PIC18F2420 MICROCHIP [Microchip Technology], PIC18F2420 Datasheet - Page 38

• Manufacturer Part Number: PIC18F2420
• Description: 28/40/44-Pin Enhanced Flash Microcontrollers with 10-Bit A/D and nanoWatt Technology
• Manufacturer: MICROCHIP [Microchip Technology]

PIC18F2420/2520/4420/4520
If the IRCF bits and the INTSRC bit are all clear, the
INTOSC output is not enabled and the IOFS bit will
remain clear; there will be no indication of the current
clock source. The INTRC source is providing the
device clocks.
If the IRCF bits are changed from all clear (thus,
enabling the INTOSC output) or if INTSRC is set, the
IOFS bit becomes set after the INTOSC output
becomes stable. Clocks to the device continue while
the INTOSC source stabilizes after an interval of
T
If the IRCF bits were previously at a non-zero value, or
if INTSRC was set before setting SCS1 and the
INTOSC source was already stable, the IOFS bit will
remain set.
FIGURE 3-3:
FIGURE 3-4:
DS39631A-page 36
IOBST
.
Note 1: Clock transition typically occurs within 2-4 T
Peripheral
Program
Counter
INTRC
OSC1
Clock
Clock
Note1: T
CPU
CPU Clock
Multiplexer
PLL Clock
Peripheral
Program
INTOSC
Counter
Output
2: Clock transition typically occurs within 2-4 T
OSC1
Clock
Q1
OST
SCS1:SCS0 bits changed
Q2
= 1024 T
TRANSITION TIMING TO RC_RUN MODE
TRANSITION TIMING FROM RC_RUN MODE TO PRI_RUN MODE
PC
Q3
Q4
OSC
Q1
; T
PLL
Q1
1
= 2 ms (approx). These intervals are not shown to scale.
T
OST
(1)
PC
2
Q2
Clock Transition
OSC
3
T
OSTS bit set
Q3
PLL
.
OSC
(1)
.
(1)
PC + 2
n-1
Q4
On transitions from RC_RUN mode to PRI_RUN mode,
the device continues to be clocked from the INTOSC
multiplexer while the primary clock is started. When the
primary clock becomes ready, a clock switch to the pri-
mary clock occurs (see Figure 3-4). When the clock
switch is complete, the IOFS bit is cleared, the OSTS
bit is set and the primary clock is providing the device
clock. The IDLEN and SCS bits are not affected by the
switch. The INTRC source will continue to run if either
the WDT or the Fail-Safe Clock Monitor is enabled.
n
Q1
1
Transition
2
Clock
n-1 n
(2)
Q2
PC + 2
Q3
Q2
Q4
 2004 Microchip Technology Inc.
Q3 Q4
Q1
Q1
Q2
PC + 4
PC + 4
Q2
Q3
Q3