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I did some experiments before and I found that for the most critical paths there were no timing difference at all before and after assigning fast input/output registers. Do you think that Quartus-II would automatically assigns fast I/O registers to the I/Os on the most critical paths when and where necessary?
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Usually fast I/O makes a significant difference. But it depends on many factors.
See the fitter report for checking which pins were actually assigned Fast I/O registers.
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I suppose that for bidirectional I/Os we have to assign them to both fast input registers and fast output registers if they are required.
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Yes, and you might want to use Fast Output Enable as well.
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