Does anyone know how OpenLane's default

IO_PCT=0.2

is derived? (Specifies the percentage of the clock period used in the input/output delays. Ranges from 0 to 1.0)

Maybe this sheds some light? Matt Liberty posted this STA document a few days ago. Page 203 https://vlsitesting.files.wordpress.com/2017/02/ref-for-unit6.pdf

Yes 20% applied by default.

If you use too small a value then you get lots of hold buffering around the inputs. 0.2 is just a heuristic value to avoid that. If you have a more realistic set of constraints you should prefer them

thanks

@Matt Liberty Sure about that (the hold buffering) ? It's a max constraint, not a min.