Diamond: WARNING - Unsupported property ASYNC_REG found
Description:
The Async_Reg is an unsupported property in the software which is why this placing attribute is ignored.
What we could suggest is that you can add an HGROUP attribute in the RTL code.
For example, a design that contains double synchronizers for clock domain crossing.
Solution:
By adding the HGROUP attribute it will make sure that the double sync registers are placed closely, at least in one PFU/PFF block. First, you can create a double sync module. And then they can add an HGROUP attribute to the module. When the module is instantiated, the logic in the module (e.g., two Flip-flops) will be placed closely in one group during PAR. It means the delay of the wire between the two Flip-flops is minimum.
The ASYNC_REG property would normally make sure that the Flip-Flops in the double sync are placed closely (e.g. in the same slice) together to achieve a high MTBF. The given example is to have the software configure the placement to put the Flip-Flops of the double synchronizer closely together.