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

Werner Almesberger werner at openmoko.org
Fri May 2 00:05:59 CEST 2008

Andy Green wrote:
> Yeah.  I think we can maybe "specify out" the need for an external
> current limit here though -->

That would be nice :)

> Right.  I think what we can do is treat the 100mA case as we should not
> use adapter power...

You mean at all or for the rest of the system ? In the former case,
this would break using the USB-debug port with an unpowered hub.

 in the later diagrams I sent I treat it as same
> situation as USB suspend there, "there is voltage but we can't take any
> power" -- that boils down to falsely reporting ADAPTSNS to absent.
> 100mA case is atypical anyway.
> 500mA is the question, can we allow to enable charge under these
> circumstances when in the worst case we have an OTG peripheral plugged
> in the other side

An empty battery should be enough to exceed 500mA. The PMU doesn't
enforce any current limit on the adapter->SYS path, so we get the
maximum battery charge current, which would be 1A.

> Maybe it turns out we should only enable adapter power usage at all
> (above just powering the FTDI) when we have an actual adapter there and
> we allow it 1A fast charge + powering the device + any OTG devices
> hanging off it.

If the on/off switch is somewhere between FTDI and PMU, that should
work, yes. Then we'd get 1A into the battery, 500mA into OTG, leaving
500mA for the system. (And we hope nobody is doing anything crazy with
the modem ;-)

- Werner

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