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AR# 18394

10.1 Timing Analysis, Constraint - Tdcmino is calculated incorrectly when the FEEDBACK constraint is too large


I added a FEEDBACK constraint to describe the length of the external feedback path to my DCM/DLL; however, if the value of the FEEDBACK constraint exceeds the value of the PERIOD constraint, the tools do not calculate the delay through the DCM/DLL (Tdcmino) correctly. The problem is that in hardware the DCM will never deskew more than one full PERIOD. However, in software, if my FEEDBACK constraint is too large, the Tdcmino value is greater than one full PERIOD.


If the value assigned to FEEDBACK plus the internal delays of the feedback path exceed the PERIOD, the correct Tdcmino value is not reported. The DCM never deskews more that one clock cycle, and the Tdcmino value should never be larger than one clock cycle. 


You can work around this by subtracting one PERIOD from the FEEDBACK constraint; negative values should work for the FEEDBACK (never tested).  




PERIOD = 10 ns, dcm2pad = 2.0 ns, pad2dcm = 3 ns, FEEDBACK = 6 ns 


This reports the incorrect Tdcmino value. To correct this, make FEEDBACK = -4 ns. You will receive the same overall result, but the Tdcmino value will now reflect hardware.

AR# 18394
Date 05/16/2014
Status Archive
Type General Article
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