USB Battery Charging without Microprocessors

First: Sorry to those c.a.e folks who abhor s.e.d -- hopefully the politics won't bleed onto this thread.

On to the real question. It's phrased oddly, but I think it gets to the point best:

If I have a dingus with a battery in it, and I want to charge that battery via USB, and I want to get the most out of the USB port, does that dingus have to have a microprocessor in it that talks on the USB?

OR, can I determine how much current I can draw via USB with some purely analog arrangement of circuitry on the "battery charge" side.

I think I know the answer, but I'd like to be surprised.

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Tim Wescott 
Wescott Design Services 
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Tim Wescott
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Whoops, answered my own question (and the answer is yes, I can do it all analogly).

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Tim Wescott 
Wescott Design Services 
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Tim Wescott

Yes. They have to negotiate power levels.

"Hi, I'm a USB device and I need 500 mA".

"Sorry, I only have 100mA."

There's always a way. Whether it's a *good* way depends.

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Les Cargill
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Les Cargill

I have a old Core-2-Duo Mini ITX MB here that will Halt and show an error in text on the screen if too much current in drawn from the USB bus. It's a pretty nasty handling of an over current condition.

Cheers

Reply to
Martin Riddle

On the plus side, the thing probably cost a whole dime less than it would have if the mobo were protected from excess USB current draw.

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www.wescottdesign.com
Reply to
Tim Wescott

It's not clear to me if Tim wants to run off a "real" USB port, off a USB- charger type of device, or both.

In the case of a "real" USB port, it'll certainly have to be negotiated.

OTOH, that doesn't work at all for most USB chargers, which don't have nearly the brains needed to support that. There are several schemes in use, all doing odd things to the D+ and/or D- lines (sinking one or the other to Vs or ground via a resistor, that sort of thing) to indicate that they support more than the standard minimum (100ma).

So determining the maximum available power on the port requires checking for the (several) schemes fiddling with D+/D-, failing that negotiating with a "real" USB host, and failing that assuming 100ma.

Fortunately there are chips that will do at least a partial job of that, but I don't know of one that supports all of the schemes in use (Sony, Apply, DCP, a couple of schemes used in China, several other custom schemes that exist mainly to sell "special" chargers at exorbitant prices) simultaneously.

Reply to
Robert Wessel

And if it's not, there's no general statement to be made, SFAIK. You've constrained the state space nicely.

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Les Cargill
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Les Cargill

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