I am about to order a BeagleBoard (waiting for Rev-C in march; is this
still the plan?), but one quick question remains open after the manual:
Documentation says:
- The OTG-port can power the board.
- The board can act as USB-host over the OTG-port (by miniA to USB-A cable).
- Always power by DC power jack in USB-host-mode.
My question: Is the last item a design limitation or is it caused by the
mA-limit of USB? Can I power by USB AND operate in USB-host-mode?
e.g. Beagle and some USB devices are connected to a selfpowered(!)
USB-hub. If the hub powersupplies the beagle board and the
keyboard,mouse,etc one power adapter would be enougth.
The OTG port can either supply power, as in host mode, or accept power, as in client mode. It cannot do both. If you are using the port to power the board, then it can’t supply power on the same line that it is receiving power on. It is bascially an electrical impossibility because the HUB or PC must supply the power and in that mode, there is no way to power the hub from the power coming from the hub. Does this make sense?.
The REV C is still on schedule of rthe end of March.
that was a misunderstanding: of course there's no bidirectional
electricity in one core
The question was, if a USB-powered beagle board can act as USB-host
for other devices at the hub simultaneously.
Or is this clashing the USB-specs?
I will need a USB-hub who is selftpowered (by an own power adapter) to
supplies the connected devices (external HDD, camera, keyboard,
mouse).
But I don't want to use - if possible - a second power adapter only
for the Beagle. I want that the board is powered by the hub.
So with the advent of Rev C, can I do all of these scenarios?
1) Be powered by the power connector and use the USB Host (up to 500
mA) and the USB OTG in Host mode (up to 100 mA).
2) Be powered by the power connector and use the USB Host and the USB
OTG in Device mode.
3) Be powered by the USB OTG in device mode, and not use the USB Host
port.
4) Be powered by the USB OTG in device mode, and use the USB Host
port too?
Correct. You can still use the host but you won’t have a lot of current left, maybe 100-125mA. IF you use a double headed USB cable, then you can get the full 500mA to the host port.
Well, no. You can for example connect it to the input port of a hub and
then connect keyboard, mouse, Ethernet, Bluetooth at the same time,
assuming of course the SW is set up to do so, but that input port will
not provide power to the port connecting back to the Beagle because it
is the input port of the Hub, providing DC, to the hub. Now if you plug
in an external supply to the hub, the DC path is removed from Beagle,
but it does not then switch and supply DC back to the Beagle. If the USB
port on Beagle is a Host, driving the Hub, it MUST provide DC and
therefore cannot receive DC. Host mode means DC is supplied.
Your best solution is to use the Rev C board, power Beagle with a DC
supply, and then use the Host Port to power the hub and all the devices
on the hub, as long as they don't go past the 500mA current supplied by
the Beagle. Now, in reality the Beagle can supply more than 500mA
assuming the DC supply can provide the extra current.
If you only want to use one power supply it is also possible to use a
usb hub and craft a cable that will have a usb connector at one side
and a round power connector at the other end. Of course you must use
the right lines, but effectively the hub then can be used to power the
beable using the power connector. I believe such a cable is mentioned
on the shopping list, but if you are somewhat handy and have the right
components you can do so yourself.
Be sure to get the right lines and polarity.
Actually I still have to do so; I'm planning on just cutting a usb
cable and I have a connector with cable from a broken PSU so it is
just a matter of connecting the right wires.
BTW some el cheapo hubs also seem to feed the beagle in OTG mode. This
was mentioned on #beagle yesterday (feb 11).
I think specifically a Hama hub was mentioned. Search the irc log for
more details.
And actually while writing I thought of a 3rd option: You could
perhaps use a powered hub and use a regular usb cable to connect to
the otg port. That would more or less make the beagle a child of
itself. Not sure if the sw can deal with that.
Note that I have never tried this myself (I obviously do not have a
rev C board), so proceed at own risk.
They do make a USB-A to DC cable. You could use the cable to connect from the Hub to the DC jack and then connect the input to the hub to the OTG port. That way, all power is supplied by the DC supply of the hub. Here is an example of one.
that was a misunderstanding: of course there's no bidirectional
electricity in one core
The question was, if a USB-powered beagle board can act as USB-host
for other devices at the hub simultaneously.
Or is this clashing the USB-specs?
I will need a USB-hub who is selftpowered (by an own power adapter) to
supplies the connected devices (external HDD, camera, keyboard,
mouse).
But I don't want to use - if possible - a second power adapter only
for the Beagle. I want that the board is powered by the hub.
