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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