Microchip Technology MCP3421DM-WS Data Sheet
© 2009 Microchip Technology Inc.
DS39632E-page 307
PIC18F2455/2550/4455/4550
FIGURE 25-4:
FSCM TIMING DIAGRAM
25.4.3
FSCM INTERRUPTS IN
POWER-MANAGED MODES
POWER-MANAGED MODES
By entering a power-managed mode, the clock
multiplexer 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 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.
multiplexer 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 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.
25.4.4
POR OR WAKE-UP 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
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 con-
siderably 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 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.
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 con-
siderably 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 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 25.3.1 “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.
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.
OSCFIF
CM Output
Device
Clock
Output
Sample Clock
Failure
Detected
Oscillator
Failure
Failure
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.
example have been chosen for clarity.
(Q)
CM Test
CM Test
CM Test
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.
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.