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 
      GTA02-Y-cable approach,
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 ;-). 
Problem solved. 
(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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