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TUSB2077A and overcurrent protection

Calum Mackinnon
Genius 4155 points
Other Parts Discussed in Thread: TUSB2077A

Hi,

I have a customer who is thinking of switching to the TUSB2077A due to hardware issues with another vendor. His current issue is -  when the device detects an overcurrent condition it disables the port but does not reinitialize the port when the overcurrent condition goes away - he needs a hard reset to get the prot active again. Does the TUSB2077A automatically recover when an overcurrent conditions is detected but goes away at some later stage? I read through the data sheet but it does not say what happens when the overcurrent condition is removed.

Cheers

calum  

  • 0
    TI__Guru 60870 points

    Hi Calum,

    USB hubs like the TUSB2077A don't really control the effects of an overcurrent condition when operating in power management mode.  They just report the overcurrent condition to the USB host which then sends a command to officially power off the port and then decides when to power it back on.   The TUSB2077A can't just power on the port once the overcurrent condition is removed since that decision has to be made by the USB host.  Your customer's best option may be to implement a USB hub without power management, that is to not connect the PWRON and OVRCUR between the hub and the power switch.  In that case, the customer will want to implement a power switch that does current limiting on its own and implements its own safety features.  There are quite a few power switches specifically geared for USB made by TI.

    Julie

  • 0 in reply to JMMN
    Genius 4155 points

    Thanks Julie,

    The  TUSB2077A just reports the overcurrent condition to the USB host which then sends a command to officially power off the port and then decides when to power it back on.

    I’m unclear how this works. When the port overcurrents, does the USB Host turn off the 5V to that port? Is that then enough that the TUSB2077A then resets its overcurrent state?

    Or does the host reset it’s port to the TUSB2077A, effectively resetting the entire Hub?

    Cheers

    Calum

  • 0 in reply to Calum Mackinnon
    TI__Guru 60870 points

     

    Calum,

    I misspoke before.  In a power switching hub application, when the hub OVRCUR input is asserted, the hub will immediately disable power to that port via PWRON and then report the condition to the host.  The host will then wait for the over-current condition to be resolved (OVRCUR input to be de-asserted and the hub to report that change in status to the host), then the host will turn that port's power back on using a SETPORTFEATURE (PORT POWER) command.

    From the USB spec:

    Host recovery actions for an over-current event should include the following:
    1. Host gets change notification from hub with over-current event.
    2. Host extracts appropriate hub or port change information (depending on the information in the
    change bitmap).
    3. Host waits for over-current status bit to be cleared to 0.
    4. Host cycles power to on for all of the necessary ports (e.g., issues a
    SetPortFeature(PORT_POWER) request for each port).
    5. Host re-enumerates all affected ports.

    The host does not reset the connection to the TUSB2077A or disable VBUS.

    Also, a closer reading of the spec indicates that the TUSB2077A (or any hub) must report overcurrent conditions to the host even if it does not support power switching, so OVRCUR input must be connected to the power switch even if PWRON is not.

    Julie