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AR# 53819

7 Series - 2D Eye scan appears skewed to one side in IBERT.


While running 2D eye scan in IBERT, the link runs error free, yet appears severely skewed to one side.


This can occur in cases where the measured eye is highly asymmetric.

The CDR has limited bandwidth and memory. This means that the alignment occurs at some lesser BER than the blue section of the eye, whichrepresents a BER of 10^-10. The eye scan is able to find this edge since it is not restricted in terms of samples or ability to store them.

It also reacts to transition density such that in cases where the link is marginal without well defined edges, the CDR will skew heavily toward the side of the eye where the contour is steeper due to the transition density there.

There is also a skew between the sampler used by the data output and the eye scan sampler, though this will be no more than 3-5 horizontal codes wide.

The edge detectors of the CDR are located at fixed HORIZ_OFFSET codes, and the eye cannot be shifted as a result. The recommendation is to further tune the link with particular focus on the pre- and post- emphasis settings on the TX side. Some severe cases can also be due to incorrect RX termination settings.
AR# 53819
Date 03/04/2013
Status Active
Type General Article
  • Kintex-7
  • Virtex-7
  • ChipScope Pro IBERT for 7 Series GTH
  • ChipScope Pro IBERT for 7 Series GTX