Stratix V Device Handbook: Volume 2: Transceivers

ID 683779
Date 11/23/2021
Public
Document Table of Contents

1.3.1.5. 8B/10B Decoder

Many protocols require the serial data sent over the link to be 8B/10B encoded to maintain the DC balance in the transmitted serial data. The PCIe protocol requires the receiver PCS logic to implement an 8B/10B decoder to decode the data before forwarding the data to the upper layers for packet processing.

The receiver channel PCS has an 8B/10B decoder after the rate match FIFO. In configurations with the rate match FIFO enabled, the 8B/10B decoder receives data from the rate match FIFO. In configurations with the rate match FIFO disabled, the 8B/10B decoder receives data from the word aligner.

In 10-bit mode, the 8B/10B decoder receives 10-bit data from the rate match FIFO or word aligner (when the rate match FIFO is disabled) and decodes the data into an 8-bit data +1-bit control identifier. The decoded data is fed to the byte deserializer or the receiver phase compensation FIFO (if the byte deserializer is disabled).

Figure 23. 8B/10B Decoder in 10-bit Mode
Note: The 8B/10B decoder is described in IEEE 802.3-2008 clause-49.

In PCIe configuration, the 8B/10B decoder operates only in 10-bit width mode. A PCIe configuration forces selection of the 8B/10B decoder in the receiver datapath.

Control Code Group Detection

The 8B/10B decoder indicates whether the decoded 8-bit code group is a data or control code group on the rx_datak signal (not shown in the figure above). If the received 10-bit code group is one of the 12 control code groups (/Kx.y/) specified in the IEEE 802.3 specification, the rx_datak signal is driven high. If the received 10-bit code group is a data code group (/Dx.y/), the rx_datak signal is driven low.

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