PIC18F452-IPT Datasheet, PDF (24/332 Pages) Microchip Technology – High-Performance, Enhanced Flash Microcontrollers with 10-Bit A/D

PIC18FXX2
2.6.2 OSCILLATOR TRANSITIONS
The PIC18FXX2 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.
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.
FIGURE 2-8:
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
3
4
5
6
7
8
Tscs
TOSC
Internal
System
Clock
SCS
(OSCCON<0>)
Program
PC
Counter
TDLY
PC + 2
PC + 4
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)
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 + 6
DS39564C-page 22
© 2006 Microchip Technology Inc.