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Clock Domain Crossing in FPGA Designs


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I'm now working on a project involving FPGA development, and I'm facing an issue with clock domain crossing.  I'm trying to synchronize signals between different clock domains, and I'm facing timing violations.

I've tried using synchronizer circuits, but I'm still having trouble there . Does anyone have suggestions for handling clock domain crossing in FPGA designs? Any  advice would be greatly appreciated!


Thank you


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Yeah, you have to provide timing exceptions to Vivado.  They belong in your XDC file.  For example, consider this line:

set_max_delay -datapath_only -from [get_cells -hier -filter {NAME=~ thedesign/GEN_ETHERNET_DECODE*u_netpath/rx_afifo/rgray_r*}] -to [get_cells -hier -filter {NAME=~ thedesign/GEN_ETHERNET_DECODE*u_netpath/rx_afifo/rgray_cross*}] 3.0

This is used to tell Vivado that the FIFO's read gray code will be read in a second clock domain into the signal rgray_cross (i.e., read gray code, but crossed clock domains).  Rather than doing proper timing analysis on this signal--which would fail because the two clocks are unrelated--the statement instead tells Vivado to call this clock domain crossing "passing" if from the time it takes to set the first signal to the time it takes to set the second can be done in less than 3ns.

There are other ways to do this as well.  They all end up being timing exceptions that you have to declare in your XDC file though.

You can let Vivado be the one to tell you which signals need exceptions like this--subject to your review.


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  • 1 month later...

Clock domain crossing can indeed be tricky in FPGA designs. Besides using synchronizer circuits, make sure to carefully analyze and understand the timing requirements of your design. You might also want to consider using FIFO buffers or asynchronous FIFOs to manage data transfer between clock domains. Another approach is to use asynchronous interfaces like handshake signals or dual-clock FIFOs to safely transfer data. Experimenting with different techniques and tools provided by your FPGA vendor's development kit can help you find the best solution for your specific design.

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