Timing-Aware Decoupling Capacitance Allocation in Power Distribution Networks

Power supply noise increases the circuit delay, which may lead to performance failure of a design. Decoupling capacitance (decap) addition is effective in reducing the power supply noise, thus making the supply network more robust in presence of large switching currents. Traditionally, decaps have been allocated in order to minimize the worst-case voltage drop occurring in the power grid. In this paper, we propose an approach for timing-aware decap allocation which uses global time slacks to drive the decap optimization. Non-critical gates with larger timing slacks can tolerate a relatively higher supply voltage drop as compared to the gates on the critical paths. The decap allocation is formulated as a non-linear optimization problem using Lagrangian relaxation, and modified adjoint method is used to efficiently obtain the sensitivities of objective function to decap sizes. A fast path-based heuristic is also implemented and compared with the global optimization formulation. The two approaches have been implemented and tested on ISCAS85 benchmark circuits and with grids of different sizes. Compared to uniformly allocated decaps, the proposed approach utilizes 35.5% less total decap to meet the same delay target. For the same total decap budget, the proposed approach is shown to improve the circuit delay by 10.1% on an average.