sivakumar_tumma,
Setup time restrictions are due to the inability of an input circuit to respond instantaneously to a change in input. Setup time allows the input circuit to settle out before the clock edge accurs.
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Hold time restrictions are due to the inability of the input circuit to respond to the positive feedbak from the output that ensures that the output will remain in a stable state. Some off the shelf devices have a minimum hold time of 0.
Regards,
Kral