PIC18F2220/2320/4220/4320
23.4.3
FSCM INTERRUPTS IN POWER
MANAGED MODES
23.4.4
POR OR WAKE FROM SLEEP
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
system clock is EC, RC or INTRC modes, monitoring
can begin immediately following these events.
As previously mentioned, entering a power managed
mode clears the fail-safe condition. By entering a
power managed mode, the clock multiplexer selects
the clock source selected by the OSCCON register.
Fail-safe monitoring of the power managed clock
source resumes in the power managed mode.
For oscillator modes involving a crystal or resonator
(HS, HSPLL, LP or XT), the situation is somewhat dif-
ferent. Since the oscillator may require a start-up 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 configured
as the system 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.
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, the device will not exit the
power managed mode on oscillator failure. Instead, the
device will continue to operate as before but clocked by
the INTOSC multiplexer. While in Idle mode, subse-
quent interrupts will cause the CPU to begin executing
instructions while being clocked by the INTOSC multi-
plexer. The device will not transition to a different clock
source until the fail-safe condition is cleared.
Note:
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 following
these events. This can be avoided by
monitoring the OSTS bit and using a tim-
ing routine to determine if the oscillator is
taking too long to start. Even so, no
oscillator failure interrupt will be flagged.
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 alter-
nate power managed mode while waiting for the pri-
mary system clock to become stable. When the new
powered managed mode is selected, the primary clock
is disabled.
DS39599C-page 250
2003 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
