PIC16F818-ESLASHML datasheet(37/176 Pages) MICROCHIP

37 / 176 page

 2004 Microchip Technology Inc.

DS39598E-page 35

PIC16F818/819

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 pack-

age 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 PIC16F818/819 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 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

These features are discussed in greater detail in

Section 12.0 “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 (Register 4-2).

4.5.1

INTRC MODES

Using the internal oscillator as the clock source can

eliminate the need for up to two external oscillator pins,

which can then be used for digital I/O. Two distinct

configurations are available:

• In INTIO1 mode, the OSC2 pin outputs FOSC/4

while OSC1 functions as RA7 for digital input and

output.

• In INTIO2 mode, OSC1 functions as RA7 and

OSC2 functions as RA6, both for digital input and

output.

OSC2/CLKO

CEXT

REXT

PIC16F818/819

OSC1

FOSC/4

Internal

Clock

VDD

VSS

Recommended values: 3 k

Ω ≤ REXT ≤ 100 kΩ

CEXT > 20 pF

CEXT

REXT

PIC16F818/819

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

oscillator block. This is regardless of

whether the actual frequency used is

INTOSC (8 MHz), the INTOSC postscaler

or INTRC (31.25 kHz).

PIC16F818-E/ML Datasheet(PDF) 37 Page - Microchip Technology

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