PIC18F242 datasheet(23/332 Pages) MICROCHIP

23 / 332 page

DS39564C-page 21

PIC18FXX2

2.6.1

SYSTEM CLOCK SWITCH BIT

The system clock source switching is performed under

software control. The system clock switch bit, SCS

(OSCCON<0>) controls the clock switching. When the

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

main oscillator that is selected by the FOSC configura-

tion bits in Configuration Register1H. When the SCS bit

is set, the system clock source will come from the

Timer1 oscillator. The SCS bit is cleared on all forms of

RESET.

OSCCON REGISTER

Note:

The Timer1 oscillator must be enabled and

operating to switch the system clock

source. The Timer1 oscillator is enabled by

setting the T1OSCEN bit in the Timer1

control register (T1CON). If the Timer1

oscillator is not enabled, then any write to

the SCS bit will be ignored (SCS bit forced

cleared) and the main oscillator will

continue 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 and T1OSCEN are in other states:

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

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