PIC18F2331/2431/4331/4431
FIGURE 23-4:
FSCM TIMING DIAGRAM
Sample Clock
Oscillator
Failure
System
Clock
Output
CM Output
(Q)
Failure
Detected
OSCFIF
CM Test
CM Test
CM Test
Note:
The system clock is normally at a much higher frequency than the sample clock. The relative frequencies in
this example have been chosen for clarity.
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
different. 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
multiplexer. The device will not transition to a different
clock source until the fail-safe condition is cleared.
Note:
The same logic that prevents false
oscillator 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.
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 system clock to become stable. When the new
powered-managed mode is selected, the primary clock
is disabled.
DS39616D-page 278
2010 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
