8 MHz clock source,

which can be used to directly drive the system clock. It

also drives the INTOSC postscaler which can provide a

range of six clock frequencies from 125 kHz to 4 MHz.

The other clock source is the internal RC oscillator

(INTRC) which provides a 31.25 kHz (32

µs nominal

period) output. The INTRC oscillator is enabled by

selecting the INTRC as the system clock source, or

when any of the following are enabled:

• Power-up Timer

• Watchdog Timer

• Two-Speed Start-up

• Fail-Safe Clock Monitor

These features are discussed in greater detail in

Section 15.0 “Special Features of the CPU”.

The clock source frequency (INTOSC direct, INTRC

direct, or INTOSC postscaler) is selected by configuring

the IRCF bits of the OSCCON register (page 40).

OSC2/CLKO

CEXT

REXT

PIC16F87/88

OSC1

FOSC/4

Internal

Clock

VDD

VSS

Recommended values: 3 k

Ω ≤ REXT ≤ 100 kΩ

CEXT > 20 pF

CEXT

REXT

PIC16F87/88

OSC1

Internal

Clock

VDD

VSS

Recommended values: 3 k

Ω ≤ REXT ≤ 100 kΩ

CEXT > 20 pF

I/O (OSC2)

RA6

Note:

Throughout this data sheet, when referring

specifically to a generic clock source, the

term “INTRC” may also be used to refer to

the clock modes using the internal oscilla-

tor block. This is regardless of whether the

actual

frequency

used

INTOSC

(8 MHz),

the

INTOSC

postscaler,

INTRC (31.25 kHz).

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