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Directed vs Random Testing

FormerMember
FormerMember

Hi

I am writing a paper looking at "myths" in functional verification. By "myth" I mean the types of things which people take as accepted truth even though there may not be much evidence to back them up. So, is the death of directed testing one such myth?

My opinion is:
- random has replaced directed as the preferred test methodology at block level (and if you want a directed test you do that via your random test bench)
- but at chip level there is still a lot of directed mainly for many reasons - the main ones being more legacy of test benches at chip level and legacy of thought (i.e. we must see the chip do this before we ship), you want to see specific integration scenarios (although random + coverage could do that too), and because you are often doing HW + SW coverification where directed is more usual

I'm looking for an active discussion plus references to good articles or papers on this topic please

Thanks

Mike Bartley

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  •  

    Hi Mike,

    Interesting topic. I agree with your thought that random & contraint random is an advanced & better way to do verification closure specially with metric driven verification based on coverage, but directed is not dead.

    However, I would like to add another aspect here. Verification is mostly done with both random and directed, and what is more common depends on the 'stage' of verification i.e. how stable is your DUV is expected to be. Typically verification is mostly started with directed testing (specific basic flow is working like device boot or reaching the initialization stage). So all the basic functions are verified using manual directed testing. Then in the middle stage of verification, it is mostly random testing where you use generate random or interesting scenarios using constraint randomization. Finally in the end the users again do directed testing to test 'corner case' situations that are specific to the device.

    Overall, I would roughly put directed-random-directed as the order of sequence of predominate way of verification.

    -Amit.

Reply

  •  

    Hi Mike,

    Interesting topic. I agree with your thought that random & contraint random is an advanced & better way to do verification closure specially with metric driven verification based on coverage, but directed is not dead.

    However, I would like to add another aspect here. Verification is mostly done with both random and directed, and what is more common depends on the 'stage' of verification i.e. how stable is your DUV is expected to be. Typically verification is mostly started with directed testing (specific basic flow is working like device boot or reaching the initialization stage). So all the basic functions are verified using manual directed testing. Then in the middle stage of verification, it is mostly random testing where you use generate random or interesting scenarios using constraint randomization. Finally in the end the users again do directed testing to test 'corner case' situations that are specific to the device.

    Overall, I would roughly put directed-random-directed as the order of sequence of predominate way of verification.

    -Amit.

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