USB host mode

Andy Green andy at
Fri Jun 27 16:18:06 CEST 2008

Hash: SHA1

Somebody in the thread at some point said:

| Was there any conclusion on whether the Y-Cable is feasible for the
| NeoRunner, or was it discussing a possible solution for a future
| hardware generation?

It's feasible in every way except fabricating one easily.  A guy on the
community list managed to do it, but my attempt ended in failure due to
the moulded mini-USB plug not having accessible ID pin.

In the mode you're discussing, you need to set the Freerunner to be in
logical host mode but not in "electrical host mode", ie, not generating
the 5V supply.  The reason is not that it makes trouble to drive 5V in
on top of the generated one, but as part of the "electrical host mode"
the freerunner takes care to defeat charging from the USB supply (since
it generates it).

To work around the final effect of "electrical host mode", you also need
to add 15K resistors to 0V on D+ and D- on your cable.  Then you're all set.

Here's the magic commands related to USB host:

# tell the USB stack the USB socket is logically a host now

~  echo host > /sys/devices/platform/s3c2410-ohci/usb_mode

# get the Freerunner to provide 5V out on the USB socket
# you don't want to do this with a Y cable

~  echo 1 > /sys/devices/platform/neo1973-pm-host.0/hostmode

- -Andy
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