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AR# 64835

Design Advisory for 1G/2.5G Ethernet PCS/PMA or SGMII v15.0 (Rev1) and earlier - Fabric Elastic Buffer overflow will cause RXBUFERR to toggle and Auto-Negotiation to restart and never complete


When the core is configured for fabric elastic buffer, and there are ppm differences between the SGMII core clock and the external PHY, Auto-Negotiation (AN) can fail to complete.

The rxbuferr signal toggles and auto-negotiation is restarted periodically, but never completes.


When elastic buffer overflow occurs, rxbuferr is used to reset the RX elastic buffer as well as the GT. 

It also causes the Synchronization statemachine to go to LOSS_OF_SYNC initially and to generate RUDI invalid.

There is a MASK RUDI counter for buffer error, which prevents reset of the AN statemachine.

When the GT comes up after the reset due to rxbuferr, the RXDATA from GT is garbled.

This causes RUDI invalid to be generated and the AN statemachine state will be AN_ENABLE.

The CDR takes longer to lock to the RX data and send a valid RXRECCLK.

If you are not using MDIO to issue a reset, then making the following changes in the gtwizard_init file will fix the issue.

Change the below line from:

gt0_gtrxreset_gt <= gt0_gtrxreset_t or gt0_gtrxreset_in;


gt0_gtrxreset_gt <= gt0_gtrxreset_t;

This stops rxbuferr from resetting the GT.

But the omission of gt0_gtrxreset_in will also remove the soft_reset ability from MDIO.

A Permanent fix for this issue is to increase the Mask RUDI Buffer timer value to match the GT CDR lock value.

This fix will be included in Vivado 2015.3.

Please note, the core is not affected if it is configured for 1000BASE-X only, or in SGMII mode without the fabric elastic buffer.
AR# 64835
Date Created 06/22/2015
Last Updated 06/26/2015
Status Active
Type General Article
  • Vivado Design Suite - 2015.1
  • Vivado Design Suite - 2015.2
  • Ethernet 1000BASE-X PCS/PMA or SGMII