PIC18FXX20
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 SCS bit is set, the processor
is frozen at the next occurring Q1 cycle. After eight syn-
chronization cycles are counted from the Timer1 oscil-
lator, operation resumes. No additional delays are
required after the synchronization cycles.
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.
FIGURE 2-8:
TIMING DIAGRAM FOR TRANSITION FROM OSC1 TO TIMER1 OSCILLATOR
Q1 Q2 Q3 Q4 Q1
Q1
Q2
Q3
Q4
Q1
Q2
Q3
Q4
Q1
TT1P
1
2
3
4
5
6
7
8
T1OSI
OSC1
TSCS
TOSC
Internal
System
Clock
TDLY
SCS
(OSCCON<0>)
Program
PC
PC + 2
PC + 4
Counter
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 crys-
tal (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 oscil-
lator 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)
Q1 Q2 Q3 Q4 Q1 Q2 Q3
Q3
Q4
Q1
TT1P
T1OSI
OSC1
1
2
3
4
5
6
7
8
TOST
TSCS
OSC2
TOSC
Internal
System Clock
SCS
(OSCCON<0>)
Program
Counter
PC + 6
PC
PC + 2
Note 1: TOST = 1024 TOSC (drawing not to scale).
DS39609A-page 26
Advance Information
2003 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
