A good timing result is obtained out of multiple implementation runs. I would like to lock some elements down per the result,so as to re-use the placement in subsequent runs.
How can I achieve this in planAhead tool?
When you import placement results from ISE software, placed instances display as unfixed in PlanAhead tool.You can lock placement in place for subsequent runs by selecting the desired logic, right clicking, and using the Fix Instances pop-up menu command.
Save the project andthe fixed logic will receive LOC and BEL constraints in the constraints file.
Or, select File -> Export Constraints to export fixed constraints only to a specified constraints file.
NOTE:Do not fix everything in place since the design is likely to change. On most designs the block RAM and DSP48 primitives have a relatively stable set of primitives and names. Re-using the placement of just the block RAM and DSP48 can help keep timing as other gates change.