PIC16HV785 Microchip Technology Inc., PIC16HV785 Datasheet - Page 34

• Manufacturer Part Number: PIC16HV785
• Description: 20-pin Flash-based 8-bit Cmos Microcontroller
• Manufacturer: Microchip Technology Inc.

PIC16F785/HV785
3.7.1
The Fail-Safe condition is cleared after a Reset, the
execution of a SLEEP instruction, or a modification of
the SCS bit. While in Fail-Safe condition, the
PIC16F785/HV785 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 Fail-Safe condition must be cleared before the
OSFIF flag can be cleared.
FIGURE 3-9:
3.7.2
The FSCM is designed to 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. If the external clock is EC or RC mode,
monitoring will begin immediately following these
events.
For LP, XT or HS mode, the external oscillator may
require a start-up time considerably longer than the
FSCM sample clock time; a false clock failure may be
detected (see Figure 3-9). To prevent this, the internal
oscillator is automatically configured as the system
clock and functions until the external clock is stable (the
OST has timed out). This is identical to Two-Speed
Start-up mode. Once the external oscillator is stable,
the LFINTOSC returns to its role as the FSCM source.
DS41249D-page 32
Note:
Sample Clock
Note:
CM Output
OSCFIF
System
FAIL-SAFE CONDITION CLEARING
RESET OR WAKE-UP FROM SLEEP
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
OSTS bit (OSCCON<3>) to verify the
oscillator start-up and system clock
switchover has successfully completed.
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
CM Test
Oscillator
Failure
© 2006 Microchip Technology Inc.
Detected
Failure
CM Test