A Hardware Engine for Analogue Mode Simulation of MOS Digital Circuits

Purely digital simulators that model digital circuits by approximations often produce poor results with even simple circuits. Analogue simulators, however, are far more accurate than necessary for digital circuits and correspondingly expensive to use. We argue that an analogue simulation is necessary, but that a simple enough approximation can be used to allow a hardware simulation engine to perform the calculations. The critical features of such an engine are discussed here, its performance analyzed, and the results are compared to a digital simulator and an analogue simulator. The use of such an engine is shown to permit approximate analogue simulation over an entire VLSI circuit with accuracy comparable to SPICE-quality analogue simulation. The speed of the engine permits interactive analogue mode simulation of circuits containing several thousand transistors.