New P2 Silicon Observations

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Comments

  • Yes, these are all completely populated boards with SPI Flash. Haven't you loaded your board yet? :)

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  • Yes, these are all completely populated boards with SPI Flash. Haven't you loaded your board yet? :)

    Aside from the P2, I just have a bypass cap on VDD and one on VIO.
  • cgraceycgracey Posts: 9,541
    edited October 10 Vote Up0Vote Down
    It would be important to make a program that simply does a GETRND and outputs it to the pins. Each time power cycles, it should show something different.

    The big PRNG is seeded from ADC noise by the ROM booter.
  • Yep. I goofed badly there. Corrected to GIps. G instructions/s
    My Prop boards: P8XBlade2, RamBlade, CpuBlade, TriBlade
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  • Did the fuses, (ie: code protection etc...) make it in in the end? I don't see any reference to them in the ROM source code (yet). Even if they did they're probably pretty far down the list of things to test as they're 'destructive' (or at least one-way) tests.

    In other news, we should probably remember to document not to rely on the PRNG seeding for anything specifically security related. It occurred to me today that just shorting rx_pin to ground while powering the chip on may make the seed predictable.
  • __red__ wrote: »
    Did the fuses, (ie: code protection etc...) make it in in the end? I don't see any reference to them in the ROM source code (yet). Even if they did they're probably pretty far down the list of things to test as they're 'destructive' (or at least one-way) tests.
    Fuses did not make the cut, as their yield was too low.

  • __red__ wrote: »
    ...In other news, we should probably remember to document not to rely on the PRNG seeding for anything specifically security related. It occurred to me today that just shorting rx_pin to ground while powering the chip on may make the seed predictable.

    We use an internal GIO calibration mode which doesn't involve the external pin. We actually measure GND, but use the noise from many measurements to seed the PRNG. Doing an ADC conversion on GND doesn't return 0, but about 1/8, averaged over many bits. We use those bits to seed the PRNG. Not only does their duty vary slightly at all times, but the positions of 1's and 0's is ever-changing.
  • regarding P2 analog, over in the prop2-fpga-files thread, jmg was saying
    -- it's not just Vio that needs to be clean, any ground noise, or ground movement, is going to 50% impose on the Analog reference too, via that CMOS gate threshold effect.
    There is no separate Analog Gnd, so even a few millivolts movement in GND will cause crosstalk.
    10 bits at 3v3 needs < 3.3mV movements.

    On the shared ground - the pads get their own ground from the inside of the exposed pad (well, the exposed ring in the current mockup), rather than getting Gnd from some path across the die. So the 'shared ground' portion is across the exposed pad and solder, after that you're on ground planes where you can tap a reference ground potential. Given typical copper ground planes are a fraction of a milliohm per square, I would think that path would all be sub-milliohm total, and if we're talking about an amp of current, you're only going to 'move' less than a millivolt.

    But if you do want something 'less coupled' for reference purposes, you could expose the pad analog ground by outputting low to a neighboring pin. The N fet seems to be around 16 ohms, and of course you can buffer that if you want an even lower impedance reference ground, for a driven shield or plane.
  • Cluso,
    Boy do I wish we were rating in Tflops :D That would be comparable to GPUs for floating point operations!
  • Roy Eltham wrote: »
    Cluso,
    Boy do I wish we were rating in Tflops :D That would be comparable to GPUs for floating point operations!

    Yeah, me too. All I can say it was after midnight and I must have been caffeine deprived :(
    My Prop boards: P8XBlade2, RamBlade, CpuBlade, TriBlade
    Prop OS (also see Sphinx, PropDos, PropCmd, Spinix)
    Website: www.clusos.com
    Prop Tools (Index) , Emulators (Index) , ZiCog (Z80)
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