PIC18F258-ESLASHPSQTP datasheet(23/402 Pages) MICROCHIP

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DS41159E-page 21

PIC18FXX8

2.6.2

OSCILLATOR TRANSITIONS

The PIC18FXX8 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.

Figure 2-7 shows a timing diagram indicating the tran-

sition from the main oscillator to the Timer1 oscillator.

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.

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), 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 2-8.

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

Q1 Q2 Q3 Q4

Q1 Q2

OSC1

Internal System

SCS

(OSCCON<0>)

Program

PC + 2

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

T1OSI

Clock

OSC2

TOST

PC + 4

TT1P

TOSC

TSCS

Counter

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