USB Power?

Started by Kevin October 28, 2005
Hey,

I was wondering, is it possible to power a BX chip by USB?

USB is 5V right? It would be great if you could, would save you that extra
socket when setting up projects, and also could solve international power
issues.

I've seen projects on hackaday for powering devices with USB, like this:
http://www.angelfire.com/games5/dsresistance/pspusb/pspusb.html

Anyone think it'd be possible.

- Kevin



I have not done it myself, ut I have seen other projects which seem to
have success with it.

I am not positive, but I have seen people writing about this, and some
indicate that the USB is not "designed" to be a source of power, and
use of it in this way is outside the specifications of USB. This means
to me that while curent USB hardware seem to tolerate acting as a power
source, future versions may well not do so, and in the future your
hardware will "malfunction".

-Tony



--- In basicx@basi..., "arhodes19044" <spamiam@c...> wrote:
> ... USB is not "designed" to be a source of power, and
> use of it in this way is outside the specifications of USB.

I haven't actually built a USB device but I've looked at the
specification briefly. The USB 2.0 specifications define "low-power
functions" and "high-power functions". A low-power function (device)
may draw up to 100mA from the bus. A high-power function draws more
than 100mA but less than 500mA from the bus. A high-power device must
have special power staging capabilities to control its demand for
power as it is powered up. I believe that devices must also limit in-
rush current when they start up but I don't know what the
specification is.

There are also specifications that relate to hubs - how much power
they should be able to supply, etc. You can download the
specifications and other information from http://www.usb.org.

Don



It seems to me that you could be conservative and use a compromise
solution.

Have the USB charge a backup battery at far less than the 100ma low-
power limit. Say 25ma. The problem appears to be that you
are 'stealing power' intended for additional hubs.

If you have a few hubs, it is likely okay. But as you increase the
number of hubs, you are stressing the system.
>
The USB 2.0 specifications define "low-power
> functions" and "high-power functions". A low-power function (device)
> may draw up to 100mA from the bus.

I have a Palm Zire72 and a Konica digital camera that appear to use the
USB as a 'battery suppliment' in situations where the battery becomes
low while connected. I suspect that is the primary purpose for
the 'high power' functions [limited period of time].

> specifications and other information from http://www.usb.org.