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marcan42's profile
Hector Martin
Hector Martin
Hector Martin
@marcan42

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Hector Martin

@marcan42

If it ain't broke, I'll fix it! I'm porting Linux to Apple Silicon Macs at @AsahiLinux. http://patreon.com/marcan  | http://github.com/sponsors/marcan 

Tokyo, Japan
marcan.st
Joined May 2009

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    1. whitequark‏ @whitequark 24 Jun 2019
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      people see "D+" and "D-" in the USB 1.1/2.0 pinout and assume it's differential whereas it's... not. it's actually a bus based on two single ended, DC-unbalanced lines that carry the inverse of each other most of the time. it only really helps with EMI and nothing more

      12 replies 20 retweets 151 likes
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    2. Kevin Hubbard‏ @bml_khubbard 24 Jun 2019
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      Replying to @whitequark

      USB 2.0 is 480 Mbps though. Surely ( don't call me Shirley ) they are true differential for USB 2.0 transfers.

      1 reply 0 retweets 1 like
    3. whitequark‏ @whitequark 24 Jun 2019
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      Replying to @bml_khubbard

      they are absolutely not.

      1 reply 0 retweets 13 likes
    4. Kevin Hubbard‏ @bml_khubbard 24 Jun 2019
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      Replying to @whitequark

      I'm perplexed how it works then. Non differential seems to poop out in the 100-200 Mbps rates in my experience without power hungry termination.

      1 reply 0 retweets 3 likes
    5. whitequark‏ @whitequark 24 Jun 2019
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      Replying to @bml_khubbard

      it's pseudo-differential: it looks an awful lot like differential most of the time (though the signals are specified relative to ground anyway), but instead of a comma with a proper line code, they use a condition of both D+ and D- being low to delimit packets

      1 reply 1 retweet 14 likes
    6. whitequark‏ @whitequark 24 Jun 2019
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      Replying to @whitequark @bml_khubbard

      that means you can't actually use a comparator to receive USB. you basically have to sample D+ and D- separately referenced to a voltage you generate locally, and then detect D+>D-, D+<D- and D+=D-.

      2 replies 1 retweet 22 likes
    7. Richard Maxwell‏ @JodiTheTigger 24 Jun 2019
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      Replying to @whitequark @bml_khubbard

      Oh ... my ... god.

      1 reply 0 retweets 8 likes
    8. whitequark‏ @whitequark 24 Jun 2019
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      Replying to @JodiTheTigger @bml_khubbard

      > I'm perplexed how it works then. this is generally what happens when you open the USB spec and read it. also, it doesn't work very well.

      1 reply 5 retweets 38 likes
    9. Hector Martin‏ @marcan42 24 Jun 2019
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      Replying to @whitequark @JodiTheTigger @bml_khubbard

      AFAIK you do use a differential receiver for HS data, it's just that you *also* need single ended comparators to detect EOP. Also this design is almost the worst possible design for audio devices. USB audio whine is now a plague, even in professional productions.

      1 reply 1 retweet 20 likes
    10. whitequark‏ @whitequark 24 Jun 2019
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      Replying to @marcan42 @JodiTheTigger @bml_khubbard

      if I remember correctly, the data lines are referenced to ground, so while you certainly -could- use a differential comparator, it doesn't give you a lot of advantage

      1 reply 0 retweets 4 likes
      Hector Martin‏ @marcan42 24 Jun 2019
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      Replying to @whitequark @JodiTheTigger @bml_khubbard

      The termination is through GND (half the differential impedance on each leg), but you still need to use a differential receiver. There's no way you can get the required noise immunity without one.

      10:34 PM - 24 Jun 2019
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      2 replies 1 retweet 17 likes
        1. Hector Martin‏ @marcan42 24 Jun 2019
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          Replying to @marcan42 @whitequark and

          They *do* use a comma (bit stuff violation) for HS EOP. But they still require a HS squelch circuit (an *absolute* differential threshold receiver that measures when the swing is too low) to detect invalid/bus idle state for other reasons.pic.twitter.com/ikUM18s5YY

          0 replies 0 retweets 13 likes
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        2. Kyle‏ @qfp64 25 Jun 2019
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          Replying to @marcan42 @whitequark and

          So it is differential after all?

          1 reply 0 retweets 0 likes
        3. Hector Martin‏ @marcan42 25 Jun 2019
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          Replying to @qfp64 @whitequark and

          It's shitty cursed differential.

          0 replies 0 retweets 9 likes
        4. End of conversation

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