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Maximum number of MSP430 HID Devices that can talk to PC through one USB 2.0 Host Controller?

simran singh
Prodigy 225 points
Other Parts Discussed in Thread: MSP430F5510

Hello!

I have 24 devices with an MSP430F5510 all connected to a USB hub; the hub is then connected to a tablet running Windows 8.1.  Each device is configured to present one HID interface to Windows.  Whenever the hub is plugged into the tablet's USB port, not all of the devices are able to successfully enumerate.  Is there any way that I can remedy this problem?  

Thanks!

  • 0
    Intellectual 670 points

    According to USB spec, each device could draw 100ma before enumeration is completed. Max current per single host (tablet) port USB2.0 is 500ma... I assume your devices consume a few ma, but be aware of current limits. There are issue with some hub vendors (in other words they might have bugs in hw) and host might limit amount of devices connected to it (AFAIR usb spec limit is 127)

    Some options you have:

    • Try to split devices into several hubs
    • try to get USB hub with power supply
    • Try different hub vendors
    • Connect all that stuff to Linux box to verify if it's a Windoze 8 issue or HW
    • connect (add) one device at a time to the hub, check if you reach device limit (unless devices fail in random)

    Good luck

  • 0
    Guru 227245 points

    When you plug the HUB to the PC, all devices get power immediately, as long as they don’t exceed the maximum power provided by the PC port (500mA once the HUB has enumerated) or the HUB (if self-powered with its own supply).
    If the HUB is not self-powered and does not switch the power off for its ports until enumerated, and the devices draw more than 100mA, then the whole thing may crash.
    However, even if power is not an issue, the host will only enumerate one device at a time. This may cause a timeout for the devices that are enumerated last. In this case the ‘connect one device at a time’ is a good advice.

  • 0 in reply to Jens-Michael Gross
    Prodigy 225 points

    Hello everybody and thanks for the replies!

    The hub that we are using is indeed self-powered, and we have confirmed that our total current draw for the devices is well under the 500 mA limit.  I will definitely try to disconnect all the devices and reconnect all of them one by one, though.

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