John,
Or another example where different companies' equivalent parts are not
equivalent-- this is one that I ran into in 1977. I had designed a
Power-On-Reset (POR) circuit using a 555 timer, and had tested the circuit on a
solderless breadboard. I was being sneaky in my design, and used the RESET,
TRIGGER, and THRESHOLD inputs as three separate inputs to the 555. When we
built the first tester the POR circuit didn't work. I then discovered that
there were two flavors of 555 timers-- TI's and most companies 555's obeyed
TRIGGER if (TRIGGER < 1/3*VCC) AND (THRESHOLD > 2/3*VCC). But the National
Semiconductor LM555, which we used when we built the tester, obeyed THRESHOLD
instead... Since this is an undefined input state for the 555, the chip
designers did as they pleased.
I have heard of many other cases where engineers have used components outside
the published specifications, and gotten burned for their trouble. Bob Pease
has written several Pease Porridge columns in Electronic Design about
specmanship, and about how Design Engineers and Applications Engineers at chip
manufacturers *try* to write honest datasheets over the objections of the
marketeers. His best advice was:
1. If in doubt, try it out.
2. If an unspecified characteristic is important to you, contact the
manufacturer and get them to guarantee it in
writing.
You may find yourself having to retest your design every time the chip
manufacturer does a die shrink or moves the chip to a different process or
plant. But you are much better off to find this out early, and have time to
find a suitable alternative part or make a lifetime buy of the older (working)
part than to suddenly have 100% of your products fail in production.
John Barnes Advisory Engineer
Lexmark International
(soon to be Chief Engineer, dBi
Corporation)
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