Studierender: Amjad Alsati
Betreuer: Alexander Sprenger
Abstract
Small delay faults (SDFs) are a significant problem in high-speed integrated circuits (ICs). Testing SDFs targeting long delay paths can be easily done at the nominal frequency of the target circuit under test (CUT). SDFs residing on short paths are hidden delay faults (HDFs) since they are not detectable at the nominal frequency. Faster-than-at-speed test (FAST) targets HDFs by overclocking the CUT to a higher frequency than the nominal frequency. On the one hand, FAST minimizes slacks of tested short paths and helps to detect HDFs. On the other hand, long paths produce unknown logic values known as X-values due to observing the signals of long paths before their arrival times. X-values obstruct the compaction of test pattern responses due to their dominance on the XOR gates of the compactors. As a result, faults detected before compaction become undetected after compaction. State-of-the-art compactors such as the stochastic space compactor (SSC) and the X-canceling MISR can tolerate those X-values. In this work, three approaches of pattern scheduling are presented to study their effectiveness in supporting the SSC in detecting faults after compaction. A probability-based schedule (PBS) approach is introduced to increase the number of detected faults after stochastic space compaction. The PBS is compared to two simpler scheduling approaches, namely naive and covering schedules. The PBS achieves better fault efficiency than naive and covering schedules while significantly utilizing fewer FAST patterns than the naive one. The covering schedule is also compared to the naive schedule to demonstrate that scheduling is not a simple covering problem due to the randomness of the stochastic space compaction. Moreover, a pattern ordering optimization technique is presented to show the effectiveness of changing the order of the patterns in supporting the SSC. The ordering optimization approach shows its effectiveness in increasing the number of detected faults after stochastic space compaction while also reducing the number of X-values on the outputs of the SSC. The case study of the ordering approach also investigates the effectiveness of pattern ordering when an X-canceling MISR is added after the SSC in the compactor chain. The study concludes that attaining a high fault efficiency and X-reduction ratio of the SSC will not always lead to a high fault efficiency when X-canceling MISR is added after the SSC. This suggests that pattern ordering should simultaneously be performed for both compactors.