PIC18F2450/4450
FIGURE 18-4:
FSCM TIMING DIAGRAM
Sample Clock
Oscillator
Failure
Device
Clock
Output
CM Output
(Q)
Failure
Detected
OSCFIF
CM Test
CM Test
CM Test
Note:
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.
considerably longer than the FCSM sample clock time,
a false clock failure may be detected. To prevent this,
the internal oscillator is automatically configured 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.
18.4.3
FSCM INTERRUPTS IN
POWER-MANAGED MODES
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-
managed clock source resumes in the power-managed
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 INTRC. An automatic transition back to the failed
clock source will not occur.
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
timing routine to determine if the oscillator
is taking too long to start. Even so, no
oscillator failure interrupt will be flagged.
If the interrupt is disabled, subsequent interrupts while
in Idle mode will cause the CPU to begin executing
instructions while being clocked by the INTRC source.
18.4.4
POR OR WAKE-UP FROM SLEEP
As noted in Section 18.3.1 “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.
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 either EC or INTRC, monitoring can
begin immediately following these events.
For oscillator modes involving a crystal or resonator
(HS, HSPLL or XT), the situation is somewhat different.
Since the oscillator may require a start-up time
© 2006 Microchip Technology Inc.
Advance Information
DS39760A-page 205
MICROCHIP [ MICROCHIP ]
