Easy USB enumeration? Request 500mA draw

[Shafto]

I'm building a USB charger for a Li-Ion cell, and I'd like to be able to achieve 500mA from the USB port. From what I've read, I need to have a device that can be enumerated, and then request 500mA.

I've seen a few USB to breadboard connectors with I/O pins and 100mA or 500mA draw, I'm just wondering if anyone has seen anything even more simple? specifically for charging duties that will enumerate itself and request 500mA?

 

[Shafto]

Hmm, seems I have wasted some space. While still looking around I read this in an article from MAXIM about USB battery charging:

"USB Ports rarely (never) turn off power: The USB spec is not specific about this, but it is sometimes believed that USB power may be disconnected as a result of failed enumeration, or other software or firmware problems. In actual practice, no USB host shuts off USB power for anything other that an electrical fault (like a short). There may an exception to this statement, but I have yet to see it. Notebook and motherboard makers are barely willing to pay for fault protection, let alone smart power switching. So no matter what dialog takes place (or does not take place) between a USB peripheral and host, 5V (at either 500mA or 100mA, or even maybe 2A or more) will be available. This is born out by the appearance in the market of USB powered reading lights, coffee mug warmers, and other similar items that have no communication capability. They may not be "compliant," but they do function."

Article:http://www.maxim-ic.com/app-notes/index.mvp/id/3241

Guess I don't have to worry about it, perfect.

 

[Haku]

So far I've not had any real trouble drawing 500mA (sometimes more) from the USB ports on my Asus Eee's and my main computer - except the left hand port on the Eee 701 won't power a slimline DVDRW or 2.5" harddrive.

The only time I've seen a USB port shut down because too much power was tried to be taken from it was on an externally powered USB hub, I tried charging+connecting an Archos AV500 and the hub freaked out at the AV500 trying to take 700mA+.

 

[hippy]

The Maxim app note probably describes the reality as it generally is and how others have observed it to be but may not be what the USB specification says is required - which would beg the question how such USB ports have passed conformance testing to be able to display the official USB logo ? I'm not familiar with the specifics of the specification.

The real question is what happens when USB 3.0 arrives ? If hardware chips or driver software handle USB ports such that they won't deliver over 150mA unless devices enumerate themselves all those 'dumb devices' which take more won't work with USB 3.0 ports. No one will know until USB 3.0 ports become more prevalent.

I suspect it will be much the same as now - 'draw whatever you want' - but if the hardware chips do handle power delivery manufacturers may simply go with the easy option and leave it to the chips.