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Undesirable buffer insertion for constant pins attached to an instantiated hard macro

My design 'foo' imports a hard macro for a subdesign 'bar', referenced via a bar.lef and bar.lib. One edge of foo abuts with an edge of bar with bar's pins on that edge passed directly to foo's ports. Some of the bar output pins are constants, and thus bar.lib lists a 'function' with a constant value for those pins. RC seems to want to buffer its version of those ports rather than just connect to the output pins of the bar (they are buffered internal to bar, so there is no ESD concern).

The problem is that there is no room to drop a buffer which leads to hilarious results (the buffer is placed many mm away). I'd like to instruct RC (and subsequent velocity runs) to connect those ports directly without any buffering. I [perhaps foolishly] tried:

set_attribute iopt_avoid_tiecell_replacement true [dc::get_ports o_*]

It didn't work (or perhaps I applied this at the wrong point in the flow)--Any suggestions?

grasshopper over 14 years ago

Hi Brannon,
did you try simply preserving (set_attr preserve true [find / -pin ...) such pins ? I believe that should suffice, If that works, I beleive you can even include the preserved/dont_touch'ed pins in your liberty model. Please report back if that works
gh-
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Brannon over 14 years ago

That didn't seem to work--it still replaces the connection to the constant pin on the hard macro with an 'assign out = 1'b0', and then sometime later it replaces all the assigns with buffers.
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Brannon over 14 years ago

I'm having better luck with:
set_attribute propagate_constant_from_timing_model false <instance>
This prevents it from noticing that the pin is constant driven from the instance and thus avoids turning this into an assign which later gets turned into a buffer.
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grasshopper over 14 years ago

Hi Brannon,
that seems like the right attibute. Thanks for reporting back
gh-
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