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You're correct that in a traditional sense, the uTsu is subtracted from the Data Arrival Time, making the slack worse(or making it harder to meet timing). I have submitted cases like this for analysis, and it comes back that it's the way it's modeled. In essence, the data path at the register might be longer than it should be, and the uTsu becomes negative to account for that. In essence, it was determined to be correct, but I get what you're saying and it is confusing.
Of course, with 19ns of slack it probably doesn't matter...:)
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This issue was raised before, see thread at:
http://www.alteraforum.com/forum/showthread.php?t=1745 I can not see how Tsu can be negative at register level. Moreover if it was negative its sign should have been there.
There is nothing scientific here. It is up to altera to sort out the confusion. is the equation wrong or the tool wrong?
Honored Contributor
