Y-cable and USB-OTG: mutually exclusive / the four connectivity solutions

Werner Almesberger werner at openmoko.org
Thu May 1 21:40:21 CEST 2008

Andy Green wrote:
> What we can't do is set current limit from adapter as we can on USB.  We
> cannot actually treat it as a second USB interface for pcf50633 purposes
> as it is I think.

Yes, we may need some current limiter. This is starting to look a bit
like the charger circuit in HXD8, which also has a third power source,
namely car power. (HXD8 is messier, though, because it uses the
PCF50606, which doesn't even know about USB.)

If you look at the HXD8 schematics, you'll find that there's a lot of
glue logic and even more analog trickery. (And as you might expect,
making all that work properly in all possible modes of operation gave
us some headache.) That experience is actually one of the things that
motivated me to want an MPU - being able to replace all that messy
glue logic (that invariably turns up) with software.

We have to consider which current limits we care about. There are:

1) 100mA: we get this one for free - if there's any voltage, we can
   draw 100mA
2) 500mA: we get this if we tell the FTDI in the EEPROM that we want
   that much power, or if we make 500mA out ceiling
3) 1A (or such, could be "unlimited", if the charger doesn't mind):
   we get this if we make it our ceiling

How do we feed the JTAG/console subsystem ? From a Vsys combined from
all three possible sources, or from the USB-debug 100mA path ?

I think we want the "high-current" path, i.e., 1A. I'm less certain
about the 500mA path.

> Yes I have a plan I will post about.  BTW I saw while looking at it that
> it seems D- USB pin on GTA02 is floating when GTA02 charger is connected.

Indeed. In SMDK2440, Samsung pull D- with 470k to GND. (Page 6/7, R67)
In SMDK2443, where the port is OTG-capable, there are no external

> but I agree external access will be more certain and better and waste
> less space.

Perfect :-)

- Werner

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