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Observer becker1
Registered: ‎10-23-2007


Hello Folks,


this subject keeps bugging me since i work with (Xilinx)FPGAs.

So,..there is an OFFSET OUT constraint one can specify for Outputs. 

And...so far i see from timing reports, PAR understands it as maximum value.


But...design i'm currently doing restricts me also for suppliying an best case clock-to-output

constraint. Seems like this is currently not possible with xilinx tools...am i right?

I need to specify an maximum and minimum value for OFFSET OUT.


All i can do is...after PAR finishes...do timing anaylsis with "Report fastest paths/verbose hold paths" check box selected.


And, also i found something like Speedgrade -0 for checking best case timing results.

(referring to http://www.xilinx.com/support/answers/4506.htm)


Can someone explain me the difference between


a) timing analysis with speed_grade 4 selected but also hold time analsyis option checked.("report fastest paths/verbose hold paths)

    So i have for each path setup and hold anaylsis.(well we use speed_grad 4 parts) 


b) timing analysis with speed_grade -0(MIN) selected. Btw, i can also select "report fastest paths/....." option here. 


Is there any speed_grade -0 parts really available? Or is it a virtual device for making best case

timing analysis. If, not...using speed_grade 4 parts and doing speed_grade -0 analysis seems nonsense to me.


So, alltogether, i have 4 anaysis for each paths: 2 with speed grade 4 selected and 2 with -MIN as speed grade selected.

Confused.....should't be there a only 2 extreme(MIN,MAX) for selected speedgrade, in my case speedgrade 4?


So, let me summarize my problem in short:


1- I have speed grade 4 parts, want to specifiy also MIN clock-to-Out constraints for some IOs.

    How can i specify MIN offset out? is it possible? 


2- I want to check the worst case/best case clock-to-output times in timing reports.

    I did once compared hold analysis results(speed grade 4 selected) with measurement

    results. Measurement results show much faster times than in Timing report.

    (so i have a good speed grade 4 unit in the hand)

    So speed grade 4 hold analysis is not the best case analysis. So what is it then?



Many Thanks for any Clarification 





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