PIC18F4550ESOSQTP datasheet(29/430 Pages) MICROCHIP

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Preliminary

DS39632D-page 27

PIC18F2455/2550/4455/4550

2.2.5

INTERNAL OSCILLATOR BLOCK

The PIC18F2455/2550/4455/4550 devices include an

internal oscillator block which generates two different

clock signals; either can be used as the microcontroller’s

clock source. If the USB peripheral is not used, the

internal oscillator may 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 device clock. It

also drives the INTOSC postscaler which can provide a

range of clock frequencies from 31 kHz to 4 MHz. The

INTOSC output is enabled when a clock frequency

from 125 kHz to 8 MHz is selected.

The other clock source is the internal RC oscillator

(INTRC) which provides a nominal 31 kHz output.

INTRC is enabled if it is selected as the device clock

source; it is also enabled automatically when any of the

following are enabled:

• Power-up Timer

• Fail-Safe Clock Monitor

• Watchdog Timer

• Two-Speed Start-up

These features are discussed in greater detail in

Section 25.0 “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 32).

2.2.5.1

Internal Oscillator Modes

When the internal oscillator is used as the micro-

controller clock source, one of the other oscillator

modes (External Clock or External Crystal/Resonator)

must be used as the USB clock source. The choice of

the USB clock source is determined by the particular

internal oscillator mode.

There are four distinct modes available:

INTHS mode: The USB clock is provided by the

oscillator in HS mode.

INTXT mode: The USB clock is provided by the

oscillator in XT mode.

INTCKO mode: The USB clock is provided by an

external clock input on OSC1/CLKI; the OSC2/

CLKO pin outputs FOSC/4.

INTIO mode: The USB clock is provided by an

external clock input on OSC1/CLKI; the OSC2/

CLKO pin functions as a digital I/O (RA6).

Of these four modes, only INTIO mode frees up an

additional pin (OSC2/CLKO/RA6) for port I/O use.

2.2.5.2

OSCTUNE Register

The internal oscillator’s output has been calibrated at

the factory but can be adjusted in the user’s applica-

tion. This is done by writing to the OSCTUNE register

(Register 2-1). The tuning sensitivity is constant

throughout the tuning range.

When the OSCTUNE register is modified, the INTOSC

and INTRC frequencies will begin shifting to the new

frequency. The INTRC clock will reach the new

frequency

within

clock

cycles

(approximately,

8* 32

μs = 256 μs). The INTOSC clock will stabilize

within 1 ms. Code execution continues during this shift.

There is no indication that the shift has occurred.

The OSCTUNE register also contains the INTSRC bit.

The INTSRC bit allows users to select which internal

oscillator provides the clock source when the 31 kHz

frequency option is selected. This is covered in greater

detail in Section 2.4.1 “Oscillator Control Register”.

2.2.5.3

Internal Oscillator Output Frequency

and Drift

The internal oscillator block is calibrated at the factory

to produce an INTOSC output frequency of 8.0 MHz.

However, this frequency may drift as VDD or tempera-

ture changes, which can affect the controller operation

in a variety of ways.

The low-frequency INTRC oscillator operates indepen-

dently of the INTOSC source. Any changes in INTOSC

across voltage and temperature are not necessarily

reflected by changes in INTRC and vice versa.

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