Timing-Test Scheduling for Constraint-Graph Based Post-Silicon Skew Tuning

Post-Silicon Tuning is an emerging technology for improving performance-yield of VLSIs under process variations. This paper focuses especially on the post-silicon timing-skew tuning (PSST) via programmable delay elements (PDEs), and proposes a novel tuning algorithm which utilizes only the result of setup and hold timing tests, not the result of costly delay-time measurements. The basic framework of our PSST consists of the construction of Control-value Constraint Graph from the results of timing-tests, and the computation of longest path lengths on this graph for finding safe PDE setting. Even though the cost for timing test is smaller than a delay-time measurement, the cost of timing-tests is still a dominant part of the PSST cost, and its reduction is a crucial problem. Longest path lengths which we need to compute depends directly on edge weights in the "longest-paths tree", but for co-tree edges, their exact edge weights are not always necessary. Based on this observation, we propose timing-test scheduling for reducing the timing-test cost for PDE tuning. The experimental simulation results show that our approach reduces the test cost by almost half or more.