PIC18LF8520-IPT301 View Datasheet(PDF) - Microchip Technology

Part Name

Description

Manufacturer

PIC18LF8520-IPT301

64/80-Pin High-Performance, 256 Kbit to 1 MbitEnhanced Flash Microcontrollers with A/D

Microchip Technology 

PIC18F6520/8520/6620/8620/6720/8720

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

T1OSI

OSC1

Q1 Q2 Q3 Q4 Q1

1

TT1P

Q1 Q2 Q3 Q4

Q1 Q2

Q3 Q4 Q1

2

3

4

5

6

7

8

TSCS

Internal

System

Clock

TOSC

TDLY

SCS

(OSCCON<0>)

Program

Counter

PC

PC + 2

PC + 4

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

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)

Q3

Q4

Q1

T1OSI

OSC1

OSC2

Internal

System Clock

SCS

(OSCCON<0>)

TOST

TOSC

TT1P

Q1 Q2 Q3 Q4 Q1 Q2 Q3

1 2 34 567 8

TSCS

Program

Counter

PC

PC + 2

PC + 6

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

DS39609C-page 26

 2003-2013 Microchip Technology Inc.

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