Compound USB device design

Greetings, 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 regarding:

- 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

regards Terry

Reply to
Terry Dawson
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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 language.

Digikey have many usb hub chips:

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as do Mouser:
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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?

Dave.

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Reply to
David L. Jones

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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.

correct.

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 straight-forward.

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.

thanks!

Terry

Reply to
Terry Dawson

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

Farnell have some too:

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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.

Dave.

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Reply to
David L. Jones

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 :)

Terry

Reply to
Terry Dawson

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.

Dave.

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Reply to
David L. Jones

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.

Terry

Reply to
Terry Dawson

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.

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Reply to
Bob Larter

important.

one

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.

MrT.

Reply to
Mr.T

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

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.

Terry

Reply to
Terry Dawson

cool. thanks! That's reassuring.

Terry

Reply to
Terry Dawson

USB

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

good

my

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

MrT.

Reply to
Mr.T

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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Reply to
Bob Larter

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