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clock tree synthesis.

gops
How should i manage CTS efficiently. I need to know what all things should be taken care in the clock tree sppecification file for an optimized clock tree. I usually create the .ctsch file from the tool itself. I just used to give the clock buffer and clock inverter foot prints for the purpose. Will this much information create the clock tree in an efficient way? if not please give me some tips to improve my clock tree. thanks gops.
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  •  Hi Alex,

    I'm a bit confused. If you don't want to build a clock tree, why are you running CTS? :-)

    The OptAddBuffer line is to control whether or not buffers/inverters are added during an optimization of the tree AFTER it is built.

    After your CTS run, the other nets you see routed are probably just trialRoutes. You will still need to route them for real with nanoRoute. You don't have to worry about the clock nets though; after being routed with nano during CTS, they are marked fixed. The more typical problem is having violations when routing the rest of the design because nano really needed to slightly move a clock net in order to connect a signal net. To allow nano to be able to move signal nets, you would do the following before calling routeDesign:

    changeUseClockNetStatus -noFixedNetWires

    This will allow nanoRoute to move a clock wire if it has to. We have never seen this affect the clock timing.

    Since it seems you don't want to build a clock tree (but I may not fully understand your intent), I would suggest just routing your clock nets first with nano:

      # ROUTE CLOCKS ONLY FIRST
      selectNet -allDefClock
      setNanoRouteMode -routeSelectedNetOnly true
      globalDetailRoute

      #unfix clock routes so nano can fix violations
      changeUseClockNetStatus -noFixedNetWires

      # ROUTE EVERYTHING ELSE
      setNanoRouteMode -routeSelectedNetOnly false
      globalDetailRoute

    - Kari
Reply

  •  Hi Alex,

    I'm a bit confused. If you don't want to build a clock tree, why are you running CTS? :-)

    The OptAddBuffer line is to control whether or not buffers/inverters are added during an optimization of the tree AFTER it is built.

    After your CTS run, the other nets you see routed are probably just trialRoutes. You will still need to route them for real with nanoRoute. You don't have to worry about the clock nets though; after being routed with nano during CTS, they are marked fixed. The more typical problem is having violations when routing the rest of the design because nano really needed to slightly move a clock net in order to connect a signal net. To allow nano to be able to move signal nets, you would do the following before calling routeDesign:

    changeUseClockNetStatus -noFixedNetWires

    This will allow nanoRoute to move a clock wire if it has to. We have never seen this affect the clock timing.

    Since it seems you don't want to build a clock tree (but I may not fully understand your intent), I would suggest just routing your clock nets first with nano:

      # ROUTE CLOCKS ONLY FIRST
      selectNet -allDefClock
      setNanoRouteMode -routeSelectedNetOnly true
      globalDetailRoute

      #unfix clock routes so nano can fix violations
      changeUseClockNetStatus -noFixedNetWires

      # ROUTE EVERYTHING ELSE
      setNanoRouteMode -routeSelectedNetOnly false
      globalDetailRoute

    - Kari
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