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I am prototyping a simple board with several (3 or 2) USB-to-Serial convertors (CP2102), a microcontroller (ATMega32U4), and one USB "output" port to connect using USB cable (in case of only 2 CP2102's). I want to use TI's TUSB2046B USB Hub controller to connect all this devices through a single microUSB input. I don't want to use an external hub to save space and to simplify the final assembly.

The question is about interconnection between USB Hub and on-board devices. The official TI's datasheet is proposed to connect several external components on the each output data lines:

  • Series resistor (27 Ohm).
  • Capacitor between the TUSB2046B pin and the series resistor (22 pF).
  • Pull-down resistor (15 kOhm) after the series resistor
  • and a USB ESD protection unit SN75240…

Also datasheet proposes to use a

  • Power Management unit (TPS2044).

And also, on the ATMega32U4 side, there should be a

  • 22 Ohm series resistor on each data line.

So, what components should I use to connection on-board devices using like a 10 centimetre length data lines?

Of course, I don't need and ESD protection module, but I'm not sure about other components.

There is a particular question about a power management unit. As I understand it tells the USB hub that something is connected to a certain port, and it does not draws too much current. So, I do not need it for on-board devices? Only for my output port, if any?

Thank you!

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  • \$\begingroup\$ Good question. Essentially, is there any difference between usb board to wire usb design, and onboard design. \$\endgroup\$ – Passerby Jul 21 '13 at 17:11
  • \$\begingroup\$ As I understand, many of this components are intended to cancel noise (low-pass filtering) and to handle transient processes after the cable is plugged in, so, some of them possibly could be removed. \$\endgroup\$ – dbolotin Jul 21 '13 at 17:34
  • \$\begingroup\$ "There is a particular question about a power management unit. As I understand it tells the USB hub that something is connected to a certain port, and it does not draws too much current. So, I do not need it for on-board devices? Only for my output port, if any?" what actually you require i didn't understand \$\endgroup\$ – user19579 Jul 25 '13 at 1:23
  • \$\begingroup\$ Sorry. I meant that I have 3 onboard devices and 1 USB socket to connect an external upstream device through it, this is the "output" port. \$\endgroup\$ – dbolotin Jul 25 '13 at 8:12
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I've more deeply studied the field, and found that 15 kOhm pull-down resistors are required to hold the data line at 0 V (D- in the full-speed mode or D+ in the low-speed mode). In my particular case, all devices are full-speed, so I will try to use them on the D- lines. I still not figured the role of the series resistors, but there are built-in resistors in CP2102 and I will add 22 Ohm resistors to the lines to my uC, just in case. CP2102 and ATMega32U4 are both have pull-up resistors on their D+ "inputs", it tells the downstream device (USB Hub in my case) that it is a full-speed device.

So, I will try to use only one pull-down resistor and will post here results, but, by now, my advice for the on-board usb interconnection is the following:

  • carefully read the documentation on the upstream device internal USB transceiver circuit. What components it already have?
  • if some external resistors (series and pull-up) are required by the upstream device, use them
  • use all resistors (series and pull-down) on the down-stream termination
  • capacitors and ESD protection are not requiered
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Treat onboard USB as a simple differential line. You may add series resistors, but a simple differential connection without any fancy components should work without any problems.

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