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the reason behind setup time and hold time is the time required for the input transistors to respond to the particular signal.... it involves charging of capacitors and switching of the transistor.....
actually setup time constraint induces metastability so the right output might be obtained.... it is not so for hold time because you are discharging or charging the capacitance while it is supposed to be the other way around and this would surely lead to a error...
It is ,the min length of time that a data i/p is stable before the active clock transition.
It is ,the min length of time that a data i/p is stable after the active clock transition.
Set-up time and hold time are the constraints from the process itself ..so this is wrong to say why they are needed. they are always there..? Only yours data should meet these constraints with respect to the trigerring signal(clk).
Requirement for set-up time: Input node takes some time to charge ( parasitic capacitance or oxide capacitance+ parasitic capacitance.so it is required to get the exact voltage at the node .
Requirement for hold time: since transistor takes finite time in triggering , so when u trigger urs transitor with respect to clk , this is time required to hold the incoming data stable till the triggers does not propagate. otherwise u will sample wrong data.
Flip-flops are bistable devices. However to switch the state you need to drive the gate for a finite time. If your input is short, the output will revert back to the original state. Thus we need hold time to successfully switch the state. for setup time, After you apply the input signal it takes some time to charge/discharge the input capacitors. Before that the input nodes will have wrong voltages. Thus set-up time is required to fully prepare the input for sampling...