L- and H-Tile Transceiver PHY User Guide

ID 683621
Date 1/30/2024
Document Table of Contents

5.9. Loopback Modes

The PMA supports three serial loopback modes:

  • Serial loopback
  • Pre-CDR reverse serial loopback
  • Post-CDR reverse serial loopback
Note: Intel® Stratix® 10 transceiver channels configured in GXT mode do not support Pre-CDR reverse serial loopback and Post-CDR reverse serial loopback.
Figure 229. Serial Loopback Mode

The serial loopback path sets the CDR to recover data from the serializer instead of the receiver serial input pin. The transmitter buffer sends data normally, but serial loopback takes the data before the buffer. Note that this also skips the receiver CTLE, but passes through the VGA before going into the CDR. The VGA can be adjusted accordingly.

Figure 230. Pre-CDR Reverse Serial Loopback Mode
Note: TX pre-emphasis is not supported in pre-CDR loopback. Intel recommends setting TX pre-emphasis to 0 for all taps.
Figure 231. Post-CDR Reverse Serial Loopback ModeThe reverse loopback path sets the transmitter buffer to transmit data fed directly from the CDR recovered data. Data from the serializer is ignored by the transmitter buffer.