Stratix V Device Handbook: Volume 2: Transceivers

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

5.3. Reverse Serial Loopback

The Reverse Serial Loopback option debugs with data through the rx_serial_data port, receiver CDR, and tx_serial_data port path.
Figure 162. Reverse Serial Loopback Datapath

Enable reverse serial loopback by accessing the register space within the reconfiguration controller through the Avalon-MM interface.

Note: For the register definitions needed to enable this functionality, refer to the Altera Transceiver PHY IP Core User Guide.

In reverse serial loopback, the data is received through the rx_serial_data port, re-timed through the receiver CDR, and sent out to the tx_serial_data port. The received data is also available to the FPGA fabric through the rx_parallel_data signal. No dynamic pin control is available to select or deselect reverse serial loopback.

You set the reverse serial loopback with the PMA analog registers in the reconfiguration controller.

The only transmitter channel resource used when implementing reverse serial loopback is the transmitter buffer. You can define the VOD and first post tap values on the transmitter buffer using assignment statements in the project .qsf or in the Quartus II Assignment Editor. You can also change these values dynamically with the reconfiguration controller.

Note: For more information about how to dynamically change these analog settings, refer to the Altera Transceiver PHY IP Core User Guide.

Reverse serial loopback is often implemented when using an external bit error rate tester (BERT) on the upstream transmitter.