PIC16F/LF1946/47
5.5.3
FAIL-SAFE CONDITION CLEARING
5.5
Fail-Safe Clock Monitor
The Fail-Safe condition is cleared after a Reset,
executing a SLEEPinstruction or changing the SCS bits
of the OSCCON register. When the SCS bits are
changed, the OST is restarted. While the OST is
running, the device continues to operate from the
INTOSC selected in OSCCON. When the OST times
out, the Fail-Safe condition is cleared and the device
will be operating from the external clock source. The
Fail-Safe condition must be cleared before the OSFIF
flag can be cleared.
The Fail-Safe Clock Monitor (FSCM) allows the device
to continue operating should the external oscillator fail.
The FSCM can detect oscillator failure any time after
the Oscillator Start-up Timer (OST) has expired. The
FSCM is enabled by setting the FCMEN bit in the
Configuration Word 1. The FSCM is applicable to all
external Oscillator modes (LP, XT, HS, EC, Timer1
Oscillator and RC).
FIGURE 5-9:
FSCM BLOCK DIAGRAM
5.5.4
RESET OR WAKE-UP FROM SLEEP
Clock Monitor
Latch
The FSCM is designed to detect an oscillator failure
after the Oscillator Start-up Timer (OST) has expired.
The OST is used after waking up from Sleep and after
any type of Reset. The OST is not used with the EC or
RC Clock modes so that the FSCM will be active as
soon as the Reset or wake-up has completed. When
the FSCM is enabled, the Two-Speed Start-up is also
enabled. Therefore, the device will always be executing
code while the OST is operating.
External
Clock
S
Q
LFINTOSC
Oscillator
÷ 64
R
Q
31 kHz
(~32 s)
488 Hz
(~2 ms)
Sample Clock
Note:
Due to the wide range of oscillator start-up
times, the Fail-Safe circuit is not active
during oscillator start-up (i.e., after exiting
Reset or Sleep). After an appropriate
amount of time, the user should check the
Status bits in the OSCSTAT register to
verify the oscillator start-up and that the
system clock switchover has successfully
completed.
Clock
Failure
Detected
5.5.1
FAIL-SAFE DETECTION
The FSCM module detects a failed oscillator by
comparing the external oscillator to the FSCM sample
clock. The sample clock is generated by dividing the
LFINTOSC by 64. See Figure 5-9. Inside the fail
detector block is a latch. The external clock sets the
latch on each falling edge of the external clock. The
sample clock clears the latch on each rising edge of the
sample clock. A failure is detected when an entire
half-cycle of the sample clock elapses before the
external clock goes low.
5.5.2
FAIL-SAFE OPERATION
When the external clock fails, the FSCM switches the
device clock to an internal clock source and sets the bit
flag OSFIF of the PIR2 register. Setting this flag will
generate an interrupt if the OSFIE bit of the PIE2
register is also set. The device firmware can then take
steps to mitigate the problems that may arise from a
failed clock. The system clock will continue to be
sourced from the internal clock source until the device
firmware successfully restarts the external oscillator
and switches back to external operation.
The internal clock source chosen by the FSCM is
determined by the IRCF<3:0> bits of the OSCCON
register. This allows the internal oscillator to be
configured before a failure occurs.
DS41414A-page 70
Preliminary
2010 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
