pwsafe: USB revisited

Werner Almesberger werner at almesberger.net
Wed Sep 11 14:15:24 EDT 2013


As I mentioned before, it seems that suitable Y-cables for powering
a device aren't easy to find. The cable would have to have:

- Micro A Male: goes into the password safe
- USB A Male: goes into a power source
- USB A Female: USB device (keyboard, etc.) plugs into this

The alternative of adding a second port on the password safe itself
isn't very tempting: if the power-only port is Micro B, then one
could plug a Micro B cable into either port, but only one of them
may work (depends also on the MCU used - some have two USB OTG
devices, so they could listen on both ports.)

That's kinda awkward and also eats precious space.

Integrating a USB A receptacle would be a cleaner solution but that
would eat even a lot more precious space.

At least for the prototype, a suitable solution may be to have an
external interface box with 2 x Micro B female receptacles for
password safe and power and one USB A female receptacle for the USB
device:

http://downloads.qi-hardware.com/people/werner/pwsafe/ybox.pdf

To use this, one would also need a Micro B (M) to USB A (M) cable,
like the one used to connect the password safe to a PC, and a
Micro A-B cable (the sort used to uinterconnect mobile phones.)

Not the most elegant solution, but I think it should be okay at
least for a prototype. However, if the password safe's case has to
grow for any other reason _and_ the MCU has two USB ports (TBD),
then it may be time to reconsider the option of adding a USB A
receptacle directly in the password safe.

In this case, no Micro A-B cable would be needed.

- Werner



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