At 08:21 AM 7/31/01 -0500, you wrote:
>Hmmmmmm....     I'm still skeptical.  You say you let the watchdog timer
>just time out every few milliseconds, rebooting the machine?  An interesting
>idea...

Except that "most cases" qualifier in his statement. ;-)

>How does a 12C508 know from millisecond to millisecond what is going on?
>Since you don't have any EEprom, can't rely on the RAM to be correct after a
>watchdog timer reboot, the processor is "starting cold" every watchdog
>period.

Unless the power is glitched, the RAM should be intact. SFRs are another
matter.

>  I suppose this might be OK for a toy or a benchtop prototype,  but
>it would not do for a product which can fail in an unsafe way, like a heater
>or a cooking appliance.

It was recommended (as an option) by Intel in one of their (very good)
early app
notes on designing for EMI with the MCS-48 and MCS-51 series. Obviously you
need a hardware safety backup (thermal cutout or whatever) whenever you
have a
micro controlling something like that, right?

>I'd still vote for a US$0.03  transistor brownout circuit.

Or better, I've used the US$0.05 version with the zener, but the supervisor
circuits are even better, bulletproof and not too expensive for lower
quantities. Depends on the temperature range you need to work with and the
supply voltage range of the micro etc.  at the frequency it's running at.

Best regards,
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Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com
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