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AR# 6766

Virtex - IOB registers clocked by the CLKDLL still use the delay elements on the data path; this causes a longer SETUP delay


Input registers that are packed into the IOB and clocked from a CLKDLL still use the DELAY element on the data path. This leads to excessive input setup delay.  


Currently, the tools issue no warning that the DELAY element is not turned off.  


This problem occurs in M1.5i, M1.5iSP1, and M1.5iSP2.


To work around this problem, add the NODELAY attribute to the input register that is clocked by CLKDLL. This can be done in the UCF file or the NCF file, using the following format: 


NET RcvData* NODELAY ;  



NOTE: This is currently also the case for Virtex-II. Please see (Xilinx Answer 11917) for more information.


This problem is fixed in the 2.1i and 3.1i software.


In version 2.1i (with Service Pack 2 and above) and 3.1i of the tools, any input IOB register that is clocked by the DLL will have NO DELAY by default.

AR# 6766
Date Created 08/21/2007
Last Updated 05/14/2014
Status Archive
Type General Article