Method of determining delay in logic cell models

Abstract

A logic-cell model accounts for nonlinear effects in determining propagation delay, thereby providing improved accuracy as compared to existing models, particularly when rise/fall times exceed several nanoseconds. Given a logic cell of the type wherein delay is a function of rise/fall time (TRL) and load capacitance (CL), the method involves choosing a plurality of discrete simulation points associated with the delay, each point also being a function of TRF and CL, after which the delay is determined in accordance with the chosen simulation points. One or more of the simulation points are preferably chosen in conjunction with both the linear and nonlinear regions of the TRL/CL space to ensure accuracy for a wide range of TRL and/or CL values. In the event of an identifiable or discontinuous transition between the linear and nonlinear regions, a discrete simulation point is also chosen with respect to the transition area. Based upon the simulation points, the invention is used to determine a plurality of constants which are then, in turn, used to solving for propagation delay on a more accurate basis. In this respect, the propagation delay, TD, may be determined in accordance with the relation