PIC16F684-I/SLG Microchip, PIC16F684-I/SLG Datasheet - Page 30

• Manufacturer Part Number: PIC16F684-I/SLG
• Description: pic, flash, Microcontrollers, Microprocessors, Semiconductors and Actives, ic, mcu
• Manufacturer: Microchip

PIC16F684
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 oscilla-
tor is stable, the LFINTOSC returns to its role as the
FSCM source.
DS41202C-page 28
Note:
Sample Clock
Note:
CM Output
OSCFIF
System
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
 2004 Microchip Technology Inc.
Detected
Failure
CM Test