AR# 3749

9.1i PAR - Multi-Pass Place and Route: The Design Score and what it means

Description

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"?

Solution

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.

http://toolbox.xilinx.com/docsan/xilinx10/books/docs/dev/dev.pdf

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.

AR# 3749
Date 12/15/2012
Status Active
Type General Article