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AR# 38015

12.x ChipScope Pro, IBERT, Spartan-6, GTP - TX buffer is Off by default with far-end loopback


When I use far-end Loopback on a Spartan-6 device, I see unexpected failures or high error rates.

What is the cause?


This can be due to channel issues, but there is also a known issue that might cause it.

In the Spartan-6 FPGA GTP, the IBERT core has the TX Buffer turned Off by default.

There is also an issue with the XCLK that is used.

This can cause high error rates and unexpected link failures.

Currently, the defaults for the following two attributes are:


These settings put the TX side of the transceiver in low latency mode, which should not be used.

The proper settings are as follows and can be set in the DRP tab:

TX_XCLK_sel_* = "TXOUT"

This issue is scheduled to be resolved in ChipScope Pro Analyzer 12.4.
AR# 38015
Date Created 09/14/2010
Last Updated 12/04/2014
Status Active
Type General Article
  • Spartan-6 LX
  • Spartan-6 LXT
  • ChipScope Pro - 12.1
  • ChipScope Pro - 12.2
  • ChipScope Pro - 12.3
  • ChipScope Pro IBERT for Spartan-6 GTP