Statistical Estimation of the Switching Activity in VLSI Circuitsy

Higher levels of integration have led to a generation of integrated circuits for which power dissipation and reliability are major design concerns. In CMOS circuits, both of these problems are directly related to the extent of circuit switching activity. The average number of transitions per second at a circuit node is a measure of switching activity that has been called the transition density. This paper presents a statistical simulation technique to estimate individual node transition densities in combinational logic circuits. The strength of this approach is that the desired accuracy and conndence can be speciied up-front by the user. Another key feature is the classiication of nodes into two categories: regular-and low-density nodes. Regular-density nodes are certiied with user-speciied percentage error and conndence levels. Low-density nodes are certiied with an absolute error, with the same conndence. This speeds convergence while sacriicing percentage accuracy only on nodes which contribute little to power dissipation and have few reliability problems. I. INTRODUCTION The advent of VLSI technology has brought new challenges to the manufacture of integrated circuits. Higher levels of integration and shrinking line widths have led to a generation of devices which are more sensitive to power dissipation and reliability problems than typical devices of a few years ago. In these circuits excessive power dissipation may cause run-time errors and device destruction due to overheating, while reliability issues may shorten device lifetime. It is especially useful to diagnose and correct these problems before circuits are fabricated. In CMOS circuits, gates draw current and consume power only when making logical transitions. As a result, power dissipation and reliability strongly depend on the extent of circuit switching activity. Hence, there is a need for CAD tools that can estimate circuit switching activity during the design phase. Circuit activity is strongly dependent on the inputs being applied to the circuit. For one input set the circuit may experience no transitions, while for another it may switch frequently.