A Switch-Level Model and Simulator for MOS Digital Systems

The switch-level model describes the logical behavior of digital systems implemented in metal oxide semiconductor (MOS) technology. In this model a network consists of a set of nodes connected by transistor "switches" with each node having a state 0, 1, or X (for invalid or uninitialized), and each transistor having a state "open," "closed," or "indeterminate." Many characteristics of MOS circuits can be modeled accurately, including: ratioed, complementary, and precharged logic; dynamic and static storage; (bidirectional) pass transistors; buses; charge sharing; and sneak paths. In this paper we present a formal development of the switch-level model starting from a description of circuit behavior in terms of switch graphs. Then we describe an algorithm for a logic simulator based on the switch-level model which computes the new state of the network by solving a set of equations in a simple, discrete algebra. This algorithm has been implemented in the simulator MOSSIM II and operates at speeds approaching those of conventional logic gate simulators. By developing a formal theory of MOS logic circuits, we have achieved a greater degree of generality and accuracy than is found in other logic simulators for MOS.