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TUSB8041A: Selecting SMBus Interface

Torsten Robitzki
Expert 2464 points
Part Number: TUSB8041A

Hi,

I try to enable the TUSB8041As SMB interface. For that, I've connected the SMBUSZ pin with 40k resistor to ground. Is this correct? I'm asking because this way, I'm seeing the TUSB8041A pulling the SMBCLK and SMBDAT line beeing pulled to ~2V. At the same time I'm seeing ~300mA drawn on 1.1V line. The GRSTZ pin is held to ground over a 40k resistor all the time in this scenario.

I'm measuring ~3.2V at the SMBUSZ pin. Does that mean, that 40k is too large? If so, how to dimension the required a pull-down resistor for SMBUSZ?

Here is the schematic of the whole project: https://docs.google.com/viewer?url=http%3A%2F%2Fwww.mikrocontroller.net%2Fattachment%2F493708%2Fbase-plane.pdf the TUSB8041A is on page 5.

Thanks in advance for any help, suggestion, pointers etc.

Torsten

  • +1
    TI__Guru 60870 points

    Hi Torsten,

    The internal pullup / pulldown resistors are 150 uA nominal or about 22K.  We recommend using 4.7K or smaller resistors to overdrive them.  If the hub is configured in SMBUS mode it will not connect to USB until the SMBUS host has configured it and set/cleared the cfgactive bit.

    The SDA/SCL lines have internal pulldowns that will need to be overdriven.  I recommend using 1K or 2K pullups on those lines.

    Schematic feedback:

    There is an internal pullup on GRSTz and it should either have an external cap or an external driver, an external pulldown is not recommended.

    There are two sets of resistor divider networks on USB_VBUS - page 2 and page 5, that would likely cause issues.

    The large 150 uF bulk caps on the 3.3V and 1.1V rails are fine, but make sure that if you are relying on a passive reset that GRSTz is held low long enough after the power rails ramp since they will ramp slowly.

    The isolation between chassis ground and digital ground at the connectors is not a requirement, and is application dependent.

    Regards,

    JMMN

  • 0 in reply to JMMN
    Expert 2464 points

    Hi JMMN,

    thank you very  much for the feedback. My plan was to connect the reset line of the USB hub to a microcontroller and for the time, the microcontroller needs to get up and running, to keep the reset at ground with the pull down resistor. Later, when everything is up, I wanted to dessert the reset.

    best regards,

    Torsten

  • +1 in reply to Torsten Robitzki
    TI__Guru 60870 points

    Hi Torsten,

    Your reset plan sounds good.  Just a FYI the hub power consumption is not in the lowest state when the hub is in reset so we do not recommend the hub be held in reset to reduce power.

    Regards,

    JMMN