Continuous Time (CT)

In the CT simulator (continuous time), actors represent components that interact via continuous-time signals. Actors typically specify algebraic or differential relations between inputs and outputs. The job of the director in the simulator is to find a fixed-point, that is, a set of continuous-time functions that satisfy all the relations.

The CT simulator includes an extensible set of differential equation solvers. The simulator, therefore, is useful for modeling physical systems with linear or nonlinear algebraic/differential equation descriptions, such as analog circuits and many mechanical systems. Its model of computation is similar to that used in Simulink, Saber, and VHDL-AMS, and is closely related to that in Spice circuit simulators.

Embedded systems frequently contain components that are best modeled using differential equations, such as MEMS and other mechanical components, analog circuits, and microwave circuits. These components, however, interact with an electronic system that may serve as a controller or a recipient of sensor data. This electronic system may be digital. Joint modeling of a continuous subsystem with digital electronics is known as mixed signal modeling. The CT simulator is designed to interoperate with other VisualSim simulators, such as Digital, to achieve mixed signal modeling. To support such mixed-modeling, the CT simulator models the discrete events as Dirac delta functions. It also includes the ability to precisely detect threshold crossings to produce discrete events. Physical systems often have simple models that are only valid over a certain regime of operation. Outside that regime, another model may be appropriate. A modal model is one that switches between these simple models when the system transitions between regimes. The CT simulator interoperates with the FSM simulator to create modal models.