Okay, I learned what I was looking for, pretty much. I raised the question
on the linux-omap list, resulting in quite a string of responses. Here's a
summary, from a non-expert's point of view.
The USB Host port of the OMAP chips operates at logic levels, but cannot
drive a USB cable. For this, an external transceiver is needed. In most
OMAP applications (e.g., Overo), this transceiver is part of the power
management companion chip, but a discrete transceiver chip can also be used.
Furthermore, sometimes it is not necessary to have a transceiver at all : if
two chips are on the same board, a short distance apart, they can be hooked
together directly using logic levels. This is called TLL , "Transceiverless
One person from TI said that the for the OMAP chips, "USB-HOST core is
primarily designed to support TLL mode". One feature specifically *not*
supported by the hardware is dynamic switching from EHCI to OHCI : this
choice must be made once and not changed.
Nonetheless, there are reports of people who have gotten EHCI working, at
least with a discrete transceiver chip.
As of today, my impression is that for us "users" the USB host port is
simply not useable, but there seems to be a renewed flurry of activity
looking into the matter - maybe it will become available Real Soon Now....
View this message in context: http://www.nabble.com/Overo-USB-host-%3A-why-%22experimental%22---tp20923238p20964350.html
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