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Compound USB device design

Discussion in 'Hobby Electronics' started by Terry Dawson, Oct 21, 2009.

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  1. Terry Dawson

    Terry Dawson Guest

    I'm designing a compound USB peripheral that will combine a USB sound
    card with two serial ports. My design objectives are in order:

    - low component count
    - small physical size
    - low power consumption
    - ideally it'd be powered from the upstream USB 2.0 interface.

    Ideally there'd be minimal programming required although it would be
    nice to have the device readily and uniquely identifiable when they're
    plugged in.

    At this stage I'm thinking of basing the design on:

    - Future Technology FT2232H dual USB UART, and a
    - Burr Brown/TI PCM2901 Audio Codec with USB interface

    with an

    - Alcor Micro AU9254

    to bind them together.

    I've had a bit of difficulty finding candidate USB hub ICs, I'm guessing
    I'm just not using the right search terms or something. I ended up with
    a couple, the Alcor seemed ok.

    I've no previous USB design experience and I'd appreciate your input

    - any potential traps or pit-falls to watch out for
    - suggestions for other USB controller ICs to use
    - any reference designs that I might plunder

  2. The drivers for the individual audio and UART chips would handle that.
    They just get treated as two seperate USB devices.
    Audio/UART device selection would in this case be based on the ease of use
    and availability of suitable drivers and example apps in your chosen
    Digikey have many usb hub chips: hub
    as do Mouser:|0&FS=True

    Not many in nice easy SO packages though, but if you want small go for QFN.

    You only need USB 1.1 for your intended devices.
    The datasheets for the chips usually have all you need hardware wise in
    their example apps.

    If this for a potential commercial product?

  3. Terry Dawson

    Terry Dawson Guest

    Thanks for those, I'll take a close look at them. I've not done business
    with either digikey or mouser before.
    Size isn't my number 1 priority, so I'm prepared to compromise.
    The datasheets have lots of good information, but many of them refer to
    "standard USB design principles" without providing a reference to them.

    Maybe I'm imagining things are there that aren't. It all seems fairly
    Did you mean "Is this" ? No, strictly hobby/personal.

    If I was wildly optimistic I could imagine enough other people being
    interested that I might consider making a batch and selling them on a
    cost-recovery basis but I'm intending to give the completed design away
    (Open Hardware). I'm basing other aspects of the design on other peoples
    work so I can't claim any sort of real ownership in any case.


  4. Delivery is the only killer for low value orders, Mouser are a fixed US$30.

    Farnell have some too:

    Some are stocked in Oz, and remember that Farnell now have free delivery and
    no minimum order, so likely a much better option than Digikey or Mouser for
    small orders.
    They are probably just referring to generic controlled impedance lines and
    power supply current limitations etc.
    It should be.
    USB hubs are complete drop-in solutions, as are the UART/Audio chips. Your
    only hurdle should be suitable drivers.
    Open Hardware is cool.
    Hope it goes well.

  5. Terry Dawson

    Terry Dawson Guest

    I've already availed myself of their free delivery, it's certainly got
    me dealing with them more seriously than I ever have before.
    ok, that could be right.
    I'm hoping drivers won't be too much of an issue either, as the vendors
    seem to supply them for their chipsets. I'll be using Linux myself but
    any broader audience might not, so I'll need to give that some thought.

    Do I need to worry about anything in PCB layout? I've seen some mention
    of avoiding running signalling traces over power planes in some
    datasheets for example. I'm assuming this is generally good practice but
    it leaves doubt in my mind that I'm missing something.
    thanks :)

  6. There would only be two things you need to be wary of.
    First is the controlled impedance lines for the USB. Plenty of info on this
    is available, Google will no doubt give countless references to what's
    required. If you keep your lines short enough the actual impedance ain't
    going to matter too much, so no need for proper controlled impedance PCB
    manufacture, do some basic calcs to get close enough to the required
    impedance and that will do. It's important to keep the USB trace lengths
    matched. But you are only running USB 1.1, so no biggie, the preverbial
    coathanger would probably work.
    You don't run controlled impedance traces over *splits* in your ground
    plane, that's a big generic no-no.

    Second would be your audio circuit. I don't kow what performance you are
    after, but keeping your analog and digital grounds seperate is important.
    They should meet only at the audio chip. Keep all your digital stuff on one
    half of the board ,analog on the other, that kind of thing.

  7. Terry Dawson

    Terry Dawson Guest

    That's precisely the sort of stuff I'm after, I'd not have given thought
    to either of these.

    Thank you for the tips.

  8. Bob Larter

    Bob Larter Guest

    For USB 1.1? - I wouldn't worry about it. Just use the usual commonsense
    rules for MHz signals (lots of decoupling caps, fast signal traces as
    straight as possible, etc) & you should be fine.
  9. Mr.T

    Mr.T Guest

    My opinion is why would you want to build such a device when separate USB
    audio devices and USB-Serial adapters are cheap?
    (and small, so size is not a real issue)
    Secondly, you will not match the audio performance of the higher quality
    audio devices given what you say above. Even the cheap ones are pretty good
    these days!
    Thirdly I wouldn't even consider USB 1.1 if you want to keep the audio
    latency at all low, especially considering the minimal savings.
    (if you don't know what that is, it probably won't affect you)

    Of course if you are just looking for an interesting project to keep you
    busy while learning something, then go for it. Personally I don't waste my
    time when cheap off the shelf solutions are already available.

  10. Terry Dawson

    Terry Dawson Guest

    heh, my initial response is "hobbies don't have to be economically

    In truth I have a small pile of $6 USB hubs and $5 USB sound cards
    sitting on the desk in front of me. I already have the thing
    "prototyped" by cobbling together off-the-shelf units. But it's not
    robust and it isn't small. USB connectors are bulky. I forgot to mention
    robust in my list of requirements, anyway, it is one :)

    I was considering at one point just ripping the guts out of these,
    desoldering the connectors, remounting the pcbs and hardwiring them
    together. That's my plan B if I run out of enthusiasm for doing it myself.
    There is no requirement for especially high-fidelity.
    Low latency isn't a requirement either.
    This isn't a commercial exercise, I'm just indulging in a hobby. In the
    absence of an original project idea I'm opting for an original
    implementation instead.

    Having fun and learning stuff is the ultimate objective.

  11. Terry Dawson

    Terry Dawson Guest

    cool. thanks! That's reassuring.

  12. Mr.T

    Mr.T Guest

    That would probably be my plan A. I'm betting you do run out of enthusiasm

    Fair enough, if you really couldn't find a more worthwhile project to waste
    your time on I guess :)

  13. Bob Larter

    Bob Larter Guest

    No worries. Glad I could help. Also, as someone else said: keep your
    analog power & ground lines as far apart as possible.
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