ISSCC 2008 Cell Processor Update Online

Article: ISSCC 2008 Cell Processor Update
By: David Wang (dwang.delete@this.not.here.com), February 25, 2008 4:48 pm
Room: Moderated Discussions
slacker (s@lack.er) on 2/25/08 wrote:
---------------------------
>David Wang (dwang@not.here.com) on 2/25/08 wrote:
>---------------------------
>>Funny enough, I can't remember now having asked that specific question to the presenter,
>>but we talked about the I/O stuff for a bit. Basically,
>>
>>1. I/O circuits don't scale well.
>>
>>2. Pads and bumps have to be at or near the same places because the package ballout
>>stayed the same (minimize system impact).
>>
>>So I'm not sure if they even tried to shrink the XDR stuff because the benefit
>>(smaller die area) would have been a lot less than what could be gotten from logic
>>shrink. Moreover, the primary target was lower power, and re-doing the XDR stuff
>>for 45 nm wouldn't get you much of anything there.
>
>It now strikes me as obvious that the I/O cells have been shrunk from the 65nm
>design. From your article, the chip dimensions are:
>
>65nm - 15.59 x 11.20 [mm^2]
>45nm - 12.75 x 9.06 [mm^2]
>
>By looking at die photos, it looks like the I/O cells stretch across the entire
>height of both designs. The simple conclusion is that the cells have been shrunk vertically from 11.20mm to 9.06mm.
>
>http://www.trustedreviews.com/images/article/inline/6744-ibmcell.jpg
>http://www.realworldtech.com/includes/images/articles/45nm-cell-3.gif
>>
>
>I am not convinced by the argument that the package ballout would limit the scaling
>of the I/O cells. From your article, the C4 bumps were redistributed after power
>analysis, so IBM would have to develop another package, anyway.

Yes, they had to develope a different package, but to use the same packaging technology, they wanted to use the same bump pitch, so you can move the bumps around, just can't move them closer together much more. Changing the aspect ratio would be somewhat limited by that constraint.

BTW, I then asked the presenter - what about 32 nm? If you're I/O limited now, what are you going to do at 32nm? (assuming there is such a 32 nm port). He gave a cryptic answer of "Well, the XDR stuff may be fairly old by the time 32 nm is ready, so we may replace it with something else." I'm not sure what that means exactly, but suppose they go with XDR2 and do a clean-sheet redesign of the I/O blocks, that would alleviate the silicon layout/bump/package constraint.

>You are right that I/O scaling is difficult, and it's often hard to shrink the
>total area of the I/O cells. But it's not the total area which is the big concern
>to me, but the aspect ratio. It just strikes me as really odd that the cells weren't
>redone with a longer aspect ratio to accommodate for the overall floorplan. Maybe
>Rambus couldn't achieve this, maybe IBM gave them a conservative estimate for chip
>height and couldn't update them until it was too late. Maybe IBM just took the hard
>IP from Rambus and did a quick-n-dirty shrink themselves. I suppose that, in the
>end, the wasted space is not defectable area, so perhaps the area (cost)-savings
>for I/O redesign simply wasn't worth it.

As I wrote, the basic floorplan had to remain unchanged, and changing the aspect ratio of the rambus blocks would probably require quite a bit of hand place/route/adjustment, and that would seem to be against the philosophy of the quick and dirty automated shrink.

< Previous Post in ThreadNext Post in Thread >
TopicPosted ByDate
ISSCC 2008 Cell Processor Update OnlineDavid Kanter2008/02/25 01:08 AM
  ISSCC 2008 Cell Processor Update Onlineslacker2008/02/25 02:49 AM
    ISSCC 2008 Cell Processor Update OnlineDavid Wang2008/02/25 10:59 AM
      ISSCC 2008 Cell Processor Update Onlineslacker2008/02/25 04:14 PM
        ISSCC 2008 Cell Processor Update OnlineDavid Wang2008/02/25 04:48 PM
          ISSCC 2008 Cell Processor Update OnlineDavid Kanter2008/02/25 06:49 PM
            ISSCC 2008 Cell Processor Update OnlineDavid Wang2008/02/25 07:04 PM
              ISSCC 2008 Cell Processor Update OnlineGabriele Svelto2008/02/27 02:22 AM
          ISSCC 2008 Cell Processor Update Onlineslacker2008/02/25 08:30 PM
            ISSCC 2008 Cell Processor Update OnlineDavid Wang2008/02/25 11:37 PM
              ISSCC 2008 Cell Processor Update Onlineanonymous2008/02/29 01:00 PM
            ISSCC 2008 Cell Processor Update OnlinePeter2008/03/01 12:03 PM
        ISSCC 2008 Cell Processor Update OnlineDavid W. Hess2008/02/25 08:40 PM
          ISSCC 2008 Cell Processor Update Onlineslacker2008/02/25 09:15 PM
            ISSCC 2008 Cell Processor Update Onlineslacker2008/02/26 10:14 PM
  ISSCC 2008 Cell Processor Update OnlineDoug Siebert2008/02/25 08:41 PM
    ISSCC 2008 Cell Processor Update Onlineend user2008/02/26 11:39 AM
      ISSCC 2008 Cell Processor Update Onlineav2008/02/26 03:49 PM
        ISSCC 2008 Cell Processor Update Onlineanonymous2008/02/29 01:16 PM
          ISSCC 2008 Cell Processor Update OnlineDavid Wang2008/02/29 05:21 PM
      ISSCC 2008 Cell Processor Update OnlineGabriele Svelto2008/02/27 02:30 AM
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