Clock buffer polarity assignment with skew tuning

Abstract

A clock polarity assignment method is proposed that reduces the peak current on the vdd/gnd rails of an integrated circuit. The impacts of (i) the output capacitive load on the peak current drawn by the sink-level clock buffers, and (ii) the buffer/inverter replacement scheme of polarity assignment on timing accuracy are considered in the formulation. The proposed sink-level-only polarity assignment is performed by a lexi-search algorithm in order to balance the peak current on the clock tree. Most of the previous polarity assignment methods that do not include clock tree resynthesis lead to an undesirable increase in the worst corner clock skew. Hence, a skew-tuning scheme is proposed that reduces the clock skew through polarity refinement and not through clock tree resynthesis. The proposed polarity assignment method with the skew-tuning scheme is implemented within an industrial design flow for practicality. Experimental results show that the worst-case peak current drawn by the clock tree can be reduced by an average of 36.5%. The worst corner clock skew is increased from 60.7ps to 76.2ps by applying the proposed polarity assignment method. The proposed skew-tuning scheme reduces the worst-case clock skew from 76.2ps to 61.5ps, on average, with a limited degradation in the peak current improvement (36.5% to 31.2%, on average).