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  3. Timing closure on cloned clock gate enable inputs

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Timing closure on cloned clock gate enable inputs

YJ202602057834
YJ202602057834 1 day ago

Our IP has a software controlled clock gate controlling an entire clock domain (several hundred flops). I noticed that in some cases, the clock gate is cloned post CTS, which create a large fanout path from the enable to the different clones. This path fails then fails timing checks. Apologies if this should not come as a surprise; I'm mostly a Verilog guy.

I have searched quite a lot online but could not find a reliable method of dealing with this kind of problem. The only method that could seemingly solve this is a set_max_fanout/set_register_duplication directive on the enable flop, but for reasons that people online do not explain, backend engineers do not like to use these directives. So:

  1. Is there an industry-standard way to deal with this kind of problems? 
  2. Can anyone tell me why set_max_fanout/set_register_duplication directives are frowned upon?

I have some ideas regarding 1 but they usually involve severe limits on the users to configure clock gating only within a reset context.

Many thanks in advance!

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