PIC18LF8620 datasheet(28/380 Pages) MICROCHIP

28 / 380 page

PIC18F6520/8520/6620/8620/6720/8720

DS39609B-page 26

 2004 Microchip Technology Inc.

2.6.2

OSCILLATOR TRANSITIONS

PIC18FXX20 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 2-8.

The Timer1 oscillator is assumed to be running all the

time. After the SCS 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.

FIGURE 2-8:

TIMING DIAGRAM FOR TRANSITION FROM OSC1 TO TIMER1 OSCILLATOR

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 2-9:

TIMING 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

SCS

(OSCCON<0>)

Program

PC + 2

Note 1: TOST = 1024 TOSC (drawing not to scale).

T1OSI

System Clock

OSC2

TOST

PC + 6

TT1P

TOSC

TSCS

Counter

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