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'When is an external watchdog timer necessary?'
1996\06\12@151350 by NEIL GANDLER

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In a response that I received from a previous question I posted, someone
commented that the internal watchdog timer within the PIC is not 100%
reliable. It doesn't always catch every type of microcontroller lock-up.
He suggested an external watchdog timer.
I decided just to be safe, to use the DS1232LP in an application where
I need high reliability. I would like to know when and if the external
component is necessary. Thanks

               Neil Gandler

1996\06\12@222536 by Mark K Sullivan

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> In a response that I received from a previous question I posted, someone
>commented that the internal watchdog timer within the PIC is not 100%
>reliable. It doesn't always catch every type of microcontroller lock-up.
>He suggested an external watchdog timer.
>I decided just to be safe, to use the DS1232LP in an application where
>I need high reliability. I would like to know when and if the external
>component is necessary. Thanks
>
>                Neil Gandler

That would have been me.  The internal watchdog is only useful if the code
screws up but the hardware is still running.  You need an external watchdog if
something can happen to the internal states of the chip.  I can tell you from
experience that the PIC does have internal states such that WDT will stop.
Examples include brownouts (VDD goes low but not too low) and external noise
(such as a burst of RF).

- Mark Sullivan -

1996\06\14@030309 by Rodger Richey

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    Neil,

    I did not read the previous message so I cannot comment, but the
    watchdog timer is very reliable.  It has it's own RC oscillator and
    runs independently of the CPU or system clock.  Since the watchdog
    timer is enabled/disabled when the device is programmed, the only
    thing your firmware can do is clear it.  The watchdog timer is
    therefore very effective for recovering from a firmware malfunction.

    The watchdog timer was not designed for brown-out situations.  If you
    expect these conditions to occur in your application, then I would
    agree to using an external device.  Otherwise use the on-chip watchdog
    timer and save some money.

    Regards,

    Rodger Richey
    Sr. Applications Engineer
    Microchip Technology Inc.


______________________________ Reply Separator _________________________________
Subject: When is an external watchdog timer necessary?
Author:  NEIL GANDLER <spam_OUTV064MB9KTakeThisOuTspamUBVMS.CC.BUFFALO.EDU> at Internet_Exchange
Date:    6/12/96 3:04 PM


In a response that I received from a previous question I posted, someone
commented that the internal watchdog timer within the PIC is not 100%
reliable. It doesn't always catch every type of microcontroller lock-up.
He suggested an external watchdog timer.
I decided just to be safe, to use the DS1232LP in an application where
I need high reliability. I would like to know when and if the external
component is necessary. Thanks

               Neil Gandler

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