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

[Marko Mäkelä]

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.

Gentlemen,

I would like to echo what I wrote a month ago at
<http://bugzilla.openmoko.org/cgi-bin/bugzilla/show_bug.cgi?id=956>
and point out that the USB Battery Charging specification
<http://www.usb.org/developers/devclass_docs/batt_charging_1_0.zip>
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
Nokia_2-mm_DC_Charging_Interface_Specification_v1_2_en.pdf
(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
current.

Best regards,

	Marko Makela
	http://www.iki.fi/~msmakela/