Tool = ISE 8.2i Target = Spartan 3 xc3s400-4tq144
I created a prorated VOLTAGE constraint using the GUI constraints editor; but it seems to be ignored when I implement my design. The PAR report quotes the default of 1.140V. When I launch Timing Analyzer, it says prorating is not available for my device and speed grade. Is there any way around this? My design achieves timing closure, but only by 1ps, which is cutting it a bit fine! Should I be worried?
The delays are for worst case process/voltage/temperature values unless the timing models support prorating and you've specified a prorated value. Since you got the "not available" message, your timing is based on worst-case. If the DCM jitter *is* included in your timing, your input clock is clean, and you don't have SSO issues that would make that input clock dirty, my understanding is that 4.999 ns value *is* worst case. I haven't had problems in my designs (which are not subject to temperature extremes) that stop trying to improve timing the moment it achieves 1 ps of margin. I have over-specified my frequency or added INPUT_JITTER to my constraints in the past but haven't had "flaky operation" to deal with as a result of marginal timing.
The tools stop trying at 1 ps margin. You should be able to rely on this OR expect the tools to keep trying.
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