PIC18F258 Datasheet, PDF (23/384 Pages) Microchip Technology – High Performance, 28/40-Pin Enhanced FLASH Microcontrollers with CAN

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, opera-
tion 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 crys-
tal (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 oscil-
lator 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
2 34
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
 2002 Microchip Technology Inc.
Preliminary
DS41159B-page 21