usability of third-party USB chargers

Shawn Rutledge shawn.t.rutledge at
Mon Mar 24 18:56:03 CET 2008

On Fri, Mar 21, 2008 at 11:23 AM, Joshua Judson Rosen
<rozzin at> wrote:
> "Shawn Rutledge" <shawn.t.rutledge at> writes:
>  >
>  > On Mon, Mar 3, 2008 at 3:30 PM, Felipe Balbi <me at> wrote:
>  > >  nope, a micro-B to standard-A cable is enough for this purpose.
>  > >  OTG Devices shall have only 1 otg port, i.e. only one micro-ab
>  > >  receptacle.
>  >
>  > I was referring to being able to use Motorola chargers and such.
>  I had the impression that such chargers were not necessarily
>  compatible with each others' devices--that they would indicate
>  `quick-charge mode' by supplying a different voltage than a standard
>  USB port, and the USB-plug-form-factor charger world had yet to settle
>  on a standard for that `different voltage'. So, devices that are
>  mechanically compatible with a given charger of a different brand are
>  most likely not electrically compatible and often ignore it.

There have been other threads about that, with links to the details.
IIRC the charger has a resistor between a couple of the pins, for
which the circuit is fairly simple to detect it.  But maybe other
companies do it a different way, I'm not sure.  Ideally we would both
follow the USB charging spec and also be able to detect one or more of
the other types of chargers (if that's not too hard when you have
designed the circuit according to the spec).  But they plan to use the
mini connector, and Felipe says you can't get the device USB certified
unless it uses the new micro connector, so I don't know what the
conclusion is, except that nobody seems to really offer
interoperability over a long enough period of time, because the
standards change with such capricious regularity.  I haven't seen
actual devices with that connector yet (but there is something small
being used for charging some Bluetooth devices.  Maybe that's it.)

AFAIK the voltage is always 5V, the disagreement is just about what
kinds of games to play with the TX and RX pins, because a dumb charger
is not going to use them for digital communication, so there can be
some sort of passive analog "communication" instead.

More information about the neo1973-hardware mailing list