Infinity Fabric = PCIe

By: Aaron Spink (aaronspink.delete@this.notearthlink.net), March 16, 2017 11:04 pm
Room: Moderated Discussions
Michael S (already5chosen.delete@this.yahoo.com) on March 16, 2017 5:13 pm wrote:
> Thanks. I never encountered it by name "disparity mechanism".
> I probably invented a dozen of various schemes for such purpose in different shipping
> products, all without knowing that I construct disparity mechanisms. :-)
>
Well the fundamental control mechanism for xb/x+2b encoding is called the "Running Disparity" for instance.

> > Even for DC coupled links, it can help with the behaviour of transmitter/receiver.
> > A popular one is, of course, 8B/10B (or 64B/66B) encoding.
>
> 8B/10B both assures high density of transitions and guarantees near-perfect DC balance.
> In DC-coupled linked the later is unnecessary.
>
This isn't actually true. There are a multitude of issues surrounding the equalizers, sense amps, jitter et al that want a probabilistic limit on the disparity within any given time frame in order to work well. Or within expected nominal BER. As you push GT/s higher, the disparity limited become more important for forward clock designs. While forward clock designs don't need super tight limits on disparity because they aren't deriving the clock from the data transitions, as they increase in performance they are effectively using a combination of the fwd_clk and the data transitions to do local clocking and jitter correction.

Something like QPI in the 8+ GT/s range is taking advantage of the bandwidth and latency savings of not needing an encoding function such as 8b/10b or 64/66 or 128/130, but in a lot of ways the actual I/O receivers are extremely similar to say a tradition SERDES I/O receiver and thus in the presence of high disparity, the circuit can fail to stay in sync. There are a variety of ways to overcome this but the most common are using various PRNGs xor'd into the data generally making use of LFSR. While this doesn't create an absolute limit on disparity, it does create a realistic/probabilistic limit on the disparity.
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    Infinity Fabric = PCIeslacker2017/03/15 02:59 PM
      Infinity Fabric = PCIeMichael S2017/03/15 04:19 PM
        Infinity Fabric = PCIeIreland2017/03/15 04:49 PM
        Infinity Fabric = PCIeRicardo B2017/03/15 10:25 PM
        Infinity Fabric = PCIeslacker2017/03/15 10:45 PM
          Infinity Fabric = PCIeRicardo B2017/03/15 11:02 PM
            Infinity Fabric = PCIeAaron Spink2017/03/16 01:08 AM
              Infinity Fabric = PCIeRicardo B2017/03/16 05:30 AM
                Infinity Fabric = PCIeAaron Spink2017/03/16 10:58 AM
        Infinity Fabric = PCIeGabriele Svelto2017/03/16 03:48 AM
          Latency vs. bandwidthDavid Kanter2017/03/16 08:04 AM
      Infinity Fabric = PCIeGroo2017/03/15 05:35 PM
        Infinity Fabric = PCIeslacker2017/03/15 11:11 PM
          Infinity Fabric = PCIeAdrian2017/03/16 02:51 AM
            Infinity Fabric = PCIeMichael S2017/03/16 04:32 AM
              Infinity Fabric = PCIeAdrian2017/03/16 07:58 AM
                Infinity Fabric = PCIeRicardo B2017/03/16 09:16 AM
                  Infinity Fabric = PCIeMichael S2017/03/16 09:54 AM
                    Infinity Fabric = PCIeRicardo B2017/03/16 10:07 AM
                      Infinity Fabric = PCIeMichael S2017/03/16 02:33 PM
                        Infinity Fabric = PCIeGabriele Svelto2017/03/17 03:35 AM
                          Infinity Fabric = PCIeAaron Spink2017/03/17 10:25 AM
                            Thanks, your posts on this thread have been extremely informative (NT)Gabriele Svelto2017/03/17 01:08 PM
                            Infinity Fabric = PCIeRicardo B2017/03/17 06:08 PM
                              Infinity Fabric = PCIeAaron Spink2017/03/17 06:27 PM
                              Quickpath papers - circuits and implementationslacker2017/03/18 06:50 PM
                                Quickpath papers - circuits and implementationRicardo B2017/03/19 08:07 AM
                    Infinity Fabric = PCIeAaron Spink2017/03/16 11:11 AM
                      Infinity Fabric = PCIeMichael S2017/03/16 02:46 PM
                        Infinity Fabric = PCIeRicardo B2017/03/16 03:07 PM
                          Infinity Fabric = PCIeMichael S2017/03/16 05:13 PM
                            Infinity Fabric = PCIeAaron Spink2017/03/16 11:04 PM
                            Infinity Fabric = PCIeRicardo B2017/03/16 11:30 PM
                              Infinity Fabric = PCIeMichael S2017/03/17 07:24 AM
                                Infinity Fabric = PCIeRicardo B2017/03/17 06:07 PM
              Infinity Fabric = PCIeDavid Kanter2017/03/16 08:36 AM
                Infinity Fabric = PCIeMichael S2017/03/16 08:50 AM
                  Infinity Fabric = PCIeAaron Spink2017/03/16 11:24 AM
                    Infinity Fabric = PCIeMichael S2017/03/16 02:45 PM
                      Infinity Fabric = PCIeAaron Spink2017/03/16 11:21 PM
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