USB Hub current and operations

Question

USB Hub current and operations

bkelly76 (Computer)

(OP)

23 Jan 08 10:56

We need to add a USB hub to our handheld unit operating under Windows CE (battery powered).  Where can I find out how much current various hubs need?  In particular, powered hubs. Where can I find out the operations of the hub with regard to all the messages that the hub sends to and from the host?

I understand that each USB device provides its current demand, but that a powered hub can supply current.  What does the host see from the hub and from the devices plugged into the hub?  How does the host and the hub manage current requests?

I don't expect a detailed answer, that is just too much information.  I am hoping for a pointer to where I can find this information.

thanks for your time,
Bkelly

IRstuff (Aerospace) · Answer 1 · 2008-01-23 12:30:05

Depends on the hub. see: http://www.usb.org/developers/compliance/check_list/compchkhub.pdf

The power checklist tells you that a bus-powered hub should only draw about 100 mA from its upstream connection, and is only required to supply 100 mA per downstream connection. If it also allows for external power, it's supposed to implement power switching to the downstream, thus limiting upstream power draw to no more than 100 mA. I didn't read it all the way through, so it's unclear whether external power will also power the hub itself, although that would seem to be a reasonable thing to do.

TTFN

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bkelly76 (Computer) · Answer 2 · 2008-01-23 13:16:37

Hello IRstuff,
That is a good checklist.

To be a bit more specific, we have a phillips ISP1362 OTG (on the go) controller chip and per the data sheet it appears to have a max current capacity of 20 ma, far below the allowed startup current of 100 ma for USB 2.0.

I am trying to figure out how the current limits are handled. Does the host check the hub and find that the hub can deliver xxx ma of current to devices, then when the device is connected to the hub, the host calculates if the hub has sufficient current to power up the device?

Finding this level is detail is a bit difficult, but I certainly have not given up.

Thanks for taking the time to respond.

MacGyverS2000 (Electrical) · Answer 3 · 2008-01-23 13:21:36

There is some back-and-forth between the two devices on initial connection deciding what each is capable of providing....

Dan - Owner
http://www.Hi-TecDesigns.com

IRstuff (Aerospace) · Answer 4 · 2008-01-23 13:23:18

The checklist mentions "fields", so referring to the USB standard would seem to be needed.

TTFN

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USB Hub current and operations