Generating Realistic Stimuli for Accurate Power Grid Analysis

Power distribution systems in integrated circuits are used to provide the voltages and currents the devices need to operate properly. As the semiconductor industry moves into deep nanometer nodes, problems like voltage drop, ground bounce and electromigration which may cause chip failures, are worsening, as more devices, operating at higher frequencies are placed closer together. Verification of a power distribution system is therefore paramount to silicon success. This type of verification is usually done by simulation, targeting a worst-case scenario, typically characterized by the almost simultaneous switching of several devices in the circuit. The definition of the worst-case situation is therefore crucial, since it influences the result of the simulation and ultimately the design target. Supposedly safe but unrealistic settings such as assuming that all signals switch simultaneously, could lead to costly over-designs in terms of die area. In this paper we describe a software tool that generates a reasonable, realistic, worst-case set of stimuli for simulation, based on timing and spatial restrictions that arise from the circuit´s netlist and placement. Generating such stimuli is akin to performing a standard static timing analysis, as is done before signoff so the tool fits well within conventional design frameworks. The resulting stimuli indicates that only a fraction of the gates change in any given timing window, leading to a more robust verification methodology, specially in the dynamic case.