PIC18F258-ESLASHSOSQTP datasheet(22/402 Pages) MICROCHIP

22 / 402 page

PIC18FXX8

DS41159E-page 20

2.6

Oscillator Switching Feature

The PIC18FXX8 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 PIC18FXX8 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 2-6 shows a block diagram of the

system clock sources. The clock switching feature is

enabled by programming the Oscillator Switching

Enable

(OSCSEN)

bit

Configuration

CONFIG1H, to a ‘0’. Clock switching is disabled in an

erased device. See Section 12.2 “Timer1 Oscillator”

for further details of the Timer1 oscillator and

Section 24.1 “Configuration Bits” for Configuration

2.6.1

SYSTEM CLOCK SWITCH BIT

The system clock source switching is performed under

software control. The system clock switch bit, SCS

(OSCCON register), controls the clock switching. When

the SCS bit is ‘0’, the system clock source comes from

the main oscillator selected by the FOSC2:FOSC0

configuration bits. When the SCS bit is set, the system

clock source comes from the Timer1 oscillator. The SCS

bit is cleared on all forms of Reset.

FIGURE 2-6:

DEVICE CLOCK SOURCES

OSCCON: OSCILLATOR CONTROL REGISTER

Note:

The Timer1 oscillator must be enabled to

switch the system clock source. The

Timer1 oscillator is enabled by setting the

T1OSCEN bit in the Timer1 Control regis-

ter (T1CON). If the Timer1 oscillator is not

enabled, any write to the SCS bit will be

ignored (SCS bit forced cleared) and the

main oscillator continues to be the system

clock source.

U-0

R/W-1

—

—SCS

bit 7

bit 0

bit 7-1

Unimplemented: Read as ‘0’

bit 0

SCS: System Clock Switch bit

When OSCSEN configuration bit = 0 and T1OSCEN bit is set:

= Switch to Timer1 oscillator/clock pin

= Use primary oscillator/clock input pin

When OSCSEN is clear or T1OSCEN is clear:

Bit is forced clear.

Legend:

R = Readable bit

W = Writable bit

U = Unimplemented bit, read as ‘0’

-n = Value at POR

‘1’ = Bit is set

‘0’ = Bit is cleared

x = Bit is unknown

PIC18FXX8

TOSC

4 x PLL

TT1P

TSCLK

Clock

Source

TOSC/4

Timer 1 Oscillator

T1OSCEN

Enable

Oscillator

T1OSO

T1OSI

Clock Source Option

for Other Modules

OSC1

OSC2

Sleep

Main Oscillator

Note:

I/O pins have diode protection to VDD and VSS.

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