Why do I see link failure after a few resets when the GTS Transceiver PMA link is placed under internal serial loopback mode?

Description

Due to a problem in the Quartus® Prime Pro Edition software version 25.1.1, if you are selecting either of the two below example designs and try to enable the internal serial loopback mode, you will see link failure, especially after one or multiple resets. The impacted example designs are:

• 1 x 1G PMA Direct Mode (System PLL Clocking) with Custom Cadence
• 1 x 3.125G PMA Direct Mode (System PLL Clocking) with Custom Cadence

This is due to the default setting of the RX Equalizer parameters (VGA gain, DFE Tap 1, and High Frequency Boost) value in the example design are not compatible during the internal serial loopback mode.
 

Resolution

To overcome this problem, you can follow the section of the GTS Transceiver PHY user guide, section 3.14.2.2, to store and restore the RX Equalizer settings. The original RX equalizer parameter value can be restored after you disable the internal serial loopback for your GTS Transceiver PMA link.
This limitation will be fixed in a future release of the Quartus® Prime Pro Edition software, meaning that when enabling the internal serial loopback mode, the effort of storing and re-storing the RX equalizer parameter settings is not required from the user end.