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PIC18F2X1X/4X1X

DS39636D-page 30

2009 Microchip Technology Inc.

2.7

Clock Sources and Oscillator

Switching

Like previous PIC18 devices, the PIC18F2X1X/4X1X

family includes a feature that allows the device clock

source to be switched from the main oscillator to an

alternate low-frequency clock source. PIC18F2X1X/

4X1X devices offer two alternate clock sources. When

an alternate clock source is enabled, the various

power-managed operating modes are available.

Essentially, there are three clock sources for these

devices:

Primary oscillators

Secondary oscillators

Internal oscillator block

The primary oscillators include the External Crystal

and Resonator modes, the External RC modes, the

External Clock modes and the internal oscillator block.

The particular mode is defined by the FOSC3:FOSC0

Configuration bits. The details of these modes are

covered earlier in this chapter.

The secondary oscillators are those external sources

not connected to the OSC1 or OSC2 pins. These

sources may continue to operate even after the

controller is placed in a power-managed mode.

PIC18F2X1X/4X1X devices offer the Timer1 oscillator

as a secondary oscillator. This oscillator, in all power-

managed modes, is often the time base for functions

such as a real-time clock.

Most often, a 32.768 kHz watch crystal is connected

between the RC0/T1OSO/T13CKI and RC1/T1OSI

pins. Like the LP mode oscillator circuit, loading

capacitors are also connected from each pin to ground.

The Timer1 oscillator is discussed in greater detail in

Section 11.3 “Timer1 Oscillator”.

In addition to being a primary clock source, the internal

oscillator block is available as a power-managed

mode clock source. The INTRC source is also used as

the clock source for several special features, such as

the WDT and Fail-Safe Clock Monitor.

The clock sources for the PIC18F2X1X/4X1X devices

are shown in Figure 2-8. See Section 22.0 “Special

Features of the CPU” for Configuration register details.

FIGURE 2-8:

PIC18F2X1X/4X1X CLOCK DIAGRAM

PIC18F2X1X/4X1X

4 x PLL

FOSC3:FOSC0

Secondary Oscillator

T1OSCEN

Enable

Oscillator

T1OSO

T1OSI

Clock Source Option

for other Modules

OSC1

OSC2

Sleep

HSPLL, INTOSC/PLL

LP, XT, HS, RC, EC

T1OSC

CPU

Peripherals

IDLEN

ostscale

8 MHz

4 MHz

2 MHz

1 MHz

500 kHz

125 kHz

250 kHz

OSCCON<6:4>

111

110

101

100

011

010

001

000

31 kHz

INTRC

Source

Internal

Oscillator

Block

WDT, PWRT, FSCM

8 MHz

Internal Oscillator

(INTOSC)

OSCCON<6:4>

Clock

Control

OSCCON<1:0>

Source

8 MHz

31 kHz (INTRC)

OSCTUNE<6>

OSCTUNE<7>

and Two-Speed Start-up

Primary Oscillator

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