Allow USB device to draw 500mA

[Aydan]

Hi there,

I'm trying to use my N900 as a "UMTS-Stick" from a powered hub, but the n900 is too well behaved USB power wise. It will only draw 100mA unless the USB host tells it it may draw 500mA. So now the problem is if there's a bit more traffic or I'm using 3G instead of 2G the battery will drain although the Hub could supply 2.5A.
Normal Computers usually give the device this information, but it seems the raspberry doesn't.
What can be done?

Regards
Aydan

PS: I'm running a fully updated Raspbian (minus desktop)

[FTrevorGowen]

What model of Pi are you using?
Trev.

[Aydan]

ModelB Rev2

Regards
Aydan

[FTrevorGowen]

ModelB Rev2
Regards
Aydan

Pity, whilst I am, by no means certain, I suspect that the combined 3-USB-port** + LAN chip used by the B probably does not provide or support the option you require. The 4-USB-port hub on a B+ may do so (since the B+ does actively limit the total current available to either 600mA or 1200mA) but I guess that only one of the designers can correctly advise you. Do you know if that type of "host control" is part of the "On The Go" functionality? (AIUI, the "built-in" USB of the Pi's SoC is an OTG device, and the A and A+ Pi's directly use that)
Trev.
** Of which only two are available

[Aydan]

AFAIK This type of current "limitation" is part of the USB Host specification.
"limitation" because it is software only and assumes the USB device will actually behave accordingly. Most devices don't. Unluckily the N900 does. E.g. it refuses to charge when plugged into a powered hub which is not connected to a host. I actually had to modify a cigarette lighter charger (resistor between USB data pins) because of that. The USB spec says that a dumb charger has to pull the data pins together. Most phones don't care and disrespect the USB spec (at least that's what the guys in the N900 forums told).

Regards
Aydan

[FTrevorGowen]

AFAIK This type of current "limitation" is part of the USB Host specification.
"limitation" because it is software only and assumes the USB device will actually behave accordingly. Most devices don't. Unluckily the N900 does. E.g. it refuses to charge when plugged into a powered hub which is not connected to a host. I actually had to modify a cigarette lighter charger (resistor between USB data pins) because of that. The USB spec says that a dumb charger has to pull the data pins together. Most phones don't care and disrespect the USB spec (at least that's what the guys in the N900 forums told).
Regards
Aydan

Aha! That suggests it's actually expecting to be connected to a "USB charger port" which can supply (according to the specs.) up to 1A (as opposed to 500mA for a standard port). These are (were) meant to be "of a different colour" to standard ports. I've not come across any in practice (although I believe that the PiHUB** does include such - maybe that's a solution). Many of the low-to-medium cost hubs (and my netbook, IIRC) appear have all (four) ports' power connections "in parallel", defeating the intended use of "Y-cables" and the "upto 500mA per port" spec.****
Trev.
** http://swag.raspberrypi.org/products/pihub
**** http://www.cpmspectrepi.webspace.virgin ... esChk.html

[Aydan]

Aha! That suggests it's actually expecting to be connected to a "USB charger port"

Not quite, it will take what current is offered.
If a dumb port is detected, it will take up to 1.2A
If an active USB port is detected it will initially take 100mA until the host allows 500mA.
Also my laptop has one of those charger ports (yellow) but it only allows full power if the host is inactive (powered down or asleep), otherwise there's still the 500 mA "limit".

Regards
Aydan