Electronic circuit simulation
Electronic circuit simulation utilizes mathematical models to replicate the behavior of an actual electronic device or circuit. Simulating a circuit’s behavior before actually building it greatly improves efficiency and provides insights into the behavior of electronics circuit designs. In particular, for integrated circuits, the tooling (photomasks) is expensive, breadboards are impractical, and probing the behavior of internal signals is extremely difficult. Therefore almost all IC design relies heavily on simulation. The most well known analog simulator is SPICE. Probably the best known digital simulators are those based on Verilog and VHDL.
Some electronics simulators integrate a schematic editor, a simulation engine, and on-screen waveforms and make “what-if” scenarios easy and instant. They also typically contain extensive model and device libraries. These models typically include IC specific transistor models such as BSIM, generic components such as resistors, capacitors, inductors and transformers, user defined models (such as controlled current and voltage sources, or models in Verilog-A or VHDL-AMS). Printed circuit board (PCB) design requires specific models as well, such as transmission lines for the traces and IBIS models for driving and receiving electronics.
While there are strictly analog <ref>Mengue and Vignat, Entry in the University of Marne, at Vallee</ref> electronics circuit simulators, popular simulators often include both analog and event-driven digital simulation<ref>P. Fishwick, Entry in the University of Florida</ref> capabilities, and are known as mixed-mode simulators <ref> J. Pedro and N. Carvalho, Entry in the Universidade de Aveiro, Portugal</ref>. 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 in mixed-mode simulators provide accurate specification of propagation time and rise/fall time delays.
The event driven algorithm provided by mixed-mode simulators 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 <ref> L. Walken and M. Bruckner, Event-Driven Multimodal Technology</ref>.
Mixed-mode simulation is handled on three levels; (a) with primitive digital elements that use timing models and the built-in 12 or 16 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. An example of a mixed-mode simulator is shown in Figure 1.
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 delay-less 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, use mixed mode capability. Each of these methods has its merits and target applications. In fact, many simulations (particularly those which use A/D technology) call for the combination of all three approaches. No one approach alone is sufficient.