The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
From: "Matthew Kendall"
Subject: Re: USB 1.0 or 1.1 -- Max current draw allowed?
X-Newsreader: Microsoft Outlook Express 5.50.4920.2300
Date: Sun, 15 Dec 2002 02:59:10 GMT
NNTP-Posting-Date: Sat, 14 Dec 2002 19:59:10 MST
Organization: Shaw Residential Internet
"J81" wrote ...
> On Fri, 13 Dec 2002 18:04:23 GMT, "Matthew Kendall"
> >Short answer: 100mA for ports on bus-powered hubs; 500mA for
> >ports on self-powered hubs (or hosts); officially no more than
> >500uA until your device has enumerated on the bus.
> Is this limited by the USB host/hub, or is it a limitation I'm
> supposed to recognize and respect in the design of my device?
The 500uA thing is something you are supposed to respect. When your device
enumerates on the bus it tells the hub how much current it wants and the hub
allows it or not. After that it can draw the amount it has requested. Bus
powered hubs will allow 100mA per port; self-powered hubs will allow up to
500mA per port. Try plugging a high-power device such as a scanner into a
bus-powered hub to see the resulting error messages.
In practice you can get away with making "dumb" peripherals that do not do
anything on the bus and just draw current. Kensington make a lamp and a fan
that work like this. This is definitely non-compliant with the USB spec. In
this situation the hub might notice your excess current and turn the port
off, but most likely not. You will likely be able to draw current up to some
margin over the limit, at which point a (self-resetting) fuse will kick in.
The only practical downside with "dumb" peripherals that just draw power is
that they will not respect standby. When all bus activity ceases a
peripheral is supposed to reduce current draw to practically zero. A "dumb"
peripheral won't do this. The upshot is that when your laptop goes into
standby mode (and bus activity ceases) your "dumb" peripheral will continue
to flatten the laptop battery.
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup