I was initially under the impression that the N800 could not supply more than 100mA of current to USB devices, but after messing around with host mode, it seems the maximum is actually ~200mA?

If I connect an externally powered device (not drawing current from the N800), dmesg shows 198mA of current available on the USB bus.

From looking at the power management ic datasheet, I think it's saying 200mA is the absolute maximum supply, with 100mA as the recommended maximum.

Is there any danger connecting devices that consume more than 100mA?

I would assume the N800 would simply disconnect any devices that require more current than can be supplied, but I'd rather not risk any damage.

So.. aside from either sucking up more juice, or being shut off, is there anything I should worry about?

Trying to draw more current than an IC is supposed to supply can permanently damage that IC. So... I'd be careful

The usb 2.0 specs state that no more than 100 mA should be drawn before negotiation. This allows the usb host pcb to be build cheaply/compact without the need of fuses or current sensing, and gives backward compatibility with previous specs.

Though, some non-compliant and badly implemented devices go directly to high power mode and ask for 500 mA or more. If you connect one of those to the N800 you will in most cases end up with a damaged pcb.

forgot to add: The 100mA margin is there to protect the device since the ground in many cases will be floating (for e.x. when you connect it with a laptop).

Easy way to find out exact answer and stop the speculation is to look at the datasheet of the USB (power) controller. In another post it has been mentioned (N800) to be Texas Instruments part TPS65030.

http://focus.ti.com/lit/ds/symlink/tps65030.pdf

In the TPS65030 datasheet they mention absolute maximum current 200mA for the Vbus output. Above this point "permanent damage may occur".

They also say that the Vbus output is current limited, with the typical current limit being at 160mA. So, if you try to overload the USB port you should see the output voltage dropping or getting turned off around 160mA point on typical device. (YMMV) Also the absolute maximum short circuit current is less than 325mA.

And that's not all yet.. (as you have heard in TV-shop... ) The chip is overtemperature protected that in case you manage to overload it up to a point of failure the chip will turn itself off before that happens.

In short the output is protected against overload and short circuit. You shouldn't be able to fry the N800 by overloading or short circuiting the USB port +5V bus.

(This is common design practise that anyting that is user accessible you should assume user can short circuit those pins/wires and the hardware should not get damaged.)

Hi Mara,

Great! thanks a lot for the info.

It is nice to see that the nokia guys took extra care to protect our devices.

Your last sentence though is not 100% correct. This is not common practice and the specs don't require protection from overloading.

From the usb spec: Actually, it is discouraged to add this kind of protection on the host (you can find this in other parts) because it might mistakenly disconnect during hot-plugging of compliant devices.

Anyway, now I am really curious to see why they chose this implementation. I am not at all familiar with the OTG and its advantages, but I suspect something about the portable/battery combination.

Chris

I checked it out, just wasn't sure if I was understanding it correctly, or if the "damage may occur" part applied to the tablets.. I'm glad there's extra protection

Thank you for the very informative post!

I have a Nokia AC-8E power charger for my N800 charging. It has 5V DC @ 890 mA output written on it.

The device I want to use on USB host mode uses 5V @ 500 mA according to technical specs I found.

My question is: If I try to use the 5V @ 500 mA consumption device on N800 while my Nokia AC-8E power charger outputs 5V DC @ 890 mA. Would my N800 be able to supply my device enough currrent as the power charger is kept connected to it? Please note that I am not using a powered usb hub.

It's not a matter of the charger, it's that the port cannot supply more than ~200mA.

There's no way to route power from charger port to usb port directly. The only path is through the charger, battery and boost circuit for usb power, so you're limited to 200mA regardless whether charger is connected or not.

I was afraid this would be case with me but was not able to decide for sure. But had to try my luck with it because if was possible, that would mean a lot, thanks