The 7 series GT wizard in ISE 14.1/Vivado 2012.1 enables the DFE (RXLPMEN=0) and auto adaptation mode for lower line rates, such as SGMII or Gigabit Ethernet protocols.
These protocols also enable 8B/10B and non scrambled data.
As explained in (UG476) v1.11, page 197, the repeated patterns can cause the DFE auto adapting algorithm to drift away from the ideal equalization settings.
Protocols such as gigabit Ethernet send out the idle sequences (repeated patterns) between the real data packet load.
The 8B10B encoded data also has spectral peaks which can cause the DFE auto-adapt to drift from its ideal settings.
This can lead to data errors when the input data changes from the repeated patterns (idle patterns) to real data (random patterns), which is the result of the equalizer auto adapting to the new random pattern data.
For 8B/10B applications in LPM mode, repeated patterns (idle patterns) with or without scrambling can both be used during adaptation.
The work-around is to change the equalization settings.
Depending on the line rate and the channel loss, enable LPM mode from the DFE mode (default) using the RXLPMEN port.
Change the auto adaptation mode to freeze or override mode using the following:
Refer to table 4-10 of (UG476) for more details.
It is recommended to take care in finding the ideal equalization settings based on the channel loss of the system.