Y-cable and USB-OTG: mutually exclusive

Joachim Steiger roh at openmoko.org
Wed Apr 30 19:28:21 CEST 2008

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

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


Joachim Steiger
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