setup and hold time with set_input_delay

Hi,

I am currently having an almost identical problem in a design.

I hope its ok to add to this post and use it as a reference.

I do not fully understand part of the first response by Rysc.

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5) Change the delay values to match your external delays. I don't follow your descriptions of "Assume a setup time of 1ns and hold time of 2ns". How much could the data come out before the clock, including board delays. If that's 4ns, then your -max delay should be 4ns and since the setup requirement is 5ns, TimeQuest is left trying to meet a 1ns setup time. If you're saying it could be delayed by as much as 1ns, then have that be the -max delay.

For the hold, if the data may leave by as much as 2ns after the clock, the the -min delay is -2ns. Since the hold requirement is -5ns, then only if the dta is held by another -3ns will it fail timing.

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I understood before that the set_input_delay constraint was to account for board delays on the tracks? On my particular interface the rising edge of the clock and the data leaving the external device at the same moment.

If in the example described by Rysc the max constraint is 4ns and the min is -2ns, basically Rysc is saying that the data could arrive to the FPGA ports 4ns after or 2ns before the clock. I do not understand what effect this constraint has. What is the advantage of telling this to Timequest? I don´t really have a clear understand of this constraint.

Also in the previous example it is claimed that it is obvious that the setup time is 5ns and the hold time is -5ns. Again this is probably the dumbest question but can someone explain to me why that is the case.

Fianlly after reading the timequest chapter in the Quartus handbook I do not understand the concept of virtual clocks. I would be very grateful if someone could explain to me in simpler terms why it is advisable to create one.

I am a beginner to timequest and have been studying the documenation over the last few days. So far I am having trouble making sense of it. Hopefully if i can get a push in the right direction and get an understanding of the basics then it will start to come together.

Many thanks for your help.