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AR# 7131

4.1i Trace (TRCE)/Timing Analyzer - 0 items analyzed on OFFSET constraints in Virtex designs on OFD/IFDs

Description

General Description:

When using OFFSET constraints on an output pad that has both the 3-state flip-flop in a particular IOB and the OUT flip-flop in the same IOB, you must apply different OFFSET constraints to the different flip-flops.

The normal method would be to create one TIMEGRP for the 3-state FFs and another for the OUT-FFs, and then to apply different OFFSET constraints to the different groups. However, when both the 3-state FF and the OUR-FF go into the IOB, one of the OFFSET constraints is ignored. ("0 items analyzed"). However, if one of those flip-flops is moved into a CLB, then both of the OFFSET constraints are analyzed properly.

Solution

To work around this problem, create a detailed timing report where it is possible to find the paths through the flip-flops situated in the IOBs, and the calculations can be performed manually.

For more information, please also see (Xilinx Answer 3285), (Xilinx Answer 2435), (Xilinx Answer 9297), and (Xilinx Answer 5965).

Another work-around is to run a detailed report with just the 3-state FF-to-pad path, then run another detailed report with just the OUT-FF-to-pad path.

AR# 7131
Date Created 08/31/2007
Last Updated 01/18/2010
Status Archive
Type General Article