[clug] A "mount" defeat - was Re: Persuading Debian to talk to an Android tablet via USB?

[Sam Couter]

David Cottrill <cottrill.david at gmail.com> wrote:
> The USB spec is 300mA without question, up to 1A on request.
> If a device doesn't ask, or the port can't supply it (in the case of a hub
> without an external power supply) then the USB port throws an error.

Your message has caused me to hunt down and peruse the USB 3.0 spec.
I've previously read the USB 2.0 spec, and what you said didn't match
what I remember.

USB power is described in terms of unit loads. This unit is 100mA
for USB 1.0, 1.1 and 2.0 and 150mA for USB 3.0. All standards
specify a port must supply one unit load (100mA for <= 2.0, 150mA for
3.0) for a device when it first connects. The device can ask for 5 units
(500mA) for USB <= 2.0 and 6 units (900mA) for USB 3.0. A compliant device
cannot use more than one unit load unless the host authorizes it.

USB 3.0 also has a Powered-B connector which simply supplies 1A @ 5V all
the time.

So that's the standard. However, in the real world negotiation is rare,
hosts usually pre-emptively supply full power and devices that expect
full power, don't ask for it and don't get it just misbehave.

TL;DR: Standards don't relate to the real world and sometimes USB devices
don't behave nicely.
-- 
Sam Couter         |  mailto:sam at couter.id.au
OpenPGP fingerprint:  A46B 9BB5 3148 7BEA 1F05  5BD5 8530 03AE DE89 C75C
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 198 bytes
Desc: Digital signature
URL: <http://lists.samba.org/pipermail/linux/attachments/20101022/fe440508/attachment.pgp>