Due to clock gating, the PMOS transistors in the clock tree often have different active probabilities, which lead to different NBTI delay degradations. To ensure that the clock skew is always zero, there is a demand to eliminate the degradation difference. In this paper, we present a critical-PMOS-aware clock tree design methodology to deal with this problem. First, we prove that, under the same tree topology, the NAND-type-matching clock tree has the minimum number of critical PMOS transistors. Then, we propose a 0-1 ILP (integer linear programming) approach to minimize the power consumption overhead while eliminating the degradation difference. Benchmark data consistently show that our design methodology can achieve very good results in terms of both the clock skew (due to the degradation difference) and the power consumption overhead.
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