Stratix V Device Handbook: Volume 2: Transceivers

ID 683779
Date 11/23/2021
Document Table of Contents Deskew FIFO

The deskew FIFO in each channel receives data from its word aligner. The deskew FIFO circuitry aligns the data across multiple channels.

Note: The deskew circuitry is available only in XAUI mode.

In the XAUI protocol, the code groups received across four lanes of a XAUI link can be misaligned with respect to one another because of skew in the physical medium or differences between the independent clock recoveries per lane.

The XAUI protocol requires the physical layer device to implement a deskew circuit to align all four channels. To enable the deskew circuitry at the receiver, the transmitter sends a /A/ (/K28.3/) code group simultaneously on all four channels during inter-packet gap (IPG). The skew introduced in the physical medium and the receiver channels can cause the /A/ code groups to be misaligned when received.

The deskew operation begins only after link synchronization is achieved on all four channels from the word aligner of each channel. When an aligned set of /A/ code groups is observed at the output of the deskew FIFOs of the four channels, the rx_channelaligned signal is asserted high, indicating channel alignment is acquired.