Native Mixed Mode SimulationUnlike SPICE 3, which is designed mainly for analog simulation, IsSpice4 includes BOTH analog and event-driven digital simulation capabilities in the same executable. This means that any simulation may contain components that are analog, event driven (digital or sampled-data), or a combination of both. An entire mixed signal analysis can be driven from one integrated schematic. All the digital models allow accurate specification of propagation time and rise/fall time delays.

The event driven algorithm in IsSpice4 is unique. It is general purpose and supports non-digital types of data. For example, elements can use real or integer values to simulate DSP functions or sampled data filters. Because the event driven algorithm is faster than the standard SPICE matrix solution simulation time is greatly reduced for circuits that use event driven models in place of analog models.

IsSpice4 Performs Digital Modeling on Three LevelsIn IsSpice4, mixed mode simulation is handled on three levels; (a) with primitive digital elements that use timing models and the built-in 12 state digital logic simulator, (b) with subcircuit models that use the actual transistor topology of the integrated circuit, and finally, (c) with In-line Boolean logic expressions.

Exact representations are used mainly in the analysis of transmission line and signal integrity problems where a close inspection of an IC’s I/O characteristics is needed. Boolean logic expressions are delayless functions that are used to provide efficient logic signal processing in an analog environment. These two modeling techniques use SPICE to solve a problem while the third method, digital primitives, uses IsSpice4’s new native mixed mode capability. Each of these methods has its merits and target applications. In fact, many simulations call for the combination of all three approaches. No one approach alone is sufficient.