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This paper addresses the problem of true delay estimation during high level design. The true delay is the delay of the longest sensitizable path in the resulting circuit, as opposed to the topological delay which is the delay of the longest path in the circuit. The existing delay estimation techniques either estimate the topological delay, which may be pessimistic if the longest path is unsensitizable or false, or estimate the true delay using gate-level timing analysis which may be prohibitively expensive. Resource sharing in high level synthesis can create false paths in the circuit implementation. Hence, determining the clock period using topological delay can be unduly conservative, resulting in excessive hardware to meet tight timing specifications. In this paper, we introduce an efficient technique to compute an estimate of the true delay. The proposed technique relies on partitioning the paths in the circuit and topological delay computation, and not on path sensitization. The paths in the implementation are partitioned into two sets given the high level information on scheduling and resource sharing: the complete determining path set (CDPR) and the nondetermining path set (NDPR). We prove that the delay of the longest path in CDPR is lower bounded by the true delay and upper bounded by the topological delay of the circuit. Consequently, an estimate of the true delay of the resulting circuit can be computed by measuring the topological delay of the longest path in CDPR. We have developed a Functional delay ESTimation tool (FEST). Experimental results on a set of benchmarks reveal the following: approximately 50% of all paths are in NDPR and can be ignored for true delay estimation, and the true delay estimates are on the average 15% less than the topological delay. The high level true delay estimates are accurate, as verified by comparing with the true delays obtained by gate-level timing analysis on actual implementations. Furthermore, results reveal that high level true delay estimation can be done very fast, even when gate-level true delay estimation becomes infeasible