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real hold time always more than 0 ns. but the clock tree can have short delays (with comparison to data paths), than more or equal than hold time of real flip-flop elements of fpga. if this delay is equal to real hold time, that we have 0 ns hold time in fpga. if more, than we can have negative hold time.
this delays are defined by engineers, who design fpga core.
Interval (t2,t4) must satisfy the setup time of the FF, and (t4,t5) must satisfy the hold time of the FF. Assume the hold time (t4,t5) is always positive for the FF.
The time intervals (t1,t2) and (t3,t5) represent propagation delays through the input buffer. Because of the delay, t3 can precede t4.
If the interval (t4,t5) satisfies the FF hold time requirement and t3 precedes t4, then the whole circuit has a negative hold time requirement for DIN with respect to CLK.
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