Safety critical devices drive fast adoption of advanced DFT
By Ron Press, Mentor Graphics Corp
Devices used in safety critical applications need be known to work and have the ability to be regularly verified. Therefore, a very high-quality test is important, as is a method to perform a built-in self-test. Recently, there has been a strong growth in the automotive market and the number of processors within each car is steadily increasing. These devices are used for more and more functions such as braking systems, engine control, heads-up display, navigation systems, image sensors, and more. As a result, we see many companies designing devices for the automotive market or trying to enter the automotive market.
2011 saw the publication of the ISO standard 26262, which specifies standard criteria for automobile electronics. Our experience is that recently two test requirements are being adopted or at least evaluated by most companies developing safety critical devices. One requirement is to perform a very high-quality test such that there are virtually no defective parts that escape the tests. The other is to perform a built-in self-test such that the part can be tested when in the safety critical application.
There are various pattern types that help support the zero DPM (defects per million) shipped devices goal. In particular, Cell-Aware test is proven to uniquely detect defects that escape traditional tests. Cell-Aware test can find defects that would escape a 100% stuck-at, transition, and timing-aware test set.. This is because it works by first modeling the actual defects that can occur in the physical layout of standard cells. Cell-Aware pattern size was recently improved and reduced, but a complete pattern set is larger than a traditional pattern set so embedded compression is used.
At Mentor Graphics, we started seeing more and more customers implementing logic BIST and embedded compression for the same circuits. Therefore, it made sense to integrate both into common logic that can be shared, since both technologies interface to scan chains in a similar manner. The embedded compression decompressor could be configured into a linear feedback shift register (LFSR) to produce pseudo-random patterns for logic BIST. Both the logic BIST and embedded compression logic provide data to scan chains through a phase shifter so that logic is fully shared. The scan chain outputs are compacted together in embedded compression. This logic is mostly shared with logic BIST to reduce the number of scan chain outputs that enter a signature calculator.
The hybrid embedded compression/logic BIST circuit is useful for meeting the safety-critical device quality and self-test requirements. In addition, since logic is shared the controller is 20-30% smaller than implementing embedded compression and logic BIST separately. As previously mentioned, we have seen this logic being adopted or in evaluation very broadly by automotive device designers.
One side effect of using embedded compression and logic BIST is that each makes the other better. For example, embedded compression can supply an extremely high quality production test. So, fewer test points are necessary in logic BIST to make random pattern resistant logic more testable, which reduces the area of logic BIST test points. Conversely, the X-bounding and any test points that are added for logic BIST make the circuit more testable and improve the embedded compression coverage and pattern count results.
Ron Press is the technical marketing manager of the Silicon Test Solutions products at Mentor Graphics. The 25-year veteran of the test and DFT (design-for-test) industry has presented seminars on DFT and test throughout the world. He has published dozens of papers in the field of test, is a member of the International Test Conference (ITC) Steering Committee, is a Golden Core member of the IEEE Computer Society, and a Senior Member of IEEE. Press has patents on reduced-pin-count testing, glitch-free clock switching, and patents pending on 3D DFT.