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USB-UART on PZCC-FMC won't enumerate

bhfletcher

We are aware of a few cases where the Silicon Labs CP2104 USB-UART won't enumerate on the PicoZed FMC Carrier. We are investigating further. A couple suggestions:

  •   The very first time you plug in the board to a PC, the driver installation may take some time. In particular, on Windows machines, Windows will attempt to look for a more up-to-date driver on Windows Update which can take several minutes. You will see an icon in your task bar indicating that the system is working on installing the driver. You can click this and save yourself some time by cancelling the Windows Update search.
  •   On the few cases we have seen, you can always get around the failure to enumerate by plugging in the microUSB cable AFTER system power is ON.

Bryan

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    We have identified a hardware design flaw in the CP2104 circuit on the PicoZed FMC Carrier. The VIO pin and the RST_N pullup need to be connected to the CP2104 VDD to properly implement bus-power mode. When the USB cable is inserted with system power off, VIO is not active and RST_N is indeterminate. These can both cause an enumeration failure and lock up the device. We have reworked a board and tested in our lab with success on a failing board.

     

    Note that this only affects a small percentage of boards and there is a very simple work-around without doing the rework:

    1.   Make sure CP2104 driver is installed on host.
    2.   All cables unplugged and power off on carrier
    3.   Plug in power cable
    4.   Turn on power to the carrier
    5.   Plug in usb-uart cable
    6.   Turn carrier on/off or soft reboot at will now. The UART will be persistent across power cycles.
    7.    

    Bryan

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    We have identified a hardware design flaw in the CP2104 circuit on the PicoZed FMC Carrier. The VIO pin and the RST_N pullup need to be connected to the CP2104 VDD to properly implement bus-power mode. When the USB cable is inserted with system power off, VIO is not active and RST_N is indeterminate. These can both cause an enumeration failure and lock up the device. We have reworked a board and tested in our lab with success on a failing board.

     

    Note that this only affects a small percentage of boards and there is a very simple work-around without doing the rework:

    1.   Make sure CP2104 driver is installed on host.
    2.   All cables unplugged and power off on carrier
    3.   Plug in power cable
    4.   Turn on power to the carrier
    5.   Plug in usb-uart cable
    6.   Turn carrier on/off or soft reboot at will now. The UART will be persistent across power cycles.
    7.    

    Bryan

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