Y-cable reinvented - for USB-OTG
joerg at openmoko.org
joerg at openmoko.org
Sun May 4 18:55:05 CEST 2008
Recently there's been a vivid discussion about external power while in
hostmode. Some of my thoughts of last 15min:
On our future micro-USB-OTG-connector (as well as now on mini-USB), we got 5
D+, D- for data (for highspeed, mustn't be touched with more than 5pF!)
Vusb, which is abused by OTG as a weird dataline, thus spoiling our
ID, which is meant to be connected to GND, to make for the "host-side" of
We are using this ID-pin to detect usual hostmode (short to GND, 100% spec
cnforming), and by our charger hack where we connect ID to GND via 47kR to
detect we got a strong OM-charger. Y-cable (of GTA02) works by using his
47k-hack to switch the logic of D+/D- to hostmode, while we are still
accepting power from Vusb instead of feeding it.
All these modes have in common: we have a R from ID-pin to +Vsys inside the
device, and we are detecting the actual voltage on ID-pin, that results from
the internal R to + and the external short or R to GND.
Now if we would hook this internal R not to V+, but instead to a GPIO, we
could still do the same tricks, but in addition can hook the internal R to
GND/0volt, what gives us exactly 0volt on ID-pin for all of the above cases.
Now if we detect there's not 0volt but instead there's 5volt on the ID-pin
while we're applying 0volt to internal R, we got a new info about what's
plugged to our USB-receptacle, and we might ->>route this 5volt to our
Got the picture?
Not yet 100% checked and verified for possible pitfalls (killing other devices
with our GTA04-Y-cable... etc), just wanted to share the idea before someone
else is coming up with it ;-)
Basically the above idea, if feasible, saves us from the need of any second
path for GTA04-power outside the one micro-USB (no additional holes need to
be "drilled in the housing").
Y-cable for GTA04 (C)jr ;-).
(Though I have to admit I still love the idea to have a Nokia style
quasi-standard barrel connector. Nevertheless there's a different approach
now. Industrial Design has to decide on this, not me)
Comments solicited! :-) Maybe there are conflicts with some of the more recent
USB-specs I'm not aware of.
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