#798 Another SDF fix

[Cary R.]

This patch adds a check or negative back annotated SDF delays. If a delay is negative then the delay would get transformed into a very large number and that is not appropriate so for this case we set the delay to zero. Negative delays are allowed in a SDF file since tools other than simulation can use it (e.g. static timing analysis). The SDF documentation specifically states that some tools may convert negative delays to 0.

This should be applied to both development and V0.9.

With this patch all delay version of my circuit run correctly! The small negative delay was for my minimum delay case and was likely need to get the various interconnect delays to work correctly. I believe interconnect delays and timing checks are the only remaining issues that need to be addressed to get back annotated delays to work 100%. Interconnect delays are obvious. The $setuphold and $recrem timing checks can also delay signals. Of course having working timing checks have other benefits as well.

[Cary R.]

patch file

[Stephen Williams]

Applied to git master.

[Stephen Williams]

Applied to git v0_9-branch.