PIC18LC658T-ESLASHCL datasheet(25/366 Pages) MICROCHIP

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 2000 Microchip Technology Inc.

Advanced Information

DS30475A-page 25

PIC18CXX8

2.6

Oscillator Switching Feature

The PIC18CXX8 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 PIC18CXX8 devices, this alternate clock

source is the Timer1 oscillator. If a low frequency crys-

tal (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

execution 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 9 for further details of the

Timer1 oscillator. See Section 22.0 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 con-

figuration bits. 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.

FIGURE 2-6:

DEVICE CLOCK SOURCES

OSCCON 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 register

(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 will continue to be the sys-

tem 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

PIC18CXX8

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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