Someone tried PCIe on the Raspberry Pi 4 already. Yes, the wiring job is horrible, and yes, of course it works, as I've said many times. Go hack on PCIe already, it's easier than you think. Now someone needs to stick a Radeon RX Vega on there, for kicks. http://mloduchowski.com/en/blog/raspberry-pi-4-b-pci-express/ …
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Thats super interesting. Also agree on the SI part. PCIe is only 100MHz which is reasonably resilient. IMO DDR is far more difficult than something like PCIe but even DDR3 is within the reach of hobbyists.
PCIe is not 100MHz, lol. The refclk is 100MHz, but PCIe 1.0 pushes 2.5 Gbaud over each lane (1.25 GHz bandwidth) and it goes up from there. But the PCIe PHY is amazing and can deal with a surprising amout of bullshit.
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Oh, interesting, I thought it was doing some neat tricks on the signal edge at refclk, but in any case, I've found it is ridiculously resilient to being pushed around. I don't really use PCIe for anything though so never looked at how it attained its line rates.
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The refclk is literally just a reference. If you don't use spread spectrum clocking, you can actually just locally generate 100MHz at each device and not have a refclk pair at all. The actual data is self-clocked 8b/10b (for PCIe 1.0 and 2.0).
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Yup. This mess of wires (probably a rather powerful unintentional emitter as well) actually reports 2.0 (5Gbps). I don't recall of the top of my head if PCIe 2.0 double the clock or the symbol density, though.
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Probably the clock if its still 8b/10b
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