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Exclude layer interfering with scripted via generation

arekhi

I have recently come across some unexpected behavior related to scripted via generation. Specifically, I'm finding that the viaGenerateViasInArea function fails to generate vias in an area in which I have previously created a polygon in the corresponding via exclude layer/purpose pair. The following minimum working example should clarify what I mean:

test_layout_view = dbOpenCellViewByType("libName" "cellName" "layout" "maskLayout" "w")
techFile = techGetTechFile(test_layout_view)
constraintGroupId = cstFindConstraintGroupIn(techFile "virtuosoDefaultSetup")
myViaOptions = viaGetViaOptions(constraintGroupId)

dbCreateRect(test_layout_view list("M1" "drawing") list(0:0 1:1))
dbCreateRect(test_layout_view list("M2" "drawing") list(0:0 1:1))
dbCreateRect(test_layout_view list("V1" "exclude") list(0:0 1:1))
viaGenerateViasInArea(test_layout_view list(0:0 1:0 1:1 0:1) myViaOptions ?topAndBottomLayers list("M1" "M2"))

If the example above is run, the vias will not be generated. This is unexpected because the exclude layer, according to my understanding, should only interact with fill generation, or at least should not interact with via generation; this is further supported by the fact that I can successfully use dbCreateRect to directly create polygons in the via layer even after the exclude layer polygon has been created. One workaround is to simply create the exclude layer polygon after any overlapping vias have already been generated, but this may not always be convenient from the standpoint of code organization.

Is there an exclude layer setting or via setting that controls this? Browsing the elements of the via options object didn't reveal anything obvious. Or could this be a bug, in either the way the PDK is set up, or in Virtuoso? Thanks in advance and let me know if you need any more info or if I should file a case with customer support.

Software: Virtuoso ICADVM20.1-64b

  • There's nothing special about the "exclude" purpose. It doesn't mean anything to Virtuoso - it's used by your physical verification solution (the fill rule decks) to prevent dummy fill in the regions specified. 

    The via generation is not using the layer-purposes, just the layers (which is the usual behaviour in OpenAccess; most rules are defined in terms of layers, not layer-purposes). So what is happening here is that it sees that there is a big square of "V1" over the region already, so no need to places vias in the region - you see the same behaviour if you'd drawn using "V1" "drawing".

    So neither dbCreateRect nor viaGenerateViasInArea take any notice of "exclude" purposes, because that doesn't mean anything to Virtuoso (it''s not even one of the built-in purposes).

    That said, there is a mechanism to tell the via engine to ignore specific purposes. You can do this by adding this line to your code:

    myViaOptions->automatic->userIgnorePurposes="exclude"

    Regards,

    Andrew

  • in reply to Andrew Beckett

    Thanks for the explanation Andrew. Good to know that rules are generally defined in terms of layers and not layer purposes. I've confirmed that the same behavior is seen if "V1" "drawing" is drawn, and that adding "exclude" to userIgnorePurposes is a working solution.

  • in reply to Andrew Beckett

    For the next time I run into something like this - is there a way to get more detailed documentation about the properties of an object returned by a built-in function? This would make it a lot easier to find solutions to these kinds of issues myself. To take this specific case as an example, the Cadence Help entry for viaGetViaOptions provides "some via options" for the returned t_viaOptionsObject, but does not describe them all in detail; searching for either t_viaOptionsObject or userIgnorePurposes in the help interface reveals nothing useful.

  • in reply to arekhi

    I've had this on my to-do list for a while, to file a detailed request asking for all the options to be documented, because many of these are rather obscured and there's little chance you'd stumble across them.

    I'd already marked the post in my inbox to follow-up on documentation and writing an article - most likely I'll get to that in a week or so (I'm on vacation after this week).

    Regards,

    Andrew

  • in reply to arekhi

    Sounds great, thanks Andrew. Enjoy your vacation.

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