[PATCH 1/1] uboot-pcf50633-default-curr-lim-1A.patch

Marko Mäkelä msmakela at gmail.com
Thu Feb 21 21:01:26 CET 2008

On Wed, Feb 20, 2008 at 02:46:32PM +0800, Andy Green wrote:
> uboot-pcf50633-default-curr-lim-1A.patch
> From: Andy Green <andy at openmoko.com>
> 1) Stop PMU killing us if we don't have a battery in (by setting
> default
>     current limit to 1A)

On Wed, Wed 20, 2008 at 05:27:28PM +0300, Harald Welte wrote:
> This has to have a BIG FAT warning and it needs to be absolutely sure
> that this code is _never_ put onto any production / 'sold' devices.  You
> cannot draw more than 100mA from USB without being configured by the
> host.  Even in the latter case, you can only draw up to 500mA, since
> this is the maximum current requirement that your config descriptor can
> indicate.


I would like to echo what I wrote a month ago at
and point out that the USB Battery Charging specification
allows devices to draw up to 1.5 amperes from a special charger.
Special chargers identify themselves by shorting the data lines.
If the data lines aren't shorted, then you are limited to 100 or 500 mA,
as Mr. Welte notes.  I hope that the OpenMoko wall charger will actually
short the data lines; I do not understand why the bug report mentions
some 48k resistor.

By the way, this is my first post on the list.  (I hope it won't reject
email from non-subscribers; I cc'd you in case it will.)  I don't have
any experience in implementing any USB stuff so far, but I happen to be
interested in mobile charging of mobile devices.  Read on if you're interested.

I've been thinking about getting myself a mobile phone that could double
as a bicycle navigator.  Because GPS navigation is quite power hungry,
I designed a simple charger for hub dynamos that I will hopefully build
within the coming weeks, as soon as I get the circuit board done.

Based on my understanding of the USB battery charging specification,
the "special charger" section of
(available from nokia.com) and on my measurements of some old
phones of the Nokia DCT-3 series (3.5mm charging connector), 
I believe that a current-limited 5.0-volt power supply at 500+ mA
will work for all three classes of devices.

For a dynamo hub, a simple low-dropout rectifier linear regulator ought
to yield 80%+ efficiency if the current drawn by the device matches the
current supplied by the dynamo.  The dynamo is an AC generator,
essentially a constant current source; the voltage varies according
to load.  The nominal output current of the SON28 that I have in my bike
<http://www.nabendynamo.de> is 580 mA.  If the current limit in the
device is set higher than that, the voltage should drop a little from
5 volts and the charge controller should automatically tune down the

Best regards,

	Marko Makela

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