PIC12F635 datasheet(39/196 Pages) MICROCHIP

39 / 196 page

Preliminary

DS41232B-page 37

PIC12F635/PIC16F636/639

3.7.1

FAIL-SAFE CONDITION CLEARING

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 PIC12F635/

PIC16F636/639 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:

FSCM TIMING DIAGRAM

3.7.2

RESET OR WAKE-UP FROM SLEEP

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 or 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.

OSCFIF

CM Output

System

Clock

Output

Sample Clock

Failure

Detected

Oscillator

Failure

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.

(Q)

CM Test

Note:

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.

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