PIC18LC658T-ESLASHCL datasheet(26/366 Pages) MICROCHIP

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PIC18CXX8

DS30475A-page 26

Advanced Information

 2000 Microchip Technology Inc.

2.6.2

OSCILLATOR TRANSITIONS

The PIC18CXX8 devices contain circuitry to prevent

"glitches" when switching between oscillator sources.

Essentially, the circuitry waits for eight rising edges of

the clock source that the processor is switching to.

This ensures that the new clock source is stable and

that its pulse width will not be less than the shortest

pulse width of the two clock sources.

A timing diagram indicating the transition from the

main oscillator to the Timer1 oscillator is shown in

Figure 2-7. The Timer1 oscillator is assumed to be

running all the time. After the SCS bit is set, the pro-

cessor is frozen at the next occurring Q1 cycle. After

eight synchronization cycles are counted from the

Timer1 oscillator, operation resumes. No additional

delays are required after the synchronization cycles.

The sequence of events that takes place when switch-

ing from the Timer1 oscillator to the main oscillator will

depend on the mode of the main oscillator. In addition

to eight clock cycles of the main oscillator, additional

delays may take place.

If the main oscillator is configured for an external crys-

tal (HS, XT, LP), the transition will take place after an

oscillator start-up time (TOST) has occurred. A timing

diagram indicating the transition from the Timer1 oscil-

lator to the main oscillator for HS, XT and LP modes is

shown in Figure 2-8.

FIGURE 2-7:

TIMING DIAGRAM FOR TRANSITION FROM OSC1 TO TIMER1 OSCILLATOR

FIGURE 2-8:

TIMING DIAGRAM FOR TRANSITION BETWEEN TIMER1 AND OSC1 (HS,XT,LP)

OSC1

Internal

SCS

(OSCCON<0>)

Program

PC + 2

Note 1:

Delay on internal system clock is eight oscillator cycles for synchronization.

T1OSI

PC + 4

Tscs

Clock

Counter

System

TDLY

TT1P

TOSC

5678

OSC1

Internal System

SCS

(OSCCON<0>)

Program Counter

PC + 2

Note 1:

TOST = 1024TOSC (drawing not to scale).

T1OSI

Clock

OSC2

TOST

PC + 4

TT1P

TOSC

TSCS

567

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