Community
cancel
Showing results for 
Search instead for 
Did you mean: 
JKastil
Beginner
910 Views

How to use DCFIFO for clock crossing?

We are using manual instantiation of the DCFIFO in the SystemVerilog for the clock crossing between two asynchronous clocks (no relation between the clock).

 

Since the write  and read clocks are not synchronous or in any other relation, we have them in two different clock groups in sdc and we do not define any other constraints for the FIFOs.

 

In the previous projects, this approach worked reliably and we did not experience any problems with designs. However, our current project exhibits strange behaviour.

 

According to the https://www.intel.com/content/www/us/en/programmable/support/support-resources/knowledge-base/soluti... the embedded constraints do not work correctly. Was the issue solved?

 

Moreover, I discovered, that when Platform Designer instantiates DCFIFO inside interconnect, it includes SDC file with following comment:

 

# Do not declare the FIFO clocks as asynchronous at the top level, or false path these crossings,

# because that will override these constraints.

 

Does this mean, that we should not have the read clock and write clock in different clock groups?

 

My current understanding of the issue is that to correct implementation of DCFIFOs, we need to remove all clock groups and place the constraints from page 23-24 of ug_fifo.pdf for all dcfifos in the design. Is this correct? It seems, that Platform Designed uses slightly different constraints in generated SDC file.

 

We have relatively large design and we are near to the project deadline, therefore before we start to rewrite our constraints, I would like to know what is the right approach.

 

I would also appreciate the answer from someone from Intel so that we may be sure.

 

Thanks for your comments

Jan

 

 

0 Kudos
1 Reply
GuaBin_N_Intel
Employee
55 Views

.... constraints do not work correctly. Was the issue solved? No for stated device families, you have to add such constraints defined in the KDB. ..Does this mean, that we should not have the read clock and write clock in different clock groups? Both read and write clock could NOT be constrained in different clock groups. Those transfers between them will be ignored in timing analysis if we doing so and may cause functionality issue. .....for all dcfifos in the design. Is this correct? Yes, user have to add those constraints for dcfifo if there is no SDC included and confirm they are valid in timing analysis . Please refer to either KDB or DCFIFO user guide.
Reply