rokafeller
Posts: 2
Joined: Sat Apr 01, 2017 7:40 am

Soldered USB dongle won't work without cable connected

Sat Apr 01, 2017 7:57 am

Hi,
I'm facing a weird issue on Pi Zero when trying to leverage a USB wifi dongle that I directly soldered to the zero pins, as described here here.
What happens is:
  • If I leave the micro-usb port empty, with nothing connected to it, the wifi dongle just won't come online.
    Image
  • If I connect a simple extension cable to the usb port, the dongle starts working perfectly. This is reversible, ie. as soon as I disconnect the extension cable, the dongle dies and the only way to have it back online is to reconnect the extension cable.
    Image
What am I missing here? I checked the extension cable and it's completely transparent. This cable trick works with 15cm cables, if I connect a longer cable the dongle stops working. If I cascade a second cable to the 15cm one, the dongle again will stop working. To me it looks like there is some sensitivity to capacitive/inductive load somewhere but still all of this sounds really weird.

any suggestions?

thanks
G.

joeygbsn
Posts: 14
Joined: Thu Sep 15, 2016 3:24 am

Re: Soldered USB dongle won't work without cable connected

Sat Apr 01, 2017 2:48 pm

Hey not entirely sure if this will fix your problem but the micro usb actually has 5 pins rather than the usual 4 for reg usb. The fifth pin is called the id pin and ive seen some people have success by grounding this pin which is what im pretty sure your otg adapter is doing. More info can be found on this pi gameboy forum http://www.sudomod.com/forum/viewtopic. ... 2ad634242e.

ait
Posts: 127
Joined: Sun Oct 30, 2011 6:51 pm

Re: Soldered USB dongle won't work without cable connected

Sat Apr 01, 2017 4:07 pm

Hello,
i would try to separate the power-cables from the data-cables and twist the data-cables.

sparkyhall
Posts: 128
Joined: Mon Aug 27, 2012 9:14 am

Re: Soldered USB dongle won't work without cable connected

Sat Apr 01, 2017 4:37 pm

Actually it's not at all weird. There is a lot more to connecting high speed data lines than just connecting them with a piece of copper wire, look up transmission line theory. The connecting cable has to have the correct characteristic impedance otherwise you get signal reflections due to impedance mismatch, this will interfere with the real signal, sometimes you can get away with it, other times not.

To get this working I would try make the data lines as short as possible (the 5V and 0V should not be as sensitive). If that doesn't work then it may be worth trying additional de-coupling (10nF capacitor) connected between the 5V and 0V on the USB dongle, again the capacitor leads must be kept as short as possible.

rokafeller
Posts: 2
Joined: Sat Apr 01, 2017 7:40 am

Re: Soldered USB dongle won't work without cable connected

Sat Apr 01, 2017 5:38 pm

Of course there is transmission line theories, but considering USB 2.0 @ 480MBit/s with NRZI coding, ie. the signal is a square wave, and considering the 3th harmonic (=a decent square wave with differential signalling) we have a signal with a wavelength around 9cm. My wiring is around 2cm, < wavelenght/4, so we can generally neglect propagation effects.

There is an interesting point: I had the zero configured to work also as ethernet gadget for failsafe scenarios.
By disabling it (commenting out dtoverlay=dwc2 from /boot/config.txt) it started working with no hardware modifications of any kind.

If you want to keep ethernet gadget feature, what suggested by joeygbsn does the trick: by shorting pin 4 and 5
Image
the wifi usb dongle works perfectly together with ethernet gadget feature enabled.

Anyways I also redid the wirings to make them a bit more structured
Image

thanks everyone, hopefully this will be useful for someone else :)

cheers
G.

MrEngman
Posts: 3838
Joined: Fri Feb 03, 2012 2:17 pm
Location: Southampton, UK

Re: Soldered USB dongle won't work without cable connected

Sat Apr 01, 2017 5:42 pm

joeygbsn wrote:Hey not entirely sure if this will fix your problem but the micro usb actually has 5 pins rather than the usual 4 for reg usb. The fifth pin is called the id pin and ive seen some people have success by grounding this pin which is what im pretty sure your otg adapter is doing. More info can be found on this pi gameboy forum http://www.sudomod.com/forum/viewtopic. ... 2ad634242e.
+1

This is your problem. Without the id pin connected to ground the Pi zero USB operates in client mode. You need to connect it to gnd to get it to operate in host mode when it will then be able to access your wifi dongle.



MrEngman
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