PIC18F6X2X Datasheet, PDF (28/386 Pages) Microchip Technology – 64/80-Pin High Performance, 64-Kbyte Enhanced FLASH Microcontrollers with A/D

PIC18F6X2X/8X2X
2.6.2 OSCILLATOR TRANSITIONS
PIC18F6X2X/8X2X 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 running 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.
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
234
5678
TSCS
Internal
System
Clock
TOSC
SCS
(OSCCON<0>)
TDLY
Program
Counter
PC
PC + 2
PC + 4
Note:
TDLY is delay from SCS high to first count of transition circuit.
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
TOST
1 2 34 567 8
TSCS
Internal
System Clock
SCS
(OSCCON<0>)
Program
Counter
PC
Note 1: TOST = 1024 TOSC (drawing not to scale).
TOSC
PC + 2
PC + 6
DS39612A-page 26
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