PIC18F2450-ESLASHMLQTP datasheet(29/320 Pages) MICROCHIP

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PIC18F2450/4450

Advance Information

DS39760A-page 27

2.2.5

INTERNAL OSCILLATOR

The PIC18F2450/4450 devices include an internal RC

oscillator (INTRC) which provides a nominal 31 kHz out-

put. 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 18.0 “Special Features of the CPU”.

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

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.

2.3

Oscillator Settings for USB

When

the

PIC18F2450/4450

used

for

USB

connectivity, it must have either a 6 MHz or 48 MHz

clock for USB operation, depending on whether Low-

Speed or Full-Speed mode is being used. This may

require some forethought in selecting an oscillator

frequency and programming the device.

The full range of possible oscillator configurations

compatible with USB operation is shown in Table 2-3.

2.3.1

LOW-SPEED OPERATION

The USB clock for Low-Speed mode is derived from

the primary oscillator chain and not directly from the

PLL. It is divided by 4 to produce the actual 6 MHz

clock. Because of this, the microcontroller can only use

a clock frequency of 24 MHz when the USB module is

active and the controller clock source is one of the

primary oscillator modes (XT, HS or EC, with or without

the PLL).

This restriction does not apply if the microcontroller

clock source is the secondary oscillator or internal

oscillator.

2.3.2

RUNNING DIFFERENT USB AND

MICROCONTROLLER CLOCKS

The

USB

module,

either

mode,

can

run

asynchronously with respect to the microcontroller core

and other peripherals. This means that applications can

use the primary oscillator for the USB clock while the

microcontroller runs from a separate clock source at a

lower speed. If it is necessary to run the entire

application from only one clock source, full-speed

operation provides a greater selection of microcontroller

clock frequencies.

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