PIC18F4550ESPQTP datasheet(29/438 Pages) MICROCHIP

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DS39632E-page 27

PIC18F2455/2550/4455/4550

2.2.4

PLL FREQUENCY MULTIPLIER

PIC18F2455/2550/4255/4550 devices include a Phase

Locked Loop (PLL) circuit. This is provided specifically

for USB applications with lower speed oscillators and

can also be used as a microcontroller clock source.

The PLL is enabled in HSPLL, XTPLL, ECPLL and

ECPIO Oscillator modes. It is designed to produce a

fixed 96 MHz reference clock from a fixed 4 MHz input.

The output can then be divided and used for both the

USB and the microcontroller core clock. Because the

PLL has a fixed frequency input and output, there are

eight prescaling options to match the oscillator input

frequency to the PLL.

There is also a separate postscaler option for deriving

the microcontroller clock from the PLL. This allows the

USB peripheral and microcontroller to use the same

oscillator input and still operate at different clock

speeds. In contrast to the postscaler for XT, HS and EC

modes, the available options are 1/2, 1/3, 1/4 and 1/6

of the PLL output.

The HSPLL, ECPLL and ECPIO modes make use of

the HS mode oscillator for frequencies up to 48 MHz.

The prescaler divides the oscillator input by up to 12 to

produce the 4 MHz drive for the PLL. The XTPLL mode

can only use an input frequency of 4 MHz which drives

the PLL directly.

FIGURE 2-6:

PLL BLOCK DIAGRAM

(HS MODE)

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 33).

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.

VCO

Loop

Filter

and

Prescaler

OSC2

OSC1

PLL Enable

FIN

FOUT

SYSCLK

Phase

Comparator

HS/EC/ECIO/XT Oscillator Enable

÷24

(from CONFIG1H Register)

Oscillator

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