PIC12F683
active and the WDT is cleared. The SCS bit
(OSCCON<0>) is not updated. Enabling FSCM does
not affect the LTS bit.
3.7
Fail-Safe Clock Monitor
The Fail-Safe Clock Monitor (FSCM) is designed to
allow the device to continue to operate in the event of
an oscillator failure. The FSCM can detect oscillator
failure at any point after the device has exited a Reset
or Sleep condition and the Oscillator Start-up Timer
(OST) has expired.
The FSCM sample clock is generated by dividing the
INTRC clock by 64. This will allow enough time
between FSCM sample clocks for a system clock edge
to occur. Figure 3-8 shows the FSCM block diagram.
On the rising edge of the sample clock, the monitoring
latch (CM = 0) will be cleared. On a falling edge of the
primary system clock, the monitoring latch will be set
(CM = 1). In the event that a falling edge of the sample
clock occurs and the monitoring latch is not set, a clock
failure has been detected. The assigned internal oscil-
lator is enabled when FSCM is enabled, as reflected by
the IRCF.
FIGURE 3-8:
FSCM BLOCK DIAGRAM
Primary
Clock
Clock
Clock
Fail
Failure
Detector
Detected
LFINTOSC
Oscillator
Note:
Two-Speed Start-up is automatically
enabled when the Fail-Safe Clock Monitor
mode is enabled.
÷ 64
Note:
Primary clocks with a frequency ≤ ~488 Hz
will be considered failed by the FSCM. A
slow starting oscillator can cause an
FSCM interrupt.
The FSCM function is enabled by setting the FCMEN
bit in the Configuration Word register (CONFIG). It is
applicable to all external clock options (LP, XT, HS, EC,
RC or IO modes).
In the event of an external clock failure, the FSCM will
set the OSFIF bit (PIR1<2>) and generate an oscillator
fail interrupt if the OSFIE bit (PIE1<2>) is set. The
device will then switch the system clock to the internal
oscillator. The system clock will continue to come from
the internal oscillator unless the external clock recovers
and the Fail-Safe condition is exited.
3.7.1
FAIL-SAFE CONDITION CLEARING
The Fail-Safe condition is cleared after a Reset, the
execution of a SLEEPinstruction, or a modification of
the SCS bit. While in Fail-Safe condition, the
PIC12F683 uses the internal oscillator as the system
clock source. The IRCF bits (OSCCON<6:4>) can be
modified to adjust the internal oscillator frequency
without exiting the Fail-Safe condition.
The frequency of the internal oscillator will depend
upon the value contained in the IRCF bits
(OSCCON<6:4>). Upon entering the Fail-Safe
condition, the OSTS bit (OSCCON<3>) is automati-
cally cleared to reflect that the internal oscillator is
The Fail-Safe condition must be cleared before the
OSFIF flag can be cleared.
FIGURE 3-9:
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.
2004 Microchip Technology Inc.
Preliminary
DS41211B-page 27
MICROCHIP [ MICROCHIP ]
