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how the PR tools identify Clock Nets? (Astro)

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sicheng163

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Hi, all
I know I have to do Clock Tree Synthesis before Route,
but actually we can't just do clock signal CTS, also we should do reset signal CTS, then I'm confused how the PR tools (Astro) Identify which net is clock net?
because sometimes Astro will give a warning that reset signal is not a clock net, then no CTS,
Thanks!!
 

For reset/enable or other special nets the tool does High Fanout Nets Synthesis!

If the tool sees that a signal goes to more than a specified number of pins (set_max_fanout <No.>) it creates a buffer tree in a special step called HFNS!
 
dementu666 said:
For reset/enable or other special nets the tool does High Fanout Nets Synthesis!

If the tool sees that a signal goes to more than a specified number of pins (set_max_fanout <No.>) it creates a buffer tree in a special step called HFNS!
Click to expand...

ok, thanks, that's clear enough!
and what's the command? or give me a clue if you can't remember the exact name!
 

Usually PnR tools performs HFN synthesis at placement stage. Command "astAutoPlace" does (by default) the HFN synthesis durirg pre-placement stage. Before going to perform placement, alway check if the reset/enable pins have ideal net attribute or not, if yes you should remove ideal net attribute (remove_ideal_network command) otherwise tool won't synthesiszes those ideal nets.

Your another question : how the PR tools (Astro) Identify which net is clock net?
Tool get this information from SDC (with command create_clock). If you do not define any clock and still need synthesis for certain nets, use "astHFCTS" command.
 
HFNS is done during preplacement optimization (astPrePS command calls pdsHFNOptimization in nontiming mode) and during in-placement optimization (astPlaceDesign command calls astFanoutSetup in timing-driven mode)!

astAutoPlace command also performs HFNS!
 

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