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Sheet目录3841

200页 201页 202页 203页 [204页]205页 206页 207页 208页

2004 Microchip Technology Inc.

DS30498C-page 35

PIC16F7X7

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) values and the

operating temperature. In addition to this, the oscillator

frequency will vary from unit to unit due to normal

manufacturing 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 varia-

tion 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 PIC16F7X7 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

s 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 38).

OSC2/CLKO

CEXT

REXT

PIC16F7X7

OSC1

FOSC/4

Internal

Clock

VDD

VSS

Recommended values: 3 k

≤ REXT ≤ 100 k

CEXT > 20 pF

CEXT

REXT

PIC16F7X7

OSC1

Internal

Clock

VDD

VSS

Recommended values: 3 k

≤ REXT ≤ 100 k

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

oscillator block. This is regardless of

whether the actual frequency used is

INTOSC (8 MHz), the INTOSC postscaler

or INTRC (31.25 kHz).

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