Disclaimer, I'm a SW guy, so please don't take anything for granted, and I would appreciate explanations in layman's terms :)

We have a custom design based on Freescale's i.mx233, and as a reference, we used a board from Olimex called Olinuxino. On the Olinuxino, they have a dual functioning LAN-USB chip that is unnecessary for our design (we only need 1 USB port for a WiFi dongle), so we figured we can just remove it, and connect a USB device directly to the processors D+ and D- lines.

Because of a separate layout mistake, the USB connector we selected cannot be used in this HW revision. I thought I could verify the design by just cutting a USB extension cable, and soldering it in connectors place, so currently it looks like this:


Now when I connect the WiFi module to the connector, it seems to start enumerating properly, and looking at the signaling, it looks decent as well (I compared to the reference board, and it look very much the same):

photo 2

However, on the software side, the driver for WiFi is complaining that it is misreading the chipset ID, and refuses to load:

[    2.390000] ieee80211 phy0: rt2x00usb_vendor_request: Error - Vendor Request 0x07 failed for offset 0x1000 with error -       
[    2.400000] ieee80211 phy0: rt2800_probe_rt: Error - Invalid RT chipset 0xc37b, rev 5108 detected
[    2.410000] ieee80211 phy0: rt2x00lib_probe_dev: Error - Failed to allocate device

The schematic part for the (original) USB port:

wlan schematic

And the actual layout for that part is simple as well (In the picture only top layer, I have hidden all other layers for clarity. I drew the D- signal trace from the bottom layer by hand, please note that in the real picture above the is rotated 180degrees):

wlan traces from AP

The exact same software package and same dongle on our reference board works, and the driver loads.. I started reading the USB2.0 Specification, and in chapter 7, they talk about the electrical signaling, and this part caught my eye:

High-speed operation supports signaling at 480 Mb/s. To achieve reliable signaling at this rate, the cable is terminated at each end with a resistance from each wire to ground. The value of this resistance (on each wire) is nominally set to 1/2 the specified differential impedance of the cable, or 45 Ω. This presents a differential termination of 90 Ω.

Have I screwed up the signaling now with my USB port ? What does this differential impedance mean ? Also, would I have had exact same problem also with the original port since we were planning just to connect the 5V, GND, D+ and D- lines to the connector? Or is this resistor supposed to be inside each end of the connection inside the silicon ? What could be wrong here ?

Thanks for any advice!

Edit: Added layout and schematic picture for more clarity

Edit 2: I have thrown away my clipped extension cable, and bent the original USB connector and I was able to place it on the board in an bit funny angle.

I also added the necessary drivers to the kernel for USB memories, and I have now verified that my 16GB HighSpeed USB memory works. During boot it is detected:

[    2.240000] usb 1-1: new high-speed USB device number 2 using ci_hdrc
[    2.430000] usb-storage 1-1:1.0: USB Mass Storage device detected
[    2.440000] scsi0 : usb-storage 1-1:1.0
[    3.460000] scsi 0:0:0:0: Direct-Access     UFD 2.0  Silicon-Power16G PMAP PQ: 0 ANSI: 4
[    3.500000] sd 0:0:0:0: [sda] 30283008 512-byte logical blocks: (15.5 GB/14.4 GiB)
[    3.520000] sd 0:0:0:0: [sda] Write Protect is off

The flash on the board is very small, but on the USB drive I can copy a 1GB file to another directory, and the speed is around 360KB/sec. The MD5 sum matches, and I dont see any problems using the memory stick.

So, I guess this means that something is now just going wrong with only the WiFi dongle.. it's strange.. If there was corruption on the DP and DM lines, wouldn't I also see corruption during the data transfers ?

Final edit While we haven't been able to pinpoint the problem with 100% certainty, when forcing the controller through registry to only work in 1.1 mode, all USB devices work without any issues, including the WLAN module. In our next layout we have followed strict differential routing rules, and hopefully this will be the end of our problems :)

  • \$\begingroup\$ I assume the required USB termination resistors are internal to Freescale iMX233 IC, because Olimex iMX233-OLinuXino-MICRO schematic shows direct connection from MX233 to USB connector. If you can find the device data sheet, they should have application test circuit to confirm. \$\endgroup\$
    – MarkU
    Commented May 20, 2014 at 6:15
  • \$\begingroup\$ On your prototype (cable pigtail connection), is it possible the D+ and D- got swapped due to layout error? Looks like your cable has Red=VBUS, White=D-, Green=D+, Black-GND, assuming the PCB layout pads are in the right order. Do other devices enumerate? \$\endgroup\$
    – MarkU
    Commented May 20, 2014 at 6:16
  • \$\begingroup\$ Actually, I tried that as well. But then the kernel gives me plenty of read errors (-71). I am quite confident that the datalines are ok now. I have tried Logitech keyboard, and it seems to be recognized by the kernel (though, I can't type on it - this might be a driver issue, I haven't verified the keyboard on olimex). The device should enumerate ok since it's starting to load the right driver ? Or how does the enumeration process exactly go.. \$\endgroup\$
    – julumme
    Commented May 20, 2014 at 7:05
  • \$\begingroup\$ @MarkU I added layout and schematic pictures to the OP, I hope this clarifies more \$\endgroup\$
    – julumme
    Commented May 20, 2014 at 14:58
  • 2
    \$\begingroup\$ On your layout, D+ and D- are different trace lengths and different inductance (D- goes through two vias, D+ has no vias.) This could affect USB signal integrity. I normally instruct CAD layout designer to route D+ and D- next to each other, equal path length, constant spacing. Similar to design rules for microstrip routing. Can you move the USB-A connector to opposite side of board, so D+ and D- can route directly, with 1 via each? \$\endgroup\$
    – MarkU
    Commented May 21, 2014 at 1:20

3 Answers 3


It's a hard question to answer as this is more of a debugging thing. But I can contribute some ideas as to what you could try.

