PIC18F258T-I/SP View Datasheet(PDF) - Microchip Technology

Part Name

Description

Manufacturer

PIC18F258T-I/SP

28/40-Pin High-Performance, Enhanced Flash Microcontrollers with CAN Module

Microchip Technology 

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

Q1 Q2 Q3 Q4 Q1

TT1P

Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1

T1OSI

OSC1

1

234

5678

Tscs

Internal

System

Clock

TOSC

SCS

(OSCCON<0>)

Program

PC

Counter

TDLY

PC + 2

PC + 4

Note 1: Delay on internal system clock is eight oscillator cycles for synchronization.

FIGURE 2-8:

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

Q3

Q4

Q1

TT1P

Q1 Q2 Q3 Q4 Q1 Q2 Q3

T1OSI

OSC1

OSC2

Internal System

Clock

SCS

(OSCCON<0>)

TOST

TOSC

1 2 34 567 8

TSCS

Program

Counter

PC

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

PC + 2

PC + 4

© 2006 Microchip Technology Inc.

DS41159E-page 21

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