Hi everyone do anyone have experience in solving max fanout

Question

Hi everyone do anyone have experience in solving max fanout violation using openlane your help is much precious pray

htamas · Answer

You could try to decrease `SYNTH MAX FANOUT` in your configuration file

dlmiles · Answer

How does a decrease of ``SYNTH MAX FANOUT`` have a positive effect on a 49 gt 10 scenario I have a few fanout=13 limit=10 to resolve myself somewhere Doesn t the flow automatically insert buffers to achieve fanout What if I create that buf clkbuf tree manually that was to be my next experiment Top google hit on this configuration item indicates cranking it all the way up to 15 has previously caused crash or halt with flow

htamas · Answer

Yosys calls abc for tech mapping in the synthesis phase which uses some combination of buffering resizing and duplication to keep fanout under `SYNTH MAX FANOUT` if `SYNTH BUFFERING` is enabled which is the default The fanout is modified by later phases in the flow so it won t necessarily stay under this value but the flag still provides some control over it

htamas · Answer

If you are curious about the details `SYNTH MAX FANOUT` is used in openlane s `scripts yosys synth tcl` to set the ` N` option for abc s `buffer` command which is in turn implemented in `Scl CommandBuffer ` in yosys `abc src map scl scl c` which calls the `Abc SclBufSize ` function in `abc src map scl sclBufSize c` where the fanout value is used as `p gt pPars gt nDegree`

htamas · Answer

By the way you can most likely ignore the warning about a fanout of 13 I m not sure about the 49 though it feels high so if it were my design I would try to tweak the config to push it down but some more knowledgeable people might tell that you that it s safe as long as it doesn t cause other errors down the line

calvin · Answer

hi htamas this is the sta i get from routing odb file is that also can using yosys to doing buffer insert inside the odb or openroad

dlmiles · Answer

I am understanding this to mean `SYNTH MAX FANOUT` is being used by an early flow step but later steps will keep on rearranging things without regard to this so the warning at the end it to show of how out of hand those later stages needed to insert elements compared to targets

calvin · Answer

hi Darryl Miles ya i understand regarding this is there anyway for solving the max fanout on the later stage so far i got to know in openroad with cmd repair design max to limit the wire length and insert buffer but in here timing violation will be a great number

Vijayan Krishnan · Answer

Check the resizer log and if fanout number is high create an issue reproducible and file a github issue

calvin · Answer

thanks from helping i will put on discussion on this matter on github