PIC18F2420-I/SOC01 Microchip, PIC18F2420-I/SOC01 Datasheet - Page 264

• Manufacturer Part Number: PIC18F2420-I/SOC01
• Description: 28 pin, 16 kb flash, 3804 ram, 25 i/o...
• Manufacturer: Microchip

managed clock source resumes in the power-managed
PIC18F2420/2520/4420/4520
FIGURE 23-4:
23.4.3
By entering a power-managed mode, the clock multi-
plexer selects the clock source selected by the OSCCON
register. Fail-Safe Clock Monitoring of the power-
mode.
If an oscillator failure occurs during power-managed
operation, the subsequent events depend on whether
or not the oscillator failure interrupt is enabled. If
enabled (OSCFIF = 1), code execution will be clocked
by the INTOSC multiplexer. An automatic transition
back to the failed clock source will not occur.
If the interrupt is disabled, subsequent interrupts while
in Idle mode will cause the CPU to begin executing
instructions while being clocked by the INTOSC
source.
23.4.4
The FSCM is designed to detect oscillator failure at any
point after the device has exited Power-on Reset
(POR) or low-power Sleep mode. When the primary
device clock is EC, RC or INTRC modes, monitoring
can begin immediately following these events.
For oscillator modes involving a crystal or resonator
(HS, HSPLL, LP or XT), the situation is somewhat
different. Since the oscillator may require a start-up
DS39631E-page 262
Note:
Sample Clock
CM Output
FSCM INTERRUPTS IN
POWER-MANAGED MODES
POR OR WAKE FROM SLEEP
OSCFIF
Device
Output
Clock
The device clock is normally at a much higher frequency than the sample clock. The relative frequencies in this
example have been chosen for clarity.
(Q)
FSCM TIMING DIAGRAM
CM Test
time considerably longer than the FCSM sample clock
time, a false clock failure may be detected. To prevent
this, the internal oscillator block is automatically config-
ured as the device clock and functions until the primary
clock is stable (the OST and PLL timers have timed
out). This is identical to Two-Speed Start-up mode.
Once the primary clock is stable, the INTRC returns to
its role as the FSCM source.
As noted in Section 23.3.1 “Special Considerations
for Using Two-Speed Start-up”, it is also possible to
select another clock configuration and enter an
alternate power-managed mode while waiting for the
primary clock to become stable. When the new power-
managed mode is selected, the primary clock is
disabled.
Note:
CM Test
Oscillator
Failure
The same logic that prevents false oscilla-
tor failure interrupts on POR, or wake from
Sleep, will also prevent the detection of
the oscillator’s failure to start at all follow-
ing these events. This can be avoided by
monitoring the OSTS bit and using a
timing routine to determine if the oscillator
is taking too long to start. Even so, no
oscillator failure interrupt will be flagged.
© 2008 Microchip Technology Inc.
Detected
Failure
CM Test