Logic-level simulators attempt to remedy the computationally intensive nature of circuit-level simulators by raising the level of abstraction to the domain of switches and logic components. Instead of manipulating continuous, analog data, logic-level simulators simply process logic values; that is, 0, 1 and X. In addition, logic-level simulators traditionally simplify the simulation process by assuming that the connecting wires have negligible resistance.
Logic level simulators can be subdivided into two further categories, switch-level and gate-level simulators. In switch-level simulators, transistors are promoted to elementary switches and very little attention is given to the intricacies of other transistor attributes. During the simulation itself, equations governing the behaviour of the circuit are greatly approximated, thereby increasing the speed at which the simulator operates. The detail that is inevitably lost as a result of this approach is not usually vital.
Gate-level simulators [9] operate at yet a higher abstraction level. Low level circuit devices such as transistors, capacitors and resistors are replaced with logic gates such as NAND, XOR and flip-flops. Circuits described at this level bear strong resemblance to data flow diagrams in which information is passed amongst interconnected components. Effective use of logic gates permit relatively high-level designs to be easily described and subsequently simulated. Because the components are increasingly abstract, more complex systems may be designed and simulated at the gate level rather than at the switch level. Traditional gate-level simulators are implemented using discrete-event simulation; as such, this report will primarily focus upon circuits described at the gate level.
There have been several successful attempts to merge switch-level simulators with simulators that operate at the gate-level and above, thereby allowing the designer to have the flexibility and speed of high-level simulators, while at the same time retaining some of the accuracy associated with switch-level simulators. Such simulators are commonly referred to as mixed-mode simulators [8].