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HUB with TUSB2077A can't enumerate downstream device

jian yang
Prodigy 70 points
Other Parts Discussed in Thread: TUSB2077A

Hello,

i hope You can help me with this.

USB HUB TUSB2077A is configured as :

i connected a pull- up 1.5k resistor on DP0.

- Self-Powered ( /BUSPWR == GND ), no external power

- No EEPROM ( /EXTMEM == VCC )

 when i connected downstream device(which is self power) to this HUB, the PC device manager shows the VID&PID of downstream device, but have a question mark on this downstream device.

then power off HUB and set the  /BUSPWR == VCC, still no external power (just trick the TUSB2077A),  the downstream device is OK to enumerate.

so my question is what is the different when i connect /BUSPWR to VCC or GND. and why this happened?

 

Jianyang

 

  • 0
    TI__Mastermind 18655 points
    Hello Jianyang,

    According to USB 2.0 spec:

    Bus-powered hubs: Draw all of their power for any internal functions and downstream facing ports from VBUS on the hub’s upstream facing port. Bus-powered hubs may only draw up to one unit load upon power-up and five unit loads after configuration. The configuration power is split between allocations to the hub, any non-removable functions and the external ports. External ports in a bus-powered hub can supply only one unit load (100mA) per port regardless of the current draw on the other ports of that hub.

    Self-powered hubs: Power for the internal functions and downstream facing ports does not come from VBUS. However, the USB interface of the hub may draw up to one unit load from VBUS on its upstream facing port to allow the interface to function when the remainder of the hub is powered down. Hubs that obtain operating power externally (from the USB) must supply five unit loads to each port. VBUS. However, the USB interface of the hub may draw up to one unit load from VBUS on its upstream.

    Since you connected the pin BUSPWR\ to VCC the hub sends a descriptor to indicate that it will be powered from an external power supply, and the host will allow to connect a high current device (500mA), but in your case, your hub is actually bus-powered since it is powered via VBUS . This configuration will work fine (VBUS is able to supply enough power) but your HUB is not complying with the USB 2.0 specification.

    Regards,
    Joel
  • 0 in reply to Joel Jimenez0
    Prodigy 70 points
    Joel
    thank you for your reply.
    but you know my downstream device is self-power. it will not draw current from the USB HUB, do you mean the HUB itself can draw enough power from host then HUB will work stable.( and when i connected /BUSPWR to GND, this may not get enough power from host, then maybe cause the downstream device enumeration failure).

    Regards
    Jianyang
  • 0 in reply to jian yang
    TI__Mastermind 18655 points
    Jianyang,

    At the process of enumeration, a USB Device will send some configuration descriptors to indicate the amount of power required. When a high-current device is connected to a bus-power hub, the host will not configure it.

    Regards
  • 0 in reply to Joel Jimenez0
    Prodigy 70 points
    thank you, Joel, that is what i want.
  • 0 in reply to jian yang
    TI__Genius 9315 points
    Hi Jian Yang,

    I would appreciate if you could send me an email with the business case for this opportunity.
    josecarlos.gil@ti.com

    Regards.
    JC