Datasheet

PIC18F2220/2320/4220/4320

2.6 Internal Oscillator Block

The PIC18F2X20/4X20 devices include an internal

oscillator block that generates two independent clock

signals. Either can be used as the system’s clock

source. This can eliminate the need for external

oscillator circuits on the OSC1 or OSC2 pins.

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

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

also drives a post-scaler that can provide a range of

clock frequencies from 125 kHz to 4 MHz. The

INTOSC output is enabled when the system clock

frequency is set from 125 kHz to 8 MHz.

The other clock source is the internal RC oscillator

(INTRC) that provides a 31-kHz output. The INTRC

oscillator is enabled by selecting the internal oscillator

block as the system clock source or by enabling any of

the following:

• Power-up Timer

• Fail-Safe Clock Monitor

• Watchdog Timer

• Two-Speed Start-up

These features are discussed in greater detail in

Section 23.0 “Special Features of the CPU”.

The clock source frequency (INTOSC direct, INTRC

direct or INTOSC post-scaler) is selected by configur-

ing the IRCF bits of the OSCCON register

(Register 2-2).

2.6.1 INTIO MODES

Using the internal oscillator as the clock source can

eliminate the need for up to two external oscillator pins.

This frees the pins to be used for digital I/O.

Two configurations are available:

• INTIO1 mode – The OSC2 pin outputs FOSC/4

while OSC1 functions as RA7 for digital input and

output.

• INTIO2 mode – OSC1 functions as RA7 and

OSC2 functions as RA6, both for digital input and

output.