18F8680 datasheet(30/496 Pages) MICROCHIP

30 / 496 page

PIC18F6585/8585/6680/8680

DS30491C-page 28

 2004 Microchip Technology Inc.

2.6.2

OSCILLATOR TRANSITIONS

PIC18F6585/8585/6680/8680 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 switch-

ing 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 2-8. The Timer1 oscillator is assumed to be run-

ning all the time. After the SCS0 bit is set, the processor

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

crystal (HS, XT, LP), then the transition will take place

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

timing diagram, indicating the transition from the

Timer1 oscillator to the main oscillator for HS, XT and

LP modes, is shown in Figure 2-9.

FIGURE 2-8:

TIMING DIAGRAM FOR TRANSITION FROM OSC1 TO TIMER1 OSCILLATOR

FIGURE 2-9:

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

OSC1

Internal

SCS

(OSCCON<0>)

Program

PC + 2

Note:

TDLY is the delay from SCS high to first count of transition circuit.

T1OSI

PC + 4

TSCS

Clock

Counter

System

TDLY

TT1P

TOSC

5678

OSC1

Internal

SCS

(OSCCON<0>)

Program

PC + 2

Note:

TOST = 1024 TOSC (drawing not to scale).

T1OSI

System Clock

TOST

PC + 6

TT1P

TOSC

TSCS

Counter

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