Analog macromodeling of digital elements in SPICE

An approach to the accurate simulation of digital elements in a circuit simulator is described. The approach gives more detailed time-domain waveforms than logic simulation. It is more than an order of magnitude faster than transistor-level simulation, but has comparable accuracy. As the simulation is completely in the analog domain, no analog-digital interfaces are required. One can freely mix analog and digital elements, and all features of circuit simulators such as sensitivity analysis and pole-zero analysis are available. Special primitives built into SPICE can greatly facilitate macromodeling of digital elements. Details of the implementation of the primitives in SPICE 3c1 are given. The modeling of chips in the TTL (transistor-transistor logic) family is considered as an example. The analysis of an analog-to-digital converter is also given to bring out the mixed signal capability of this approach