PIC18F242 datasheet(22/332 Pages) MICROCHIP

22 / 332 page

PIC18FXX2

DS39564C-page 20

2.6

Oscillator Switching Feature

The PIC18FXX2 devices include a feature that allows

the system clock source to be switched from the main

oscillator to an alternate low frequency clock source.

For the PIC18FXX2 devices, this alternate clock source

is the Timer1 oscillator. If a low frequency crystal (32

kHz, for example) has been attached to the Timer1

oscillator pins and the Timer1 oscillator has been

enabled, the device can switch to a Low Power Execu-

tion mode. Figure 2-7 shows a block diagram of the

system clock sources. The clock switching feature is

enabled by programming the Oscillator Switching

Enable (OSCSEN) bit in Configuration Register1H to a

’0’. Clock switching is disabled in an erased device.

See Section 11.0 for further details of the Timer1 oscil-

lator. See Section 19.0 for Configuration Register

details.

FIGURE 2-7:

DEVICE CLOCK SOURCES

PIC18FXXX

TOSC

4 x PLL

TT1P

TSCLK

Clock

Source

TOSC/4

Timer1 Oscillator

T1OSCEN

Enable

Oscillator

T1OSO

T1OSI

Clock Source option

for other modules

OSC1

OSC2

SLEEP

Main Oscillator

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