Logic Simulation - Event Simulation Versus Cycle Simulation

Event Simulation Versus Cycle Simulation

Event simulation allows the design to contain simple timing information – the delay needed for a signal to travel from one place to another. During simulation, signal changes are tracked in the form of events. A change at a certain time triggers an event after a certain delay. Events are sorted by the time when they will occur, and when all events for a particular time have been handled, the simulated time is advanced to the time of the next scheduled event. How fast an event simulation runs depends on the number of events to be processed (the amount of activity in the model).

While event simulation can provide some feedback regarding signal timing, it is not a replacement for static timing analysis.

In cycle simulation, it is not possible to specify delays. A cycle-accurate model is used, and every gate is evaluated in every cycle. Cycle simulation therefore runs at a constant speed, regardless of activity in the model. Optimized implementations may take advantage of low model activity to speed up simulation by skipping evaluation of gates whose inputs didn't change. In comparison to event simulation, cycle simulation tends to be faster, to scale better, and to be better suited for hardware acceleration / emulation.

However, chip design trends point to event simulation gaining relative performance due to activity factor reduction in the circuit (due to techniques such as clock gating and power gating, which are becoming much more commonly used in an effort to reduce power dissipation). In these cases, since event simulation only simulates necessary events, performance may no longer be a disadvantage over cycle simulation. Event simulation also has the advantage of greater flexibility, handling design features difficult to handle with cycle simulation, such as asynchronous logic and incommensurate clocks. Due to these considerations, almost all commercial logic simulators have an event based capability, even if they primarily rely on cycle based techniques.