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AR# 19656

LogiCORE XAUI core - What causes Rx Local Fault or the link to go down when using the XAUI core (or XGMAC with XAUI interface)?


Keywords: XAUI, XGMAC, 10 Gigabit, Ethernet, MAC, lane, align, alignment, sync, synchronization, local, fault, link, status, BER

Urgency: Standard

General Description:
What causes a Rx Local Fault with the XAUI core (or 10 Gigabit Ethernet MAC with XAUI interface)?


The most likely causes for a Rx Local Fault are:

1. At least one of the lanes is not synchronized - SYNC_STATUS[3:0]
2. The lanes are not properly aligned - ALIGN_STATUS.

There are a few reasons that might cause the lanes to lose synchronization or alignment and lead to an Rx Local Fault.

1. Following a reset (or power-up), the XAUI core tries to initialize. However, before this happens the lanes are not synchronized or aligned and as a result the XAUI core always shows a Rx Local Fault initially. This can be observed in Bit 1 of STATUS_VECTOR or the appropriate register if the MDIO interface is used.

Because Rx Local Fault and Rx Link Status are defined as latching error indicators by the IEEE spec, they must be cleared with the Reset Local Fault bit (Bit 2 of CONFIGURATION_VECTOR) and Reset Rx Link Status bit (Bit 3 of CONFIGURATION_VECTOR), respectively. Therefore, you must always clear these upon initialization.

2. If the link comes up but then goes down again or never comes up following a reset, the most likely cause for a Rx Local Fault is a BER (Bit Error Rate) that is too high. A high BER causes incorrect data to be received, which leads to the lanes losing synchronization or alignment. Trying different combinations of the Pre-Emphasis and Differential Swing Control attributes of the MGT might reduce the BER.

Note that, unlike the Local Fault and Link Status bits, the SYNC_STATUS[3:0] and ALIGN_STATUS bits are not latching. Therefore, it is possible for the Rx Local Fault to be latched low indicating an error but for the alignment and synchronization status bits to reflect no issues. Once again, the Rx Local Fault and Rx Link Status bits must be cleared as described in the previous situation.
AR# 19656
Date Created 07/09/2004
Last Updated 07/09/2004
Status Active
Type General Article