5.1. Serial Loopback
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.
You can enable serial loopback using the PHY IP Parameter Editor or the reconfiguration controller, depending on which PHY IP mode you select. 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.
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.
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