Test the Tester
When chips come out of fab, the designer sometimes gets to go to the
test floor to be there when the chips are first tested. One time that I
was there, the functional test patterns seemed to run flawlessly the
very first time they were tried. "Hurray! Wow - that's great. Don't
think that I've ever seen it work first time before; we're getting
pretty good at this." But then we started to wonder. "Uh,
try running the functional test on a test site instead of a design site.
Oh, it still passes?" It turned out that the test program was missing a
statement to specify when the outputs should be checked, so they were
never checked.
So, don't just test whether something works, test the tester. This means that
you should be skeptical about the way you test something, just as you
are about whether the 'something' works. You may need to deliberately
break the 'something' in order to see whether the test procedure is
checking properly. It may be necessary to break it in many
different ways.
cbenz 17-Jun-2003
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