Easy Question about USB Hardware Design

According to the USB 2.0 specification, section 7.2.2, the maximum allowed voltage drop allows for a minimum voltage of 4.35V at the input of a low-power device.

So, 3.3V would not be ok.

Le Thu, 13 Mar 2008 19:44:29 -0700, Paul a écrit:

Host Device USB 1.1 (and possibly USB 2.0 too) delivers VBUS = 5V (no loads) and Imax = 500mA. If you expect using this Power Rail you have to use a 3.3V LDO (VBUS --> LDO --- 3.3V/500mA max). Many USB Gadgets use this for self-powered capabilities.

Paul,

I do wish you wouldn't ask questions in "assertion","right?" form - it comes across to us Brits as rather agressive - it's a culture thing and I'm sure you don't mean it.

To answer your question - assuming you mean what the USB spec calls VBUS, then a device is entitled to expect 4.35V minimum at the end of the wire. This means that 3.3V is out of spec. Many devices use a 3.3V internal supply and a low drop regulator from VBUS. Depending on exactly how this is implemented a device may or may not work from 3.3V VBUS.

If your 3.3V is subject to tolerances as well the situation is worse - my expectation is that you would have trouble.

Michael Kellett

No, it wouldn't, the lowest permissible voltage on Vbus is 4.35V for a low powered device. A low powered device may only ever draw 1 unit load (100mA).

Andrew

third strike: plonk

It may work on occasion but is not reliable design practice. Even if a device is 3.3v internally, it will have a regulator on the USB VBUS input, and that regulator may or may not work at all with such a low input voltage.

You'd be more likely to get away with it with devices that are not bus powered.

In short, if it's for one-off personal use and testing indicates its reliable enough for your needs you can do it. Otherwise - product, anything where you have a responsibility to others, etc you must not violate specifications to such a degree.

es

I'm OT here, but that's a fascinating fact. "Assertion","right?" is certainly a common idiom in America, and I doubt anybody here (USA) gives it a second thought (as you acknowledge). Anyway, thanks for pointing something out that I never would have guessed.

Mike

As another Brit, I'd agree with Michael - the "assertion", "right?" idiom sounds like the poster is confident that he is correct, and is challenging others to prove him wrong. I'd take it as an indication that the questioner has a bad attitude (unless it really is something he can be confident about) - he's looking for confirmation of his fixed ideas, not help and advice. Of course, it depends on the rest of the context of the post.

The assertion of "right?", is an Americans _lack_ of confidence that is showing.

American TV has used this as a question of our lack of confidence that has made its way into everyday use.

Go figure.

donald

Thanks for clarifying this to the Brits, Don.

Asking "Right?" at the end certainly means you are not really sure yourself. And I'm most certainly not.

But people usually see bits of themselves when they judge others on a few words.

Right?

:)

That's what i thought, Thanks to everyone who chipped in.

Now i have ammunition for the silly supervisor who wants to get rid of everything!

It's interesting to hear that, as the idiom sounds (to British ears) the opposite.

I guess I don't watch that sort of American TV (I watch a few American series, but not things with chatter - game shows, talk shows, "reality" shows, etc.)

There's another phrase that is totally alien to non-USAnians (I know what it means - it just sounds nonsensical).

It's been said that the USA and the UK are two cultures separated by a common language. It's also been said that one of the differences between us is that in the UK we speak English, in the USA they don't.

I'm only commenting on how it appears to a non-American - not on how you meant it to sound. And I suppose my reading is coloured by your previous "I know USB better than the experts" posts.

Wrong.

People's impressions, especially of a few words, are subjective - but that doesn't mean they judge others by their own standards.

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Yeah, most Brits think that even the white American english is "wrong".

But funny, white Americans think the black version of english is "wrong" too!

But 1000 years from now, people will be hard pressed to read this sentence.

Languages do NOT stand still....it's all constantly changing.

you are not

Dude, why would i ask if i knew better?!

I simply had issue with the "he's in the weaker position because he has admited that he doesn't know something I do, so now i can give him s*it" mentality.

But, that's how the world is.

when they judge

Well, you were clearly wrong when you typed this:

"As another Brit, I'd agree with Michael - the "assertion", "right?" idiom sounds like the poster is confident that he is correct, and is challenging others to prove him wrong."

No, we think it is *different*, not wrong. You speak American English, I speak British English, also known simply as "English". If you write "color", then it's correct in American English - it's only wrong if you are trying to write British English and spell it "color".

If they think the black speakers are "wrong", then it's just because many Americans have serious racial problems. Different groups of people speak different dialects - they are only "wrong" if they use a dialect but claim it to be standard language (this is mostly relevant only in written language).

Yes, I'm aware of that. I am also aware that in most cases of spelling differences between American and British English, the American version pre-dates the British version.

I'm not at all sure why you bothered to ask - you certainly don't seem to listen to or appreciate the technical help you were given.

Somebody told you you were doing things all wrong, and gave you advice on better methods - you seem to have taken it all as a gratuitous personal insult. No one else saw the other poster's posts in that way.

What part of "sounds like" do you have problems understanding?

What you mean by the things you write, and what I think you mean as I read them, can be different things. It might be that you expressed yourself badly, it might be that I misread your statements, or it might just be differences in cultural idioms. That's why I *don't* "judge others on a few words" - I'll look at a wider context before thinking ill of anyone. And it's also why it's useful to learn a little about such idioms - now you will not use "assertion, right?" in an international newsgroup to avoid giving the wrong impression, and I'll know what is really meant if I read it from another poster.

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Don.

es

h,

I don't think you speak for all Brits....I know quite a few consider British English the "proper" english.

ple

Plenty of race issues in the UK, from what other Brits have told me....

Ebonics and the Hip-Hop culture have gone far into the mainstream......the younger generation embraces change....

ng

Yeah, wonder what Shakespeare would think about the present English dialects.

ns you are not

There is where you are once again misperceiving my words. Or you were being very selective about which posts you read. I was very gracious to a couple of cool cats on my past posts, and also to everyone on this one, so go back and re-read if you must.

y.

There's something called tact, that many people lack. And the dude was spouting off, without answering the question directly.

ext -

I'm not going to change my writing style just for the Brits out there.....