Timing Analysis


Setup & Hold Time Check please tell me why for the conclusion listed as follow: 1.The Maximum data path is used to determine if setup constraint is met. (why is Maximum data path used to determine....., when i do synthesis,how it work?) 2.The Minimum data path is used to determine if hold time is met.

Asked By: chen214
On: Apr 17, 2005 10:33:09 AM

Comments(3)



thanks for help
Why maximum data path is used to determine if setup constarint is to met???? Let us suppose we have two flipflops with combinational block between them. Let the combinational block delays be (min=6 ns, typ = 7 ns, max = 8 ns). Let the setup time for second flop is 2 ns. For the time being let us suppose the clock to Q delay of first flop is 0 ns. Now Let us caluclate the clock period required. with min delay (6 + 2) = 8 ns. Now we are using 8 ns clock for this circit. Let us suppose now the delay of cicuit became 7 ns(i.e. the typical value). So the data will arrive at the input of second flop at 7 ns and the time left for the setup is (8 - 7) = 1 ns (but set up is 2 ns) so there is a set up violation. If we have considered max delay at the time of calculating clock period it will be (8+2) = 10 ns. So even if min or typ delays happes to occus there will be enough time for setup. So we consider max data path or max delay for setup. Similarly for the hold also.
the minimum time b4 clock the data shd be ready to get an actual op is tsu and by all means it is relevant to max delay.intuitive one.even if it is min delay its correct because the data will arrive well b4 and that is even better. the minimu time after the clock is th and this is minimal path because on some cases the data may change taking the short route and the op cannot be justified correctly if u consider the max path.
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