Generating realistic stimuli for accurate power grid analysis

Abstract

Power analysis tools are an integral component of any current power sign-off methodology. The performance of a design's power grid affects the timing and functionality of a circuit, directly impacting the overall performance. Ensuring power grid robustness implies taking into account, among others, static and dynamic effects of voltage drop, ground bounce, and electromigration. This type of verification is usually done by simulation, targeting a worst-case scenario where devices, switching almost simultaneously, could impose stern current demands on the power grid. While determination of the exact worst-case switching conditions from the grid perspective is usually not practical, the choice of simulation stimuli has a critical effect on the results of the analysis. Targetting safe but unrealistic settings could lead to pessimistic results and costly overdesigns in terms of die area. In this article we describe a software tool that generates a reasonable, realistic, set of stimuli for simulation. The approach proposed accounts for timing and spatial restrictions that arise from the circuit's netlist and placement and generates an approximation to the worst-case condition. The resulting stimuli indicate that only a fraction of the gates change in any given timing window, leading to a more robust verification methodology, especially in the dynamic case. Generating such stimuli is akin to performing a standard static timing analysis, so the tool fits well within conventional design frameworks. Furthermore, the tool can be used for hotspot detection in early design stages.