Cyclone V Device Handbook: Volume 2: Transceivers

ID 683586
Date 10/24/2018
Document Table of Contents

6.1. Serial Loopback

This section describes the use of serial loopback as a debugging aid to ensure the enabled PCS and PMA blocks in the transmitter and receiver channels function correctly.

Serial loopback is available for all transceiver configurations except the PIPE mode. You can use serial loopback as a debugging aid to ensure that the enabled physical coding sublayer (PCS) and physical media attachment (PMA) blocks in the transmitter and receiver channels are functioning correctly. Furthermore, you can dynamically enable serial loopback on a channel-by-channel basis.

The data from the FPGA fabric passes through the transmitter channel and is looped back to the receiver channel, bypassing the receiver buffer. The received data is available to the FPGA logic for verification.

Figure 108. Serial Loopback Datapath

When you enable serial loopback, the transmitter channel sends data to both the tx_serial_data output port and to the receiver channel. The differential output voltage on the tx_serial_data port is based on the selected differential output voltage (VOD) settings.

The looped-back data is forwarded to the receiver clock data recovery (CDR). You must provide an alignment pattern for the word aligner to enable the receiver channel to retrieve the byte boundary.

If the device is not in the serial loopback configuration and is receiving data from a remote device, the recovered clock from the receiver CDR is locked to the data from the remote source.

Note: For the phy_serial_loopback register access description and addressing, refer to the “Loopback Modes” section of the Transceiver Reconfiguration Controller chapter in the Altera Transceiver PHY IP Core User Guide.

If the device is placed in the serial loopback configuration, the data source to the receiver changes from the remote device to the local transmitter channel—prompting the receiver CDR to start tracking the phase of the new data source. During this time, the recovered clock from the receiver CDR may be unstable. Because the receiver PCS is running off of this recovered clock, you must place the receiver PCS under reset by asserting the rx_digitalreset signal during this period.

Note: When moving into or out of serial loopback, you must assert the rx_digitalreset signal for a minimum of two parallel clock cycles.