Who in this profession hasn’t heard of DFT?  It is, of course, Design for Testability… or is it Design for Test?  Shouldn’t it be DfT?  Well, regardless, we all know what it means, so let’s not get bugged down on details.

This was exactly what I thought when we attempted to define some basic terms and get consensus from the testability professionals at the Testability Management Action Group (TMAG).  It was in a meeting of the “Beyond DFT Committee” that we first attempted to quickly glaze over simple definitions, such as DFT and BIST (Built-In Self Test).  I steadfastly defended the definitions I proposed while Dr. Scott Davidson of Sun Microsystems and Dr. Tony Ambler of the University of Texas were just as committed to their views of what DFT was and was not.  When we finally lowered our voices a few decibels, we were able to agree on the following definitions of simple testability terms:

Testability is a property of a circuit that enables one to test it easily, or in some cases to test it at all, by being able to control and observe signal nodes that are buried within the circuit.

Design for Testability (DFT) is a methodology incorporated in the design of electronic circuits which takes into consideration the post-design testing phase, and which attempts to reduce the effort and cost of testing.

Structured Design for Test (Structured DFT) is a design technique, usually for ICs, which enables tests to be created automatically or algorithmically.  One example of is the design of an IC with a scan structure that enables test for structural faults using a predefined test methodology.  While the test may be long, it can be generated without test engineering involvement and test patterns created by computer programs can ensure nearly 100% fault detection of certain fault types.

Built-In Self-Test (BIST) is a method of design – generally for ICs – whereby the mission circuit tests itself.  Though this is seldom performed strictly without additional circuitry, if the entire circuitry performing the test is contained within an IC, we call it self-test, in situ test, or built-in self-test.

Built-In Test (BIT) is similar to BIST in that it performs test of the circuit it resides in, but it is generally used at board and system levels and often uses extra hardware, software, and/or firmware to implement the test. When the added circuitry is substantial, it may be called embedded test.  If BIT is implemented in software, it is called BIT software.

So we tentatively adopted these terms for the Committee, but as the Committee Chair, Dr. Steve Pateras of LogicVision was not in attendance that day as well as other key members, we thought we will rubber stamp them soon.  Ron Press of Mentor Graphics saw our “work” and showed us that his company’s glossary disagreed with some of our views, so we decided to send these few definitions out for the entire TMAG membership for consensus.  It didn’t take long before we received some valid objections.  Craig DePaul, President of DSI International felt that as a system-level DFT tool maker, the definitions we offered were too circuit oriented and were not inclusive enough of systems that may utilize non-electronic parts as well. 

But it was Dr. Kenneth Parker of Agilent Technologies, Inc. who demonstrated that we have a great deal more work to do.  He asked whether we can actually tackle such definitions before we have consensus on some more basic terms such as “Defect,” “Failure,” “Fault,” and yes even “Test.”   He wrote,  “If you do not have clarity at this first level down, you will compromise the usefulness of any additional work.”  I agree.

The problem, in my opinion, stems from the evolution of the field.  We started with probing, moved on to scanning, compressing, while we tested for verification, for diagnoses, for repair, and for characterization.  We did this all in a hurry, always trying to catch up with the latest circuits that left previous methods obsolete.  There were terms we didn’t use long enough to define.  The definitions themselves changed.  A good example is Built-In Test (or is it Built-In Self Test, or is it Embedded Test?)  Today many circuits use BIST, neither for go/no-go test, nor for diagnoses, not even for prognoses.  They use BIST to achieve built-in self repair (BISR?).  So, even if the circuit is the same, how can we call a BISR a mere built-in tester?

If  we do not have a common language, we will not be able to communicate.  We need our definitions.  Ken Parker referred us to “Advances in Electronics Testing” (chapter 11) edited by Gizopoulos, Springer, 2005 and it does a nice job of defining some basic terms.  There are, of course, many other text with important definitions, but we need an industry-wide standard.  This is where you, members of the BestTest community can help.  We have an extensive Test Dictionary right here in the BestTest Knowledge Base with over 2,750 test terms. You can search definitions and you will find that it is always includes the source.  In many cases there are multiple definitions of the same term and that is quite all right.  By examining the source, you can make your decision of which one to use.

In keeping with the BestTest community approach, however, we need your input.  Searching for a definition is one thing, but being able to contribute your own definition is your civic duty to our community.  If you search for a definition in The BestTest Definitions and do not find it you will likely look elsewhere.  When you do find it, please come back and share it with the rest of us.  It takes less than 2 minutes to post a definition, just copy and paste.  Leave us the source (with a linked web site if you like) so we can give credit where credit is due.

In the meantime, can anyone define the term “Test?”  The BestTest Directory offers the following:

Test 

This issue's focus is Design for Test vs. Design for Testability.  Compare worldwide web results for Design for Test, Design for Testability and DFT Test.

• If your friend forwarded this newsletter to you, please register as a member and receive The BestTest Newsletter -- absolutely free!
• If you wish to update your news preferences or cancel the subscription, please unsubscribe.
• If you have any questions, please email experts@BestTest.com

Get the widest selection of test related books and software at the BestTest Online Store.

Introducing
The Testability Director Version 3.2
 

Reach the thousands of test professionals we mail to.
Place Ad here.