From: David Brownell <[EMAIL PROTECTED]>
To: linux-usb-devel@lists.sourceforge.net
CC: "Steve Calfee" <[EMAIL PROTECTED]>
Subject: Re: [linux-usb-devel] Re: Question about OTG operations
Date: Tue, 28 Feb 2006 23:42:35 -0800
On Tuesday 28 February 2006 11:03 pm, Steve Calfee wrote:
> Yes, it is a huge drawback to the spec. Being host does not say you
provide
> power, being "A" connector does. How do you communicate the fact that
your
> handheld can use and get power from a device if it is plugged in
properly.
That one's easy: there's now a "battery powered" device feature
(from wireless usb). Use that ...
What's harder is the other way around: "I have power to spare,
from the AC outlet." Because the handheld gadget would likely
be the one to detect that case and suggest switching the cable.
Of course one could apply heuristics too. Most printers have
AC power, so if your handheld hooks up to an OTG printer in host
role, and its battery is low enough, then suggest re-cabling.
OTG printer? Presumably they exist by now ...
It is very hard to find OTG devices. They also don't seem to be advertised
even if a device supports OTG. Google shows some lists, many cameras, a few
printers. A camera/printer pair makes sense because both can be devices to
some PC, but maybe you would like to print directly from a camera, so OTG
could handle that.
Someone said that the Sony PSP (handheld game) was OTG, presumably mainly
used to connect two PSPs for linked play.
> Probably having
> both A and B connectors is a more sane and clear indication of power and
> connections.
I thought only hubs were allowed to do that, though...
Think of a Linux PC with a gadget card. it has both A and B connectors and
does not violate the USB bus topology because they are really on two USB
buses.
Increasingly many of the "SOC" chips now becoming available (such as the
samsung s3c2410 ARM chip) have multiple device and host cores so being both
a host and a device is quite possible. I have not seen one (yet) with a
built in OTG phy, so you would have to use an external phy to do OTG. So as
a system designer you could choose either or both of OTG or having both A
and B connectors.
Regards, Steve
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