Hi, I just overheard about a kind of standard lib component F/F, which are used to "smash" X-values at the module level boundary. This components are said to have a quality of either pushing "0" or "1" at the outputs instead of "x" or "X". I would like to request you all, to throw some light on this. As I searched in Google it did not result in any kind of convincing information.
Please reply.
Akhilesh
Well, in the real world there really isn't such a beast as "X". Everything is either a "1" or "0". Sometimes you may not know whether its a "1" or "0" .. in virtual land that is one of the things that "X" tells you. SInce there is no "X" in the real world where real library components are used, then I do not believe there is such a library cell that banishes this condition.
You can always edit the models for the cell libraries such that they don't produce an "X" anymore. You can also edit the SDC files so that timing violations no longer occur as well (that is another cause of the wicked "X").
Mike
X's are your friend. Filter them away and you filter away your bugs.
The place where you want to filter aways X's is when crossing clock domains. Here you would like to propagate 0 or 1 to mimic metastability flops.
Petter
-- A: Because it messes up the order in which people normally read text. Q: Why is top-posting such a bad thing? A: Top-posting. Q: What is the most annoying thing on usenet and in e-mail?
In news: snipped-for-privacy@www.gratismegler.no timestamped 27 Jul 2007
16:52:54 +0200, Petter Gustad posted: |---------------------------------------------------------------------| |" Akhil writes: | | | |> I just overheard about a kind of standard lib component F/F, which | |> are used to "smash" X-values at the module level boundary. | | | |X's are your friend. Filter them away and you filter away your bugs. | | | |The place where you want to filter aways X's is when crossing clock | |domains. Here you would like to propagate 0 or 1 to mimic | |metastability flops." | |---------------------------------------------------------------------|Why not use Std_ULogic instead?
Why not eradicate every X?
Regards, Colin Paul Gloster
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