PIC18F8722 datasheet(39/446 Pages) MICROCHIP

39 / 446 page

DS39646C-page 37

PIC18F8722 FAMILY

2.7

Clock Sources and Oscillator

Switching

The PIC18F8722 family of devices includes a feature

that allows the device clock source to be switched from

the main oscillator to an alternate clock source. These

devices also 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 FOSC<3:0>

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.

The PIC18F8722 family of devices offers 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 13.3 “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 PIC18F8722 family of devices

are shown in Figure 2-11. See Section 25.0 “Special

Features of the CPU” for Configuration register details.

FIGURE 2-11:

PIC18F8722 FAMILY CLOCK DIAGRAM

PIC18F6527/6622/6627/6722/8527/8622/8627/8722

4 x PLL

FOSC<3:0>

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

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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