PIC18C242 MICROCHIP [Microchip Technology], PIC18C242 Datasheet - Page 19

• Manufacturer Part Number: PIC18C242
• Description: High-Performance Microcontrollers with 10-Bit A/D
• Manufacturer: MICROCHIP [Microchip Technology]

2.6.2
The PIC18CXX2 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:
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.
FIGURE 2-9:
Program Counter
7/99 Microchip Technology Inc.
Note 1: T
Internal System
Note 1: Delay on internal system clock is eight oscillator cycles for synchronization.
(OSCCON<0>)
T1OSI
OSC1
Internal
System
Clock
SCS
(OSCCON<0>)
Program
Counter
OSCILLATOR TRANSITIONS
T1OSI
OSC2
OSC1
Clock
SCS
Q1
OST
T
Q2
OSC
= 1024T
PC
Q3
TIMING DIAGRAM FOR TRANSITION FROM OSC1 TO TIMER1 OSCILLATOR
Q3
TIMING FOR TRANSITION BETWEEN TIMER1 AND OSC1 (HS,XT,LP)
PC
Q4
Q4
Q1
OSC
T
DLY
(drawing not to scale).
1
Q1
2
T
T
1
P
3
T
OST
4
Tscs
PC + 2
Preliminary
5
T
OSC
6
PC + 2
7
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 SCS bit is set, the pro-
cessor 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.
If the main oscillator is configured for an external crys-
tal (HS, XT, LP), then the transition will take place after
an oscillator startup 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 2-9.
1
8
2
Q1
3
Q2
4
T
T
T
SCS
1
Q3
P
5
6
Q4
7
PIC18CXX2
OST
Q1
Q1
8
) has occurred. A timing
Q2 Q3
Q2
PC + 4
DS39026B-page 19
Q3
Q4
Q1 Q2
Q4
PC + 6
Q1
Q3