Optimization of overdrive signoff

In modern SOC implementations, multi-mode design is commonly used to achieve better circuit performance and power across voltage-scaling, “turbo” and other operating modes. Although there are many tools for multi-mode circuit implementation, to our knowledge there is no available systematic analysis or methodology for the selection of associated signoff modes. We observe that the selection of signoff modes has significant impact on circuit area, power and performance. For example, incorrect choice of signoff voltages for required overdrive frequencies can result in a netlist with 15% suboptimality in power or 21% in area. In this paper, we propose a concept of mode dominance which can be used as a guideline for signoff mode selection. Further, we also propose efficient circuit implementation flows to optimize the selection of signoff modes within several distinct use cases. Our results show that our proposed methodology provides 5-7% improvement in performance compared to the traditional “signoff and scale” method. The signoff modes determined by our methods result in only 0.6% overhead in performance and 8% overhead in power after implementation, compared to the optimal signoff modes.