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2010 Microchip Technology Inc.

Preliminary

DS41350E-page 15

PIC18F/LF1XK50

2.0

OSCILLATOR MODULE

2.1

Overview

The oscillator module has a variety of clock sources

and features that allow it to be used in a wide range of

applications, maximizing performance and minimizing

power consumption. Figure 2-1 illustrates a block

diagram of the oscillator module.

Key features of the oscillator module include:

System Clock Selection

- Primary External Oscillator

- Secondary External Oscillator

- Internal Oscillator

Oscillator Start-up Timer

System Clock Selection

Clock Switching

4x Phase Lock Loop Frequency Multiplier

CPU Clock Divider

USB Operation

- Low Speed

- Full Speed

Two-Speed Start-up Mode

Fail-Safe Clock Monitoring

2.2

System Clock Selection

The SCS bits of the OSCCON register select between

the following clock sources:

Primary External Oscillator

Secondary External Oscillator

Internal Oscillator

TABLE 2-1:

SYSTEM CLOCK SELECTION

The default state of the SCS bits sets the system clock

to be the oscillator defined by the FOSC bits of the

CONFIG1H Configuration register. The system clock

will always be defined by the FOSC bits until the SCS

bits are modified in software.

When the Internal Oscillator is selected as the system

clock, the IRCF bits of the OSCCON register and the

INTSRC bit of the OSCTUNE register will select either

the LFINTOSC or the HFINTOSC. The LFINTOSC is

selected when the IRCF<2:0> = 000 and the INTSRC

bit is clear. All other combinations of the IRCF bits and

the INTSRC bit will select the HFINTOSC as the

system clock.

2.3

Primary External Oscillator

The Primary External Oscillator’s mode of operation is

selected by setting the FOSC<3:0> bits of the

CONFIG1H Configuration register. The oscillator can

be set to the following modes:

LP: Low-Power Crystal

XT: Crystal/Ceramic Resonator

HS: High-Speed Crystal Resonator

RC: External RC Oscillator

EC: External Clock

Additionally, the Primary External Oscillator may be

shut-down under firmware control to save power.

Note:

The frequency of the system clock will be

referred to as FOSC throughout this

document.

Configuration

Selection

SCS <1:0>

System Clock

Internal Oscillator

Secondary External Oscillator

(Default after Reset)

Oscillator defined by

FOSC<3:0>

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