Computer Aids for VLSI Design
Steven M. Rubin
Copyright © 1994

Chapter 6: Dynamic Analysis Tools

This chapter was contributed by Robert W. Hon, Cadence Design Systems Inc.

Section 8 of 8

6.8 Summary

The state of the art demands that designers make compromises in size of design versus level of detail in simulation. The best performing simulators can simulate large designs, but not in sufficient detail to catch subtle errors of the type likely to occur in a process that still involves a great deal of low-level design. Circuit-level simulators can provide all the detail that a designer is likely to need, but cannot process large designs.

A particularly effective approach is to use detailed (circuit-level) simulation to check critical functional or timing paths, and less-detailed simulation for the remainder of the design. The critical sections can be determined using human insight, or through the use of higher-level simulations that identify potential problems in the design.

Although thorough static-analysis techniques proliferate, they still do not replace the actual use of data in debugging a design. Simulation is therefore of continued importance in a CAD system. Simulation tools will have to keep up with the rapidly changing needs of design environments, both in their capabilities and in their style of use.


  1. Invent a more abstract logic block that could replace the cross-coupled NOR gates of the example in the section, Event-Driven Simulation. Give its extended truth table.
  2. Show the event queue that results from simulating your logic block under the same assumptions as before (unit gate delay, zero wire delay).
  3. Build a truth table for an AND gate that includes the four states: 0, 1, X, and Z (resistive-pullup high).
  4. What problems occur when simulating bidirectional MOS transistors? How can these be overcome?
  5. How can more accurate models of time improve the quality of simulation that is concerned only with logic values at nodes (and not time)?
  6. How can hierarchy aid in simulation?


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Steven M. Rubin
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