Neo Sound and USB Questions

Tilman Baumann tilman at
Sun Aug 19 03:04:24 CEST 2007

Am 17.08.2007 um 19:19 schrieb danimanns at

> Tilman Baumann wrote:
>> danimanns at wrote:
>>> Are there plans to provide a car adapter for the Neo?
>> This is a general problem. Where are USB-Power Adator out there.  
>> Also for cars.
>> But most of them just power the 5V line. This is bad with 99% of  
>> the bad behaving usb gadgets out there.
>> But Neo is prolite and asks the host if it could draw it's 500mA  
>> and if not only takes 100mA out of the bus.
>> This is how the usb specs tell you to do it.
>> But most devices (like usb microovens and the like) just draw  
>> power as there would be no tomorrow and don't care how much the  
>> host can privice. If your usb-powersupply burns out - bummer. :)
> Don't know if I understood right. Do you mean,

Sorry. I don't remember being drunk when i wrote this. But you are  
right, it's completely bad written. :)

> the Neo asks the power supply how much power it's able to provide?
> Why should the Neo do this?
This is part of the USB power management system. Which is clearly  
defined in the USB specs.

> Like I described below I guess
> the Neo uses the 5V channel only for charging, and so
> the 500mA are always okay.
500mA is quite a lot power if you drain it from a bus which is maybe  
provided by a mobile device. Or imagine dozens of such devices on one  

Therefore the host (where the power comes from) can tell each USB  
device how much power it is allowed to use.
This is needed, because the cost can not just cut power from the  
ports that are overloading, because it is a bus. (All devices sit in  
one boat. So they have to cooperate.)
(Most, but not all USB chips have build in fuses to prevent actual  
overloading of the whole bus. But not all!)

> Why not use a normal 500mA
> power supply then?

Mainly, because USB was never intended to be a power supply.

Sure the stable and most the time quite powerful 5V from USB proved  
very useful for many gadgets. And it is quite normal to just hook  
your average USB coffee machine or cooking stove on the 5V line and  
don't care. But in reality, this is just bad.

But the Neo is not built by assholes that don't care. ;-)
And it is just sophisticated enough to do it right anyway. (And  
implementing the USB power management at least in a rudimentary way  
is the normal way to go for everything that has some smart silicon in  

I see the problem with this attitude. and as i know FIC and the  
openmoko guys do too.
So i guess there will be a solution soon. Either a "don't be smart  
and don't care about specs" overwrite button for charging on a dumb  
USB power supply, or a advice which charger to buy.

For the mean time. Use you computer to charge your moko and never let  
it run out of power so you don't need the 100mA charging while the  
device is in 'dump' (as in switched off) mode.


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