Maximum Flow Algorithms for Maximum Observability During FPGA Debug

Due to the ever-increasing density and complexity of integrated circuits, FPGA prototyping has become a necessary part of the design process. To enhance observability into these devices, designers commonly insert trace-buffers to record and expose the values on a small subset of internal signals during live operation to help root-cause errors. For dense designs, routing congestion will restrict the number of signals that can be connected to these trace-buffers. In this work, we apply optimal network flow graph algorithms, a well studied technique, to the problem of transporting circuit signals to embedded trace-buffers for observation. Specifically, we apply a minimum cost maximum flow algorithm to gain maximum signal observability with minimum total wirelength. We showcase our techniques on both theoretical FPGA architectures using VPR, and with a Xilinx Virtex6 device, finding that for the latter, over 99.6% of all spare RAM inputs can be reclaimed for tracing across four large benchmarks.