PIC18F6520-I Datasheet, PDF (28/380 Pages) Micrel Semiconductor – 64/80-Pin High-Performance, 256 Kbit to 1 Mbit Enhanced Flash Microcontrollers with A/D

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).
DS39609B-page 26
 2004 Microchip Technology Inc.