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TUSB2036: downlink device not enumerating (any longer): USB device needs more power than the port can supply

Severin Voelkl
Prodigy 50 points
Part Number: TUSB2036
Other Parts Discussed in Thread: PCM2900C,

Hello forum,

I'm struggling with an issue I cannot comprehend. We respun a PCB featuring a TUSB2036 with two permanent bus-powered downlink devices. One is a PCM2900C, the other a third party HID device connected via X1. We respun it to have better access to the PCMs HID pins, improve signal integrity and exchanged USB mini B for USB C. For the other device we didn't change much because it already worked fine.

With the new PCB the hub and PCM enumerate perfectly, but other device fails. However communication is possible as VID and PID are read correctly. What could possibly cause this error message?

Items marked with green boxes have been added during redesign.

When searching through the forum for insights (and applying different sorts of possible fixes) I came across that bus-powered devices are sensitive to overall VBUS decoupling (which I already played around with). Could this be the issue?

Summary:
- Communication to device works in the first place (VID, PID are read)
- Problem code 10 (failed start) is issued
- OS signals overpower situation
- Device has worked fine with old PCB revision with only few changes made in it's circuitry (yes, I already bypassed the series Rs in uplink and removed DP/DM decoupling caps → no change)

Any hints are appreciated.

Best regards

Severin

  • 0
    TI__Guru 60870 points

    Hi Severin,

    Did you say you get an overcurrent error from the OS?  That could be a problem with in-rush current on the X1 connector.  Can you add 22 uF of bulk capacitance to VBUS?  What is tied to pin 9 on the X1 connector?

    Regards,

    JMMN

  • 0 in reply to JMMN
    Prodigy 50 points

    Hi JMMN,

    thanks for your reply. Yes, overcurrent from OS. How does it know? Does it supervise VBUS at root hub? If this is the way it works, then there's definitely a voltage drop at device start-up. I have indeed tried some electrolytics (up to 100 uF) bodged between X1.1 (VBUS) and GND plane to rule it out, but they didn't make any difference. According to datasheet pin X1.9 is called USBEN, which as its name implies could be important here. Interestingly enough the old PCB still works when leaving X1.9 floating.

    Regards,

    Severin

  • 0 in reply to Severin Voelkl
    TI__Guru 60870 points

    Hi Severin,

    The host port may be monitoring the VBUS at the root hub and sensing an overcurrent due to inrush current that way.  It is more likely that the USB host sees that the TUSB2036 reports as bus powered, and by default a bus powered USB 2.0 hub is budgeted 100 mA per downstream port.  If the device that is getting connected reports that it is a bus powered device requiring more than 100 mA, the USB driver may flag it as an overcurrent event even if there is no actual overcurrent. 

    If the same devices were used on the old PCB and the TUSB2036 was configured to report as bus-powered, I'm surprised the error didn't occur there too.  Can you try pulling up the BUSPWR/ pin on the TUSB2036 to see if the problem goes away?

    Regards,

    JMMN

  • 0 in reply to JMMN
    Prodigy 50 points

    Hi JMMN,

    I'll try that and let you know.

    Regards,
    Severin