PIC16LF18854 Datasheet, PDF (118/668 Pages) Microchip Technology – C Compiler Optimized RISC Architecture

PIC16(L)F18856/76
6.4 Fail-Safe Clock Monitor
The Fail-Safe Clock Monitor (FSCM) allows the device
to continue operating should the external oscillator fail.
The FSCM is enabled by setting the FCMEN bit in the
Configuration Words. The FSCM is applicable to all
external Oscillator modes (LP, XT, HS, EC and
Secondary Oscillator).
FIGURE 6-9:
External
Clock
FSCM BLOCK DIAGRAM
Clock Monitor
Latch
SQ
LFINTOSC
Oscillator
31 kHz
(~32 s)
÷ 64
488 Hz
(~2 ms)
Sample Clock
R
Q
Clock
Failure
Detected
6.4.1 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 6-9. 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.
6.4.2 FAIL-SAFE OPERATION
When the external clock fails, the FSCM switches the
device clock to the HFINTOSC at 1 MHz clock
frequency and sets the bit flag OSFIF of the PIR1
register. Setting this flag will generate an interrupt if the
OSFIE bit of the PIE1 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, by writing to the NOSC and NDIV bits of the
OSCCON1 register.
6.4.3 FAIL-SAFE CONDITION CLEARING
The Fail-Safe condition is cleared after a Reset,
executing a SLEEP instruction or changing the NOSC
and NDIV bits of the OSCCON1 register. When
switching to the external oscillator or PLL, the OST is
restarted. While the OST is running, the device
continues to operate from the INTOSC selected in
OSCCON1. When the OST times out, the Fail-Safe
condition is cleared after successfully switching to the
external clock source. The OSFIF bit should be cleared
prior to switching to the external clock source. If the
Fail-Safe condition still exists, the OSFIF flag will again
become set by hardware.
DS40001824A-page 118
Preliminary
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