Report Number: CSL-TR-91-465
Institution: Stanford University, Computer Systems Laboratory
Title: Analysis of Power Supply Networks in VLSI Circuits
Author: Stark, Don
Date: March 1991
Abstract: Although the trend toward finer geometries and larger chips has produced faster systems, it has also created larger voltage drops and higher current densities in chip power supply networks. Excessive voltage drops in the power supply lines cause incorrect circuit operation, and high current densities lead to circuit failure via electromigration. Analyzing this power supply noise by hand for large circuits is difficult and error prone; automatic checking tools are needed to make the analysis easier. This thesis describes Ariel, a CAD tool that helps VLSI designers analyze power supply noise. The system consists of three main components, a resistance extractor, a current estimator, and a linear solver, that are used together to determine the voltage drops and current density along the supply lines. The resistance extractor includes two parts: a fast extractor that calculates resistances quickly using simple heuristics, and a slower, more accurate finite element extractor. Despite its simplicity, the fast extractor obtained nearly the same results as the finite element one and is two orders of magnitude faster. The system also contains two current estimators, one for CMOS designs and one for ECL. The CMOS current estimator is based on the switch level simulator Rsim, and produces a time-varying current distribution that includes the effects of charge sharing, image currents, and slope on the gate's inputs. The ECL, estimator does a static analysis of the design, calculating each gate's tail current and tracing through the network to find where it enters the power supplies. Extensions to the estimator allow it to handle more complex circuits, such as shared current lines and diode decoders. Finally, the linear solver applies this current pattern to the resistance network, and efficiently calculates voltages and current densities by taking advantage of topological characteristics peculiar to power supply networks. It removes trees, simple loops, and series sections for separate analysis. These techniques substantially reduce the time required for solution. This report also includes the results of running the system on several large designs, and points out flaws that Ariel uncovered in their power networks.