PIC18F2331 MICROCHIP [Microchip Technology], PIC18F2331 Datasheet - Page 284

• Manufacturer Part Number: PIC18F2331
• Description: 28/40/44-Pin Enhanced Flash Microcontrollers with nanoWatt Technology, High Performance PWM and A/D
• Manufacturer: MICROCHIP [Microchip Technology]

PIC18F2331/2431/4331/4431
FIGURE 22-4:
22.4.3
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.
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.
DS39616B-page 282
Sample Clock
Note:
CM Output
OSCFIF
System
FSCM INTERRUPTS IN POWER-
MANAGED MODES
Output
Clock
(Q)
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.
FSCM TIMING DIAGRAM
CM Test
Preliminary
22.4.4
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
system 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
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.
As noted in Section 22.3.1 “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.
CM Test
Note:
Oscillator
Failure
POR OR WAKE FROM SLEEP
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.
 2003 Microchip Technology Inc.
Detected
Failure
CM Test