Y-cable and USB-OTG: mutually exclusive

Carsten Haitzler (The Rasterman) raster at openmoko.org
Thu May 1 02:31:16 CEST 2008

On Wed, 30 Apr 2008 19:28:21 +0200 Joachim Steiger <roh at openmoko.org> babbled:

> sorry for jumping into this thread so late...
> we are at the point of considering having 2! external accessible usb
> connectors for reasons of not wanting to sacrifice charging while using
> usb client OR host as well as providing power to usb host if wanted as
> well as having a internal debugboard.
> this all got me thinking.. especially all this nonsense of 'usb
> certifications' (if you ask me, do whats reasonable and helps people to
> use stuff in the real world, and ignore all the nonsense parts)
> - i really like my mini-usb. (for mechanical, practical and spread in
> the real world)
> - i have till now not seen a single micro-usb wallwart charger or carkit
> in the real world (nor a device which uses that connector at all).
> - the N8xx series is the only device i know i were near which does OTG
> as far as i know.
> thus my questions:
> - why not put a mini-usb-b like we have now onto the device?
> * do charging and client mode there. no host
> * be usb compliant when it comes to 100/500mA with usb enum
> * detect and use chargers with either id-resistor of known value and
> thus current
> * AND be able to detect usb charging spec (which is just nailed down
> what was in use before and now) by checking short on D+ and D-
> (eventually you can switch them to gpio and wiggle one and watch the other?)
> = > compliant with all carkits and wallwarts which i know of.
> means 'it will work' (and charge) in any case i can imagine and not
> endanger any fuses on mainboards ;)
> to the usb-host part:
> we are talking about a second connector there.
> - why not make it usb-(mini)-A ?
> * i would rather like A due to the more widespread of cables and
> devices, but its propably too big
> * provide 100 or 500mA there.
> * put a single 2-positions switch below the battery flap to switch this
>  whole port to jtag.
> = > would enable directly plugging loads of usb client stuff.
> - also makes the schematic much easier. less complicated cornercases.
> - the _only_ point where somebody needs a nonstandard cable is when
> using jtag ((mini)-A to A). and it is only a cable, no electronics.
> => ALL other usecases are available without any non-usb standard cables,
> devices, pinouts or whatever. like total compliance with everything of
> usb that makes sense AND keeping our beloved 'can do all' features while
> still knowing what people will plug together when plugs fit and make it
> work.
> additional thoughts:
> * put both connectors to one side of the case (enable docking both with
> fixed plugs in some kind of cradle
> * leave enough space between for silly stuff which is wider than a
> simple connector.
> please let me know what you think

i agree on all - mini not micro usb, and the switch sounds nice (though from an
ID point of view sean is mentioning having the back held in by screws so
changing that switch may be a little hard, but then again... how often will an
end user really need jtag?).

i definitely think we need to have the usb connector at the bottom of the case
- not the side. docking/cradles really would demand this.

> -- 
> Joachim Steiger
> Central Services
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Carsten Haitzler (The Rasterman) <raster at openmoko.org>

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