V-Series Transceiver PHY IP Core User Guide

ID 683171
Date 7/26/2022
Document Table of Contents

17.32. Loopback Modes

The PMA analog registers allow you to enable pre- and post-CDR serial loopback modes.

You can enable the pre- and post-CDR reverse serial loopback modes by writing the appropriate bits of the Transceiver Reconfiguration Controller pma_offset register described in PMA Analog Registers. In pre-CDR mode, data received through the RX input buffer is looped back to the TX output buffer. In post-CDR mode, received data passes through the RX CDR and then loops back to the TX output buffer. The RX data is also available to the FPGA fabric. In the TX channel, only the TX buffer is active.

Figure 102. Pre- and Post-CDR Reverse Serial Loopback PathsIn this figure, grayed-out blocks are not active in these modes. The number (2) shows the post-CDR loopback path and the number (3) shows pre-CDR reverse serial loopback path.

In addition to the pre-CDR and post-CDR loopback modes available in the Transceiver Reconfiguration Controller register map, all the of PHYs, with the exception of PCI Express, support serial loopback mode. You enable this mode by writing the phy_serial_loopback register (0x061) using the Avalon-MM PHY management interface except for the Native PHY IP. In Native PHY IP, you can enable the serial loopback mode by driving rx_seriallpbken input port to 1'b1. Also, PCI Express supports reverse parallel loopback mode as required by the PCI Express Base Specification.

The following figure shows the datapath for serial loopback. The data from the FPGA fabric passes through the TX channel and is looped back to the RX channel, bypassing the RX buffer. The received data is available to the FPGA fabric for verification. Using the serial loopback option, you can check the operation of all enabled PCS and PMA functional blocks in the TX and RX channels. When serial loopback is enabled, the TX channel sends the data to both the tx_serial_data output port and the RX channel.

Figure 103. Serial Loopback