PIC18FXX8 Datasheet, PDF (23/402 Pages) Microchip Technology – 28/40-Pin High-Performance, Enhanced Flash Microcontrollers with CAN Module

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
 2004 Microchip Technology Inc.
DS41159D-page 21