This section uses a conceptual approach to describe the important processes and analyses that are involved in circuit simulation as they relate to the Multisim Simulation Core.

Multisim contains an analog simulation engine and a tightly-coupled digital simulation engine. Accordingly, the following sections discuss the simulation of purely analog circuits, purely digital circuits, and mixed-mode circuits.

Schematic Capture Circuit

For the purposes of simulation, schematic capture is used to graphically construct the netlist, which is a text-based representation of the circuit, from interconnections of components and instruments. The associated component and instrument models are automatically inserted into the netlist. The netlist is generated and loaded into the simulation core upon invoking any analysis dialog or starting an interactive simulation.

Analysis Dialogs

Analysis dialogs set up analysis simulation mode simulation. They automatically generate simulation commands from the settings within them. Refer to the Analyses section for more information.

Instruments

Instruments have multiple uses including signal generation and data analysis. Although they are primarily designed for the interactive simulation mode, certain instruments such as the Function Generator can be used in the analysis simulation mode as well. Refer to the Instruments section for more information.

Command Line Interface

The XSPICE Command Line dialog box is used as an alternative to schematic capture and analysis dialogs. Use the XSPICE Command Line dialog box to load an external, pre-defined netlist and a set of commands into the simulation core. Refer to the Command Line Interface section for more information.

Interactive Events

Certain interactive components and instruments can alter the state of the circuit while a simulation is in progress by sending interactive events directly to the simulation core. Refer to the Interactive Simulation section for more information.

Grapher

The Grapher is used to analyze the simulation output data. Although it is designed primarly for the analysis simulation mode, certain instruments send their data to the Grapher during the interactive simulation mode. Refer to The Grapher for more information.

Indicator Components

Indicator components display output values or change their appearance when used in an interactive mode simulation. Indicator components are not responsive in the analysis simulation mode. Commonly-used animated components include the Ammeter, the 7-segment display and the Probes.

Simulation Core

The Simulation Core carries out the specified analysis. The core is based on the XSPICE project from Georgia Technical Research Institute. XSPICE adds a native event-driven digital simulator to the SPICE3 analog simulator from the University of California at Berkeley. Both XSPICE and SPICE3 are public domain projects.

The Multisim Simulation Core contains many improvements to the XSPICE core, especially in the areas of convergence and Cadence® PSpice® compatibility. The Multisim Simulation Core also contains an Interactive Event Manager.

Simulation Core Subsystems

The Simulation Core is composed of the following subsystems: the Netlist Parser, the Command Interpreter, the Interactive Event Manager, the Analog and Digital Simulation Engines, and the Analog and Digital Device Library.

The Netlist Parser interprets the circuit netlist and builds an internal representation of the circuit from the device models. Using appropriate syntax, the models in the netlist must reference devices that are in Multisim's Analog and Digital Device library.

The Analog Device Library contains many models that closely resemble the behavior of actual electronic elements such as resistors, capacitors, diodes, and MOSFETs. The library also includes more abstract elements, such as voltage sources, controlled sources and ideal switches, which can be used to construct macromodels, such as opamps.

The Digital Device Library includes common elements found in gate-level digital circuits such as NAND gates, NOR gates, and flip-flops. It also includes elements that are customizable with user-defined logic behavior. Refer to the Multisim SPICE Reference section for more information about the libraries.

The Command Interpreter interprets the commands and sets up the simulation internally.

The Interactive Event Manager waits for events from interactive elements and changes the circuit state during transient analysis.

The Analog and Digital Simulation Engines perform the actual calculations in accordance with the analysis specified. The Simulation Methods section provides more details on the internal processes of the analyses.