PIC18F6585/8585/6680/8680
DS30491C-page 28
2004 Microchip Technology Inc.
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
FIGURE 2-8:
TIMING DIAGRAM FOR TRANSITION FROM OSC1 TO TIMER1 OSCILLATOR
FIGURE 2-9:
TIMING FOR TRANSITION BETWEEN TIMER1 AND OSC1 (HS, XT, LP)
Q3
Q2
Q1
Q4
Q3
Q2
OSC1
Internal
SCS
(OSCCON<0>)
Program
PC + 2
PC
Note:
TDLY is the delay from SCS high to first count of transition circuit.
Q1
T1OSI
Q4
Q1
PC + 4
Q1
TSCS
Clock
Counter
System
Q2
Q3
Q4
Q1
TDLY
TT1P
TOSC
2
1
3
4
5678
Q3
Q4
Q1
Q2
Q3
Q4
Q1
Q2
OSC1
Internal
SCS
(OSCCON<0>)
Program
PC
PC + 2
Note:
TOST = 1024 TOSC (drawing not to scale).
T1OSI
System Clock
TOST
Q1
PC + 6
TT1P
TOSC
TSCS
12
3
4
5
6
7
8
Counter