PIC18F6585 Datasheet, PDF (30/496 Pages) Microchip Technology – 64/68/80-Pin High-Performance, 64-Kbyte Enhanced Flash Microcontrollers with ECAN Module

PIC18F6585/8585/6680/8680
2.6.2 OSCILLATOR TRANSITIONS
PIC18F6585/8585/6680/8680 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 switch-
ing 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 run-
ning all the time. After the SCS0 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), 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 2-8:
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
SCS
(OSCCON<0>)
TDLY
Program
Counter
PC
PC + 2
PC + 4
Note:
TDLY is the delay from SCS high to first count of transition circuit.
FIGURE 2-9:
TIMING FOR TRANSITION BETWEEN TIMER1 AND OSC1 (HS, XT, LP)
Q3
Q4
Q1
T1OSI
OSC1
TOST
TT1P
Q1 Q2 Q3 Q4 Q1 Q2 Q3
1 2 34 567 8
TSCS
Internal
System Clock
SCS
(OSCCON<0>)
Program
Counter
PC
Note:
TOST = 1024 TOSC (drawing not to scale).
TOSC
PC + 2
PC + 6
DS30491C-page 28
 2004 Microchip Technology Inc.