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AR# 21327

14.x Timing - Why is the Tsetup/Thold spread different with and without a DCM/PLL/MMCM in the design?

Description

Why is the Tsetup/Thold spread different with and without a DCM/PLL/MMCM in the design?

Solution

Relative minimum delays are used when calculating the setup and hold window. For setup times, the clock path uses relative minimum delays. For hold times, the data path uses relative minimum delays. Because the design has the same data path both with and without a DCM, only the clock path can account for the difference in the setup/hold window. The difference is caused by two characteristics associated with using a DCM. The first characteristic is that the actual path the clock signal takes when using a DCM is different than without a DCM. This new path adds delay to the clock path delay to the design. The second characteristic of using a DCM is that the Tdcmino, or delay removed by the DCM, is different for the setup and hold checks. Given the larger clock path, using a DCM and a pessimistic timing analysis to prevent in-system failures across PVT, the setup and hold requirements at the I/O register will be larger when using a DCM (compared to when not using a DCM).
AR# 21327
Date Created 09/04/2007
Last Updated 12/15/2012
Status Active
Type General Article
Tools
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