The first thing I thought was that your pigtail connections (the 1-2cm loose wires coming out of the cable and onto your board) might be too long for 480 MBps operation. Ideas to try:

  • Force your host to run as USB 1.1 (1.5 or 12 MBps). Maybe you can run it through a USB powered hub or something to force it to the lower speed.
  • Use a real USB connector, glue it upside down or something, and make really tight short connections from connector pins to the PCB.
  • Maybe you can find a way to cut the cable pigtails length to 1/5 or something in that order.
  • Make sure you are not testing with the oscilloscope probe attached.
  • Maybe try really short and really long USB cable.
  • Maybe try changing temperature (warmer makes the edges slower and colder makes the edges faster).

A note to some comments here: Don't worry about number of vias and the length matching of those PCB traces. Done right, vias and a small bit of length mismatch have never been a problem for USB 2.0. And this is insignificant compared to what you do with the cable pigtails.

Some typical general errors to check for include:

  • Clocking. Verify clock frequency and jitter is within spec - including any PLL's.
  • Power. Verify (with 1-2 GHz BW oscilloscope) that your Vcc ripple is within spec etc.

Also I would not rule out software just yet. Look for differences - like in the config data etc.

And don't be too proud to ask an experienced hardware guy for help :-)

Update - Note on measuring Vcc ripple:

Taken from my answer to this question: How do I verify that my 3.3v power rail meets the requirements for an Ember EM357 SoC?

The best paper I know of that describes how to do this measurement is this one: http://www.electrical-integrity.com/Quietpower_files/Quietpower-21.pdf

In short: Use a coax cable soldered directly to your board. Run the 50R coax into your oscilloscope set to 50R input impedance. Select AC-coupling. A bandwidth that is adequate (minimum 500 MHz). And infinite persistence.

If you make the measurement using a high impedance probe with a long "pig-tail" for ground - you may have extra noise not related to your Vcc noise picked up. When in doubt, always do the null-experiement: touch the probe tip to the ground point, so both tip and ground of the probe touches the same point on the board. If you don't get a flat line, something is being picked up by inductive coupling into the loop formed by probe and ground lead.

So do you have too much noise? Suppose the datasheet of this device calls for 3.3V +/-5% for the Vcc supply. That means you have +/-165mV as the limit. Let's assume you have a 2% accuracy of your 3.3V DC regulator. And let's assume you have a 0-1% distribution drop in the connections between the regulator and the device (cables, connectors, traces, filters etc.). That leaves 2% to the AC-noise/ripple or +/-66mV (132mVpp).

  • \$\begingroup\$ Hi Rolf, thanks for your advice. I have indeed now bent a real USB connector, and added another edit explaining that USB memory now works. Can you give me advice how I can measure the Vcc ripple ? We are currently powering the board with a regular cellphone charger, and usb also receives 5V directly (but the voltage never drops under 4.9V when I checked with scope) \$\endgroup\$
    – julumme
    Commented May 29, 2014 at 7:07
  • \$\begingroup\$ Great that you could use my advice. I added a note about how to do Vcc ripple measurement. \$\endgroup\$ Commented May 30, 2014 at 13:01

You could try tacking the USB cable shield to the USB ground and see if that improves the situation. High speed USB is touchy about things like cable impedance.

  • \$\begingroup\$ thanks for the suggestion. I tried this, but unfortunately there doesnt seem to be any effect on the signaling.. \$\endgroup\$
    – julumme
    Commented May 20, 2014 at 14:56

Hi Regarding your question "What does this differential impedance mean ?" The differential impedance of the line is actually the resistance of the PCB trace up to the connector, since USB is differential it will be a differential impedance. So a single track impedance of for example D+ will be 45 Ohms and the combined impedance of D+ & D- will be 90Ohm, now just real quick you can't measure this with a simple digital voltmeter (DMV) so you probably won't be able to check this unless you have a TDR somewhere near you. Regarding your issue, it is not likely to be caused by an impedance mismatch a first sight look at you signal it doesn't seem to be very bad.

If you have questions about electrical testing and problems you can always take a look at http://www.testusb.com/ this is a website me and some of my colleagues wrote.

Could you maybe share a screenshot of your schematic ? So we can take a deeper look here.

  • \$\begingroup\$ Thanks for your explanation. I have also added the pictures to the original posting, I hope they could help you \$\endgroup\$
    – julumme
    Commented May 20, 2014 at 14:57
  • \$\begingroup\$ Could it be that you switched the D+ & D- Lines ? Looking at the scoop image I see that your signal is inverted to the other one and you say that your connector is turned 180°. D- is green and D+ is white now looking at the connector diagram the D- should be near VCC and D+ near GND looking at you board connection this is the opposite. Could be that I'm mistaking something here but it's worth double checking ! \$\endgroup\$
    – Jan Claes
    Commented May 21, 2014 at 7:54
  • \$\begingroup\$ Jan, you must have been right with this. I have now removed the hacked extension cable, and bent a real connector to a funny angle, but now USB memory works in HighSpeed. I still have same problem with WiFi dongle, though.. \$\endgroup\$
    – julumme
    Commented May 29, 2014 at 7:09

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