Research On Power Grid Integrity Analysis Of VLSI

In an integrated circuit,all the devices get power supplies through Power/Ground network. Because of the presence of resistance,when currents flow through P/G network,there are unavoidable voltage drop on it. We call this voltage drop IR drop. IR drop on power grid may decrease switching speed and noise immunity of the circuits. It may even cause the circuit to fail. With the advance of semiconductor manufacturing,circuits with increasingly higher speed are being integrated at an increasingly higher density. These trends make analysis and verification of power grid integrity more important.There are many aspects of power grid integrity analysis,for example,analysis of IR drop and analysis of electromigration. This paper focuses on IR drop analysis. Power grid integrity analysis can be implemented in three steps. First,to extract the RC parameters of the power grid. Second,to extract cell currents of all the devices in the design. Finally,to construct the nodal analysis equation of the power grid and solve it.There are two types of IR drop analyses:static IR drop analysis and dynamic IR drop analysis. Static IR drop analysis is based on the average current of the devices. According to different ways of current extraction,static IR drop analysis can be categorized into two major types:area based IR drop analysis and activity based IR drop analysis. Dynamic IR drop analysis is based on input vectors,it can report IR drop of power grid under all the input vectors,thus show a more accurate result.Power grid integrity analysis is a global problem in a sense that IR drop of one part of the design is closely relative to the current drawn by the other parts. Additionally,for a million gates design,nodes of power grid are typically more thanone million. Traditional linear system solver can't deal with such a large network whether in speed or in memory. All the above reasons make the solving of the network a challenging work.With a thorough research on the structure of power grid,we come up with a network reduction algorithm. This algorithm can simplify the circuit and decrease its' dimension. Based on the research work on matrix solution techniques,we propose a incomplete Cholesky pre-conditioning based conjugate gradient solution technique. This technique can take advantage of the special properties of power grid and solve the circuits in a fast speed.Static IR drop analysis is independent with input vectors,so the speed is very fast. Dynamic IR drop analysis is dependent on input vectors,so it's time consuming. For a large scale integrated circuit,full chip full input vectors analysis is impossible. In order to solve this problem,we give a definition of critical node to identify those suspected nodes. We can find out the critical nodes using static analysis,and then apply dynamic IR drop analysis on the critical nodes.We have developed a tool suite using C/C++ language on Sun Ultra60 workstation. The name of this tool suite is IRdrop2001. It can be used to accomplish the analysis of power grid integrity. This tool suite consists of four components:tools for power grid parameter extraction,tools for cell current extraction,a circuit solver and a plotter. So far,this tool suite can accomplish area based and activity based IR drop analysis.