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Subject: Re: Why use resistors to tie a logic input to ground?
Date: Thu, 17 Oct 2002 19:46:18 +0100
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"fred bartoli" wrote:
>I used to work for Alstom, building boards that travel in trains and such
>Hard tying inputs was strictly forbidden on the assumption that it could
>lead to non detected future field failures that would have been otherwise :
>even if you don't use it, a dead input or gate is a bad sign for the future
>life of the part.
Only if you bother to test them. Resistive tie up/downs are required to
allow node forcing during board test.
I suggest the above argument is bullshit, just how many manufacturing test
failures where down to faulty chips and what tiny proportion of these were
faults only detected by stimulating unused pins? I suggest that the
additional components and solder joints for pull up/down resistors would
have a far worse impact on reliability than not testing unused pins.
I am reminded of an old story I read somewhere about a Japanese manufacture
being required to supply an American company some chips with a 99.9% AQL.
On delivery they supplied the 0.1% of faulty chips in a separate package
with a covering note.
I suspect this argument was used to justify forced node testing of
individual chips which means the test programmers can just verify
individual chips against existing models instead of having to devise a
proper functional test for the board.
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