I'm building a device using

  • TUSB2046B hub controller
  • 4x FT230X USB to UART controller

First, let's look on Hub controller sample application:

TUSB2046B Application Note

Note D:

All USB DP, DM signal pairs require series resistors of approximately 27 Ohm to ensure proper termination. An optional filter capacitor of about 22 pF is recommended for EMI suppression. This capacitor, if used, must be placed between the hub terminal and the series resistor, as per section 7.1.6 of the USB specification.

Then let's look on FT230X sample application:

FT230X Application Note

My questions:

  • What is the purpose of 15kOhom resistors to ground on first image? Are they necessary in single pcb layout?
  • How should I connect those two devices assuming single pcb layout with no more then 5cm distance?

This is the most intuitive idea:


simulate this circuit – Schematic created using CircuitLab

And some questions:

  • Can I safely remove one 27 Ohm resistor on each line? Must I change the remaining one to 54 Ohm?
  • Is there any difference whether capacitors are 22pF or 47pF?
  • Are those capacitors necessary?

1 Answer 1

  1. The 15kOhm pull-down resistors are most definitely required. The slave device has internal pull-up resistors on one or both of the DM/DP lines which are used by the host to detect that a slave has been attached and its type - low, full or high speed. You need these pull-down resistors to ensure the line isn't left floating or this detection won't work properly.
  2. You can omit the termination resistors and capacitors. They have two purposes: the first is to prevent reflections from the end of the lines which would cause fading interference with the signal. However, since you are only running the bus in full speed mode (12Mbps) and the line length (5cm) is much much shorter than the wavelength of the signal and its main harmonics, reflections aren't going to be an issue. The second purpose is as a line filter to reduce the EMI generated by the USB cable. You might need some sort of filter to pass EMC tests, but you should just treat the bus lines as you would any other digital lines you have on your board. Note that most USB drivers are slew rate limited so you are already much better off EMC wise than a GPIO line driving your LCD or whatever.
  • \$\begingroup\$ Ad 1. Why there is no pull-down resistor on hub-to-pc minus line? There is only pull-up on plus (for speed negotiation). \$\endgroup\$
    – peku33
    Commented Sep 3, 2015 at 23:36
  • 2
    \$\begingroup\$ @peku33. USB is not a symmetric bus - the role of hosts and devices is different. A hub is a device to the up-stream PC, and a host to the down-stream devices connected to it. So the pull-up on the hub-pc connection is to tell the pc (host) that the hub is a full speed device. That probably sounds a bit confusing. Have a read of the first couple of chapter of USB Complete (Jan Axelson) and it should help you a lot. \$\endgroup\$
    – Jon
    Commented Sep 4, 2015 at 8:15
  • \$\begingroup\$ Ok, I understand that part about pullup for speed negotiation, but I still don't understand the role of 15k pull-down. Without pulldown on hub upstream, minus line is left floating. \$\endgroup\$
    – peku33
    Commented Sep 4, 2015 at 10:35
  • \$\begingroup\$ The up-stream 15k pull-downs are inside the pc. \$\endgroup\$
    – Jon
    Commented Sep 4, 2015 at 10:52
  • \$\begingroup\$ So what happens when the data cable is disconnected, but hub is powered up? \$\endgroup\$
    – peku33
    Commented Sep 4, 2015 at 13:34

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.