Gilmore on USB power

[Jim Gettys]

On Wed, 2006-09-13 at 21:23 -1000, Mitch Bradley wrote:
> John Gilmore and I were having a conversation about OLPC.  He had some
> interesting comments regarding the power available to USB devices.
> (Forwarded to this list with his approval.)
> 
> Mitch:
> 
> The crux of the problem is that full support for arbitrary USB devices 
> blows our power budget right out of the water.
> 
> John:
> 
> OK, but devices should not end up with *flakey* power.  The USB protocol
> has the device specify how much power it needs.  If the system can't
> provide the amount of power that the device requests, it should turn
> off the device and report the error -- rather than providing it "some
> of" the power it requests.

In fact, recent Linux systems have this behavior; they do "all or
nothing" behavior. This has been somewhat controversial, as it has
sometimes resulted in the system refusing to support sets of devices
that have apparently worked in the past.

> 
> Is the OLPC violating the USB spec by failing to supply the requested
> power?  You may not be able to use the trademarked USB name, logo, or
> patented connector if your device violates their specs.

Dunno.  If we behave as recent Linux systems do, we will provide the
power, or refuse to try.  I'm not familiar with the letter of USB
trademark requirements.
> 
> When the OLPC is plugged into a wall, can it provide more power than
> when it's running from a battery?  Or is the current limitation in
> some intermediate circuit like a voltage regulator?  All the current
> "A" boards are running on wall warts, but reporting problems anyway.  Why?

IIRC (Mark can say for sure), the limitation is in the voltage
regulator.  To be able to provide 4 more watts would have presented both
cost problems and more thermal problems.  This would have required a
very substantially larger supply; 4 watts on our scale is a significant
increase in the size of power supply, rippling through the regulator
back to the power brick (which would also have to be bigger and heavier,
and to what powers the machines).

> 
> And whose "power budget" is this, anyway?  If the user would rather
> have an hour of usage with a USB device, rather than have a day of
> usage with no USB device, why is the design team making that choice
> for the user?  Pop up a message if you insist -- but don't just say,
> "Sorry, we have the power, but we won't use it for *that*!"  The
> user may need to copy one minute's worth of data to or from an external
> hard drive, or hook to an Ethernet for long enough to handle queued
> emails, or something; the design would deny them that opportunity?

My understanding is that OLPC is not unique in this limitation.

> (Of course you'll run into devices that actually draw more than what
> they ask for -- that's the device's bug, and should be reported
> back so that they lose their USB certification if they don't fix it.
> But that problem shouldn't be too widespread, though you won't know
> til you start enforcing it.)
> 
> 
> _______________________________________________
> Devel mailing list
> Devel at laptop.org
> http://mailman.laptop.org/mailman/listinfo/devel
-- 
Jim Gettys
One Laptop Per Child