PIC18F258 MICROCHIP [Microchip Technology], PIC18F258 Datasheet - Page 23

• Manufacturer Part Number: PIC18F258
• Description: High Performance, 28/40-Pin Enhanced FLASH Microcontrollers with CAN
• Manufacturer: MICROCHIP [Microchip Technology]

2.6.2
The PIC18FXX8 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 2-7 shows a timing diagram indicating the tran-
sition from the main oscillator to the Timer1 oscillator.
The Timer1 oscillator is assumed to be running all the
time. After the SCS bit is set, the processor is frozen at
the next occurring Q1 cycle. After eight synchronization
cycles are counted from the Timer1 oscillator, opera-
tion resumes. No additional delays are required after
the synchronization cycles.
FIGURE 2-7:
FIGURE 2-8:
 2002 Microchip Technology Inc.
Internal System
(OSCCON<0>)
Note 1: T
T1OSI
OSC1
Internal
System
Clock
SCS
(OSCCON<0>)
Program
Counter
Note 1: Delay on internal system clock is eight oscillator cycles for synchronization.
Program
Counter
T1OSI
OSC1
OSC2
Clock
SCS
OST
Q1
OSCILLATOR TRANSITIONS
= 1024 T
T
OSC
Q2
PC
Q3
Q3
OSC
PC
Q4
TIMING DIAGRAM FOR TRANSITION FROM OSC1 TO TIMER1 OSCILLATOR
TIMING DIAGRAM FOR TRANSITION BETWEEN TIMER1 AND OSC1 (HS, XT, LP)
(drawing not to scale).
Q4
Q1
T
DLY
1
Q1
T
2
T
1
P
3
T
4
OST
Tscs
PC + 2
Preliminary
5
T
OSC
6
PC + 2
7
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), the transition will take place after an
oscillator start-up time (T
diagram indicating the transition from the Timer1 oscil-
lator to the main oscillator for HS, XT, and LP modes is
shown in Figure 2-8.
1
8
2
Q1
3
Q2
4
T
T
SCS
T
1
5
Q3
P
6
Q4
7
PIC18FXX8
OST
Q1
8
Q1 Q2 Q3 Q4 Q1 Q2
) has occurred. A timing
Q2
PC + 4
DS41159B-page 21
Q3
Q4
PC + 4
Q1
Q3