Digital Simulator Test

Can a Simulator Correctly Simulate a Digital Circuit?

The marketing hype for a CAD tool typically claims that their tool is the greatest invention since sliced bread. The tool has super speed and can simulate very large circuits in a short time on your workstation. How good is the tool? Is it as good as the marketing advertisement implies, or is it limited to what the user license agreement guarantees?

By correct, I mean logically correct in the strict mathematical or philosophical sense. The answer to the question depends on the circuit and the test performed on the circuit. The test performed on this Web page is intentionally designed to cause a Simulator to fail. The purpose is to illustrate some limitations that exist in most Simulators.

All tools have limitations. Knowing what a tool can and can not do will allow the tool to be used effectively. This test highlights two deficiencies that are present in most Simulators:

The inability to calculate the initial state and

the inability to simulate asynchronous behavior.

The test circuit is a simple logic circuit, consisting of five two-input NAND gates. The circuit has never been correctly simulated. If you have confidence in your simulator that you do not wish to be shattered, leave now. If you would like to test your simulator, proceed to the Test Circuit.

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Motivation: The purpose of the Web page is to correct misconceptions. Too many people assume that the expensive tool that they are using will correctly simulate logic. What they do not realize is that the software takes short cuts that are not always valid. My main objection is that the capabilities of the tool are never stated.

Is there an ulterior motive? Yes. I have been told too many times that I must be wrong and that the Simulator is correct. Since this comment usually comes from someone that does not understand logic, it is very disturbing to someone that considers logic to be simple common sense.

Disclaimer: The test circuit has been chosen to illustrate the inability of Simulators to correctly evaluate the initial state and simulate asynchronous circuit behavior. The test is not intended to be comprehensive. I expect all Simulators to fail.

Assume that that the standard software user license agreement applies to this web page.

Credits: The Web page was created with Microsoft Word. The Simulation results, figures, and waveforms come from MicroSim PSpice.

This Web space is provided by 50webs.com .

Thanks to Gabe Sellers for checking the links. Gabe provided the correct value for V on the first try.