Logic simulation is the use of a computer program to simulate the operation of a digital circuit. Logic simulation is the primary tool used for verifying the logical correctness of a hardware design. In many cases logic simulation is the first activity performed in the process of taking a hardware design from concept to realization. Modern hardware description languages are both simulatable and synthesizable.
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Because simulation is a general technique, a hardware design can be simulated at a variety of levels of abstraction. Often it is useful to simulate a model at several levels of abstraction in the same simulation run. The commonly used levels of abstraction are gate level, register transfer level (RTL), and behavioral (or algorithmic) level. However, it is possible to incorporate lower levels like transistor level or even lower physical levels as well as higher levels such as transaction levels or domain-specific levels.
Simulation is the key activity in the design verification process. That is not to say that it is an ideal process. It has some positive attributes:
Prospective way to accelerate logic simulation is using distributed and parallel computations. [1]
On the negative side, simulation has two drawbacks, one of which is glaring:
Every design project must answer the question “have we simulated enough to find all the bugs?” and every project manager has taped out his design knowing that the truthful answer to that question is either “no” or “I don’t know”. It is this fundamental problem with simulation that has caused so much effort to be spent looking for both tools to help answer the question and formal alternatives to simulation.
Code coverage, functional coverage and logic coverage tools have all been developed to help gauge the completeness of simulation testing. None are complete solutions, though they all help. Formal alternatives have been less successful. Just like in the general software world, where proving programs correct has proven intractable, formal methods for verifying hardware designs have still not proven general enough to replace simulation. That is not surprising, since it is the same problem.
The second drawback motivates most of the research and development in simulation. That is, simulation is always orders of magnitude slower than the system being simulated. If a hardware system runs at 1GHz, a simulation of that system might run at 10-1000 Hz, depending on the level of the simulation and the size of the system. That is a slowdown of from 106 to 108! Consequently, many people have spent a lot of time and effort finding ways to speed up logic simulation. The straightforward approach is to emulate the circuits on FPGA. FPGAs are the fine-grain supercomputers, intentionally designed for logic simulation. The emulation is only 10 times slower than the circuit implemented in true silicon (ASIC). Though, a lot of information is lost from observation -- only external I/O is available.
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).
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.
While event simulation can provide some feedback regarding signal timing, it is not a replacement for static timing analysis. 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.
Considering both the advantages and disadvantages of logic simulation, it really is quite a good tool for verifying the correctness of a hardware design. Despite its drawbacks, simulation remains the first choice for proving correctness of a design before fabrication, and its value has been well established. [2]
For functional simulation of program source code, see Emulator.