Algorithms and Low Cost Architectures for Trace Buffer-Based Silicon Debug

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Virginia Tech


An effective silicon debug technique uses a trace buffer to monitor and capture a portion of the circuit response during its functional, post-silicon operation. Due to the limited space of the available trace buffer, selection of the critical trace signals plays an important role in both minimizing the number of signals traced and maximizing the observability/restorability of other untraced signals during post-silicon validation. In this thesis, a new method is proposed for trace buffer signal selection for the purpose of post-silicon debug. The selection is performed by favoring those signals with the most number of implications that are not implied by other signals. Then, based on the values of the traced signals during silicon debug, an algorithm which uses a SAT-based multi-node implication engine is introduced to restore the values of untraced signals across multiple time-frames. A new multiplexer-based trace signal interconnection scheme and a new heuristic for trace signal selection based on implication-based correlation are also described. By this approach, we can effectively trace twice as many signals with the same trace buffer width. A SAT-based greedy heuristic is also proposed to prune the selected trace signal list further to take into account those multi-node implications. A state restoration algorithm is developed for the multiplexer-based trace signal interconnection scheme. Experimental results show that the proposed approaches select the trace signals effectively, giving a high restoration percentage compared with other techniques. We finally propose a lossless compression technique to increase the capacity of the trace buffer. We propose real-time compression of the trace data using Frequency-Directed Run-Length (FDR) code. In addition, we also propose source transformation functions, namely difference vector computation, efficient ordering of trace flip-flops and alternate vector reversal that reduces the entropy of the trace data, making them more amenable for compression. The order of the trace flip-flops is computed off-chip using a probabilistic algorithm. The difference vector computation and alternate vector reversal are implemented on-chip and incurs negligible hardware overhead. Experimental results for sequential benchmark circuits shows that this method gives a better compression percentage compared to dictionary-based techniques and yields up to 3X improvement in the diagnostic capability. We also observe that the area overhead of the proposed approach is less compared to dictionary-based compression techniques.



Silicon Debug, Logic Implications, Trace selection, State Restoration, Trace Compression