When I run Multi-Pass Place and Route, what does the "Design Score" mean? Why do the tools not save the iteration with the best "Timing Score"?
The design score is derived from a formula that takes the following into account:
- The number of unrouted nets
- The number of timing constraints not met
- The amount (ns) by which the constraints were not met
- The maximum delay on a net with a weight greater than 3
- Net weights and priorities
- The average of all maximum delays on all nets
- The average of the maximum delays on the 10 worst nets
For information on the exact formula, please see PAR Reports in the System Reference Guide.
The timing score is simply a summation in picoseconds of all timing violations. This score can be misleading; for example, you may have a design with one net that violates a constraint by 10 ns that would result in the same timing score as a design with 100 nets violating constraints by 0.1 ns. Therefore, the timing score is not necessarily a good indication of which result will run fastest.
The design score attempts to judge more than just the current frequency obtained. It is a comprehensive analysis that also indicates how much potential the design has for improvement with continued routing effort. There is a margin of error in this score, however, so it is best to try re-entrant routing on several of the top design score results to find the best possible overall result.