I agree with what bschaitanya posted. This is possibly the best explanation for your question.
Vijay - "But, think about it, if you have to a design which u need to constraint without any prior/other specs then how do we do it"
Based on your last post, it looks like you have started recently. In the academic world they don't tell you everything. In the real work environment it is simply impossible to start a design without any spec.
There are several large engineering departments involved in building a chip. The systems engineer will usually put a certain basic spec req. Then the circuit guys will give process related specs. Then Logic design engineers will take care of design related specs. + the test engineers will have their set of specs for test mode. All these specs put together will form the basis for constraints.
To answer your question - We simply cannot build a chip without any prior spec. It would be the dumbest thing to try in the real world.
If you have only clock period in hand and if you are working in a company, Then ask your boss or the system engineer to give the spec. You can start with the system engineer and then discuss with the rest.
If you are a student then you can simply assume any value for clock uncertainty, jitter etc to get timing closure and finish your project. You can assume a drive strength of BUF4 for Input ports and a output load of 200fF as a starting point. There is not a lot of theory behind these values.
I have a question. What would be the specification/constraints if we are doing an academic project and wanted to tape out the design? How do we derive the spec/constraints?
Thank you.
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