12-16-2013 11:06 AM
Is the speed grade information used while creating an ngc file if the project doesn't include a xcf file?
Assume I am providing a user with an ngc file to be used in his design. The user will take the ngc file and create his own project with additional design units added to the design and run the place & route.
If I were to create the ngc file with a different speed grade, would this be problem? Would this result in any inefficient designs?
12-16-2013 02:57 PM
12-16-2013 01:48 PM
Hmmm... Interesting question.
So, first, the design will be "correct" regardless of which speed grade you used in generating the ngc, as long as your customer uses the correct speed grade in his project. The ngc is merely a netlist describing the basic elements (BELs) used to implement the design and their connectivity. It can carry some constraints (that's why its ngC - the C is for constraints), but the constraints are not written unless the -write_timing_constraints option to xst is used (which, as far as I know, no one does - particularly if you don't have an .xcf!)
As for it being inefficient... Well, that's a tougher question. In theory XST is timing driven, so it is theoretically possible that it will optimize your design differently on different speed grade parts (maybe a faster speed grade would result in a smaller design, since the tool has less trouble meeting timing, and can hence reduce area). Of course, this is based on two assumptions
1) That the tools have a goal. Since you haven't given an xcf file, they don't. and
2) that XST is really timing driven (or does something sane based on timing constraints). In my experience, the verdict on this is still out (and probably always will be). I have never found XST to be particularly strongly timing driven
Since, in your case, neither of these things seem to be true, its not likely that you will be able to find any difference in efficiency. However, the results will be different - changing the speed grade is an "independent variable" to the synthesis process, and even without an .xcf file this will likely change the resulting design. Its impossible to know if it will be better or worse, but it will likely be "different".
12-16-2013 02:57 PM
12-19-2013 11:29 AM
Thank you both avrumw and muzaffer. Both your answers were very nice and explanatory. I appreciate your help.
I accepted muzaffer's explanation as the answer though avrumw's answer was sufficient as well.