Intel® Arria® 10 Transceiver PHY User Guide

ID 683617
Date 3/28/2022
Document Table of Contents 8B/10B TX Disparity Control

The Disparity Control feature controls the running disparity of the output from the 8B/10B Decoder.

To enable TX Disparity Control, select the Enable TX 8B/10B Disparity Control option. The following ports are added:
  • tx_forcedisp—a control signal that indicates whether a disparity value has to be forced or not
  • tx_dispval—a signal that indicates the value of the running disparity that is being forced
When the number of data channels is more than 1, tx_forcedisp and tx_dispval are shown as buses in which each bit corresponds to one channel.

The following figure shows the current running disparity being altered in Basic single-width mode by forcing a positive disparity /K28.5/ when it was supposed to be a negative disparity /K28.5/. In this example, a series of /K28.5/ code groups are continuously being sent. The stream alternates between a positive running disparity (RD+) /K28.5/ and a negative running disparity (RD-) /K28.5/ to maintain a neutral overall disparity. The current running disparity at time n + 3 indicates that the /K28.5/ in time n + 4 should be encoded with a negative disparity. Because tx_forcedisp is high at time n + 4, and tx_dispval is low, the /K28.5/ at time n + 4 is encoded as a positive disparity code group.

Figure 152. 8B/10B TX Disparity Control

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