Hello,

I have an usb wifi nic, that I want to be able to power by battery, so I got a battery usb power supply. I didn't have a Y cable around so I have a stack of usb cables and deviced to make a "Y" cable, something like this:

N810 ------------------------ Wifi Nic
|
----------------- USB Power supply


Currently, I have the white and green data cables between the netpad and nic straight through.

I have the ground from all three connected together, and the red 5v line from the USB power supply connected to the red 5v line to the nic.

The final red 5v from the netpad is not connected to anything.

With this configuration, the Nokia recognises the wifi card, but I get "rejected 1 configuration due to insufficient available bus power"

I have confirmed the 5v is present from the USB power supply. It appears that the usb host on the netpad doesn't recognise it is getting power from elsewhere.

I have guessed at how to wire the voltage together. Is there something else I need to do to get this working?

Hmm, the formatting didn't work, the line from the USB power supply should intersect the line between the n810 and the wifi card at the middle.

I don't think it's a great idea to connect the 5V of the tablet to the external 5V.
my DIY Y cable goes this way:

(X : not connected, --- : connected)

Male USB A (USB power pack) to Female USB A (device):
5V ------------- 5V
Green -X X X- Green
White --X X X- White
Gnd ------------ Gnd


Male Mini/Micro USB (tablet) to the same Female USB A (device):
5V -X X X X X- 5V
Green --------- Green
White ---------- White
Gnd ------------ Gnd

so on the Female cable you should only have Gnd connected to both cables. Data connected to the NIT, and 5V connected to the power pack.

The downside is that you can use this cable only with a power pack, it can't draw power from the tablet.
If you know what you're doing, you can put a low voltage-drop diode (Schottky?) on the 5V/5V cable connected to the tablet, but you still lose some hundreds of milliwatts. I don't know what is the exact output voltage of the USB host port on the tablets, but if it's too low it can make your USB devices not work.


I did this mod, and mine works.

[Edit] is your NIC powered by something else than USB though? if you don't connect the power pack's 5V to the NIC, it may not be supposed to work.

Your y cable is the same as mine. Thanks for confirming.

What are you plugging into it? The nokia rejects the wifi usb because of an absence of power, but I don't think that is real...

The problem with your setup is that you're trying to cheat the hardware; USB, and more specifically the Kernel support for USB on the tablets, are designed with power negotiation protocols and handshakes that have to happen in order for a device to be "powered up". Essentially the way it works is:
1) When in host mode, the tablet keeps a very small amount of power available on the USB port.
2) You plug in a USB device. The device uses that small amount of power to send a message to the tablet saying "I need X amount of power to work properly".
3) The tablet (kernel) receives this information and decides based on the hardware information whether that much power is available to put out the USB port.
4) If the kernel decides it's okay, then it turns up the USB port, subtracts the amount of power it's just allocated to the device from the total power available, and goes back to 1)
5) On the other hand, if the kernel says "nope, not enough power available" then it refuses to activate the device (to prevent USB over-draw and power problems) and goes back to 1).

It's a bit more complicated than that in reality, but that's the concept. The problem with your homemade Y cables are that they don't take this power negotiation into mind; that is, although technically you've provided enough power straight to the device itself, when the tablet tries to initialize it the device still asks for it's power level and the tablet still thinks (rightfully so) that it doesn't have enough power to activate the USB device on it's own.
Anyone who's had success with this type of Y cable is using it with either 1) a device that doesn't really need an external power source as teh tablet would be capable of powering it on it's own, but the power source is there for other reasons (longer battery life,e tc), 2) a device that does not report it's real power consumption level during negotiations 3) has a custom hacked kernel with USB power management disabled.

The proper way to do this is to use an externally powered (or battery powered) USB hub. The hub has logic circuits in it that will allow for proper power negotiations with the tablet (essentially the hub connects and tells the tablet "I've got this much power available to me, so use that as the value to check against for anything plugged into one of my ports").

Thanks,
-Rob

Cheers, that confirmed my suspicions.

So given that I know that I have enough power, I echoed "1" to the bConfigurationValue, which turns of power management.

The wifi card came up and I can scan for networks.

My deal here was to cut out the powered hub from the situation.

I did that when I tried my external hard drive, I should try again without the hub.

Thanks for the course on usb power negociation, I didn't know about it (I found some info on wikipedia too).