Critical Warning: Register-to-register paths between different clock domains is not recommended if one of the clocks is from GXB receiver channel.

Description

The critical warning is produced in the Quartus® II software to warn against possible metastability when data crosses clock domains and one of the clocks is sourced from a transceiver configured as a receiver.

To remove the warning and safely cross clock domains you can use the following methods.

• Data synchronous to the receiver that can change on every cycle should cross clock domains using a dual clocked FIFO.
• Status signals can cross clock domains with synchronization register chains. When synchonized correctly, you can add a TimeQuest® constraint that cuts the path between the registers clocked by different sources.

You can read more about metastability in the Managing Metastability with the Quartus II Software chapter of the Quartus II Software Handbook.