While much engineering effort in a VLSI design project goes into checking whether the HDL models developed do indeed capture the desired functionality, this does not suffice to make sure the design data submitted to fabrication are correct. Design flaws may also creep in during netlist synthesis and layout preparation. The extra checks required to uncover such problems and the EDA tools available to do so are the subject of this chapter. Section 12.1 is concerned with locating timing problems while section 12.2 analyzes the accuracy of the timing models being used. Electrical rule check (ERC) and other static design verification techniques are the subject of section 12.3. Section 12.4, finally, addresses post-layout design verification.

Uncovering timing problems

As underlined in earlier chapters of this text, getting the timing right is vital for making VLSI circuits work as intended; catching potential timing problems is thus very important indeed. One is easily tempted to accept an error-free simulation run as a proof for a workable design, yet this is not so. We will first demonstrate why before introducing a more effective approach in section 12.1.2.

What does simulation tell us about timing problems?

Abstracting from the fine points that make up the differences between the various clocking disciplines, almost all timing-related difficulties that may occur in a synchronous (sub)system can be attributed to one or more of the causes listed below.