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From: rsd@sei.cmu.edu (Richard S D'Ippolito)
Newsgroups: sci.electronics
Subject: The 555
Message-ID: <10014@as0c.sei.cmu.edu>
Date: 11 Jan 91 18:55:50 GMT
Organization: Carnegie-Mellon University (Software Engineering Institute), Pgh, PA
Lines: 19


If my memory is serving me correctly, I seem to recall (from 10 or more
years back!) that there is a fundamental design flaw with the 555 timer
chip, in that there are certain operating conditions and external part
values which could cause part of the chip circuitry to latch up (like a
thyristor), a condition requiring power removal to reset, making the 555
definitely unsuitable for a watchdog design.  My further recollection is
that the flaw was publically aired in some of the professional/trade
magazines, such as EDN, causing no little excitement for the original
manufacturer.

Can anyone verify or refute this and point me to the information?  Am I
thinking of another chip?  If true about the 555, does anyone know if this
design was corrected or know of a prefered replacement part?


Thanks,

Rich