PIC16F636T-I/SL(2005) View Datasheet(PDF) - Microchip Technology

Part Name

Description

Manufacturer

PIC16F636T-I/SL
(Rev.:2005)

8/14-Pin Flash-Based, 8-Bit CMOS Microcontrollers with nanoWatt Technology

Microchip Technology 

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.

FIGURE 3-9:

FSCM TIMING DIAGRAM

Sample Clock

System

Clock

Output

CM Output

(Q)

OSCFIF

The Fail-Safe condition must be cleared before the

OSFIF flag can be cleared.

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.

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.

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.

© 2005 Microchip Technology Inc.

Preliminary

DS41232B-page 37

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