We propose a new problem formulation for low power clock network design that takes rise time constraints imposed by the design into account. We evaluate the utility of inserting buffers into the clock route for satisfying rise time constraints and for minimizing the area of the clock net. In particular, we show that the classical H-tree is sub-optimal in terms of both area and power dissipation when buffers may be inserted into the tree. We show that the power minimization problem is NP-hard and propose a greedy heuristic for power-optimal clock network design that utilizes the opportunities provided by buffer insertion. Our algorithm inserts buffers and designs the topology simultaneously. The results we obtain on benchmarks are significantly better than previous approaches in terms of power dissipation, wire length, rise times and buffer area. Power dissipation is typically reduced by a factor of two, rise times are four times better and buffer area requirements are an order of magnitude smaller.
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