12.13  Summary

A logic synthesizer may contain over 500,000 lines of code. With such a complex system, complex inputs, and little feedback at the output there is a danger of the “garbage in, garbage out” syndrome. Ask yourself “What do I expect to see at the output?” and “Does the output make sense?” If you cannot answer these questions, you should simplify the input (reduce the width of the buses, simplify or partition the code, and so on). The worst thing you can do is write and simulate a huge amount of code, read it into the synthesis tool, and try and optimize it all at once with the default settings.

With experience it is possible to recognize what the logic synthesizer is doing by looking at the number of cells, their types, and the drive strengths. For example, if there are many minimum drive strength cells on the critical path it is usually an indication that the synthesizer has room to increase speed by substituting cells with stronger drive. This is not always true, sometimes a higher-drive cell may actually slow down the circuit. This is because adding the larger cell increases load capacitance, but not enough drive to make up for it. This is why logical effort is a useful measure.

Because interconnect delay is increasingly dominant, it is important to begin the physical design steps as early as possible. Ideally floorplanning and logic synthesis should be completed at the same time. This ensures that the estimated interconnect delays are close to the actual delays after routing is complete.

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