newbie question

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On Wed, 29 Apr 2009 11:27:06 -0400
"King, Mike" <MKing@...> wrote:

> What problems will be solved by a watchdog circuit?

It recovers a system, by putting it a knonw state after it has gone out
of control, whatever the cause of the loss of control... For example,


> will it only protect against rare software bugs.

be it designer/software induced problems like bugs...


> Do CPU glitch?

... or hardware problems. Yes the CPU can misbehave, it's hardware and
sometimes they have design flaws, it designed by human beings after
all. But usually this is rare and well documented. Like the floating
point problems in the old Pentium, causing incorrect calculation in some
cases, which can of course cause the software, in turn, to misbehave.
I don't know if the AVR CPU's have known bugs, though... but the
experienced on this list will chime in I am sure ;-)

Another, more likely source of problems that could cause the
hardware (CPU or whatever peripheral on the chip) is interferences from
the environment. Like electronics used in satellites, which are
sensitive to the sun's activity. When the sun sends a burst of
radiation, the satellites must be tilted/turned around, to protect the
electronics.

So a watchdog timer can help recover the system by resetting it to a
known state, from mostly software bugs (designer's fault), or
environemental "aggressions" so to speak (not the designers
fault, assuming said interferences could not be foreseen from the
specified use cases of the products).

--
Vince


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On Wed, Apr 29, 2009 at 9:33 AM, Vincent Trouilliez
<vincent.trouilliez@...> wrote:
> Another, more likely source of problems that could cause the
> hardware (CPU or whatever peripheral on the chip) is interferences from
> the environment. Like electronics used in satellites, which are
> sensitive to the sun's activity. When the sun sends a burst of
> radiation, the satellites must be tilted/turned around, to protect the
> electronics.

And it doesn't have to be quite so harsh as space, either.  I can
think of a project where code redesign was done to work around faults
related to environmental effects.  We had a product that would
experience random reboots in the field.  Turned out it was the
watchdog randomly expiring.  We spent months trying to capture the
cause of these events, certain it was a software fault of some sort.
We finally nailed it down to an alpha hit corrupting the memory in the
processor we were using.  The corrupted memory caused the processor to
occasionally "get lost" and fail to tickle the watchdog, causing the
hardware reset.

In the end, we had to move all the code out of the internal processor
memory and use external memory that was more robust with a lower FIT
rate.  I'm not sure what the AVR FIT rates are for their internal RAM.
 I did a quick perusal of their quality website
(http://atmel.com/quality/quality_reliability.asp), but it's not clear
to me whether the memory numbers are for flash only, or also RAM.  But
if they include RAM, their FIT rates are excellent.

Before this, I thought watchdogs were overkill.  Perhaps in consumer
applications it might be, but certainly in industrial, scientific, or
military applications I think a watchdog is invaluable.  Far better
for a critical device to reboot to a known state than to get lost and
act erratically.

Pete
--
--
"To love for the sake of being loved is human;  to love for the sake
of loving is Angelic."  -- Alphonse de Lamartine


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"King, Mike" wrote:

> What problems will be solved by a watchdog circuit?  For example, will it only
> protect against rare software bugs.  Do CPU glitch?

http://www.ganssle.com/spcl_reports.htm
Look at the paper "Great Watchdogs" about half way down the above page with
a list title of "Watchdog Timers".

-Preston




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