PIC16F87 Datasheet, PDF (146/214 Pages) Microchip Technology – 18/20/28-Pin Enhanced FLASH Microcontrollers with nanoWatt Technology

PIC16F87/88
15.12.4 FAIL-SAFE OPTION
The Fail-Safe Clock Monitor (FSCM) is designed to
allow the device to continue to operate even in the
event of an oscillator failure.
FIGURE 15-10:
Peripheral
Clock
FSCM BLOCK DIAGRAM
Clock Monitor
Latch (CM)
(edge-triggered)
SQ
INTRC
Oscillator
÷ 64
31.25 kHz
488 Hz
(32 µs)
(2.048 ms)
CQ
Clock
Failure
Detected
The FSCM function is enabled by setting the FCMEN
bit in Configuration Word 2.
In the event of an oscillator failure, the FSCM will gen-
erate an oscillator fail interrupt and will switch the sys-
tem clock over to the internal oscillator. The system
will continue to come from the internal oscillator until
the fail-safe condition is exited. The fail-safe condition
is exited with either a RESET, the execution of a
SLEEP instruction or a write to the OSCCON register.
The frequency of the internal oscillator will depend
upon the value contained in the IRCF bits. Another
clock source can be selected via the IRCF and the
SCS bits of the OSCCON register.
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.
On the rising edge of the postscaled clock, the moni-
toring latch (CM = 0) will be cleared. On a falling edge
of the primary or secondary system clock, the monitor-
ing latch will be set (CM = 1). In the event that a falling
edge of the postscaled clock occurs, and the
monitoring latch is not set, a clock failure has been
detected.
While in Fail-Safe mode, a RESET will exit the fail-
safe condition. If the primary clock source is config-
ured for a crystal, the OST timer will wait for the 1024
clock cycles for the OST time-out, and the device will
continue running from the internal oscillator until the
OST is complete. A SLEEP instruction, or a write to the
SCS bits (where SCS bits do not = 00), can be
performed to put the device into a low-power mode.
Note:
Two-Speed Start-up mode is automatically
enabled when the fail-safe option is
enabled.
If RESET occurs while in Fail-Safe mode and the pri-
mary clock source is EC, or RC, then the device will
immediately switch back to EC or RC mode.
15.12.4.1 Fail-Safe in Low-power Mode
A write to the OSCCON register, or SLEEP instruction
will end the fail-safe condition. The system clock will
default to the source selected by the SCS bits, which
is either T1OSC, INTRC, or none (SLEEP mode).
However, the FSCM will continue to monitor the sys-
tem clock. If the secondary clock fails, the device will
immediately switch to the internal oscillator clock. If
OSFIE is set, an interrupt will be generated.
FIGURE 15-11: FSCM TIMING DIAGRAM
Sample Clock
System
Clock
Output
CM Output
(Q)
OSCFIF
Oscillator
Failure
Failure
Detected
Note:
CM Test
CM Test
CM Test
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.
DS30487B-page 144
Preliminary
 2003 Microchip Technology Inc.