USB, Toshiba

[Lennart Sorensen]

On Thu, Jan 20, 2011 at 07:03:33AM -0500, Scott Allen wrote:
> Does anyone here know if /how this is physically enforced? Do
> motherboards, hubs and other USB hosts actually have two stage current
> limiting circuitry, or will they always provide the maximum current
> (possibly protected with a thermal fuse or other current limiter) even
> if not negotiated?

Depends on the board design.  Some do more than others.  The USB standard
simply says what devices are allowed to do without asking, and what the
port should be able to provide (providing more is of course permitted,
but not required).

> I bet many of the USB gadgets and toys that use the interface strictly
> for power don't do any power negotiation and may not even have data
> lines connected. In this case, do they limit themselves to 100mA?

They might use more, but the system is allowed to limit it to 100mA if
it wants to.  The system would even be completely within it rights to
shut off the power to the port if more than 100mA is being used too.

Now sure what a 5V 100mA cup warmer is good for, but oh well.  0.5W heat
is more than 0W heat.

Many things just treat USB as a 5V power source that can do a couple
of watts.

-- 
Len Sorensen
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