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TUSB8020B: Power control of downstream devices

Mark Forbes
Prodigy 110 points

Part Number: TUSB8020B
Other Parts Discussed in Thread: SN65LVPE502, TUSB522P

I have a USB3 device which I need to be able to switch on and off. It obtains all of its power from the USB port, and does not have any external supply connection. I intend to implement an on-board 2-port hub using the TUSB8020B, and it is not clear to me how one turns the power on and off. I see power switching controls and polarity selectors on the hub component, but I don't know how they are controlled.

In my particular case, I do not want the upstream USB host to be in control of the power switching. I have a local microcontroller which should do that. Can I connect the power switch directly to my microcontroller, and have it switch the power on and off as needed? It seems to me that if the power is turned off to the downstream device, it appears as though it has been removed, and would later re-initialize when power is restored.

I'm using a hub because past experience has shown that trying to implement a pass-through pair of connectors without an intermediate hub tends to be fraught with peril....impedance matching, connector losses, etc. Has anyone managed to make that work? All I really need to do is to interrupt the power; a hub is just a further (but perhaps necessary) complication.

MGF

  • 0
    TI__Guru 60870 points
    Hi Mark,

    Unfortunately USB hubs are essentially slave devices, they are controlled by the USB host which controls the downstream port power. Hubs can also leave power always on, but that doesn't work here. As you mentioned you can turn power off and on with your microcontroller but you will see the device enumeration occur. Typically, unless the downstream port power is turned off when VBUS to the hub is off (or in low power mode), you will always see that re-enumeration process occur.

    A hub is the safest way to implement a pass-through board since it retimes and redrives all the signalling upstream and downstream. Depending on your application (how much cable / trace you are driving, if the application is close to the host or the device, etc), you may be able to get away with using redrivers only.

    Regards,
    JMMN
  • 0 in reply to JMMN
    Prodigy 110 points
    Distances will be measured in inches, since this is all going inside a lab instrument. Given that, I'm thinking maybe redrivers are the right approach. I see the SN65LVPE502 dual-channel redriver listed on your website and on DigiKey. Would that be the preferred device for this kind of application? I have an embedded PC motherboard, a controller board with my MCU sitting next to it, and a camera connected via a short USB3 cable. All I need to do is to have a way to 'break into' the USB cable to interrupt the power to the camera, in case it requires a hard reset. From past experience I know that just putting two connectors down back-to-back can work for USB 1.x or even USB2, but I'm thinking that at USB3 speed that may be an impedance-mismatch-too-far for a simple wired connection.

    MGF
  • 0 in reply to Mark Forbes
    TI__Guru 60870 points
    Hi Mark,

    I would recommend using the TUSB522P device instead since the SN65LVPE502 is marked as NRND (not recommend for new designs). You may also consider putting in a USB 2 redriver if you are worried about meeting USB compliance for signal quality.

    Regards,
    JMMN