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TUSB8042A: What are the internal pull-down resistors on SM-Bus I / O for

h.schwaer
Prodigy 60 points
Part Number: TUSB8042A

Hallo,

can someone explain that to me? Why pull-down resitors at an open-drain net?

Kind regards

Holger Schwär

  • 0
    TI__Guru 60870 points

    Hello Holger,

    Can you reattach the pictures?  I can't tell which pins you are talking about?

    Thanks,

    JMMN

  • 0 in reply to JMMN
    Prodigy 60 points

    Hello JMMN,

    I´m talking about Pin 38 and 37.

    Kind regard

    Holger

  • 0 in reply to h.schwaer
    TI__Guru 60870 points

    Hi Holger,

    The internal pull-up / pulldowns on the configuration pins of the hub are only enabled during the sampling period after power on reset. The pulldowns on SDA/SCL help the hub to determine if an EEPROM is connected or not.

    Regards,

    JMMN

  • 0 in reply to JMMN
    Prodigy 60 points

    Hallo JMMN,

    the problem is the pull-downs are still active after reset. But the hub works fine. Maybe a config problem? I want to access TUSB8042A via I2C were TUSB8042A is the slave.

  • 0 in reply to h.schwaer
    TI__Guru 60870 points

    Ah, you are correct these devices leave the SDA/SCL pulldowns enabled after power on reset, the other configuration pins are released.  To use the I2C interface you will need strong external pullups, 1K-2K.  If you want to access the hub as a I2C device from an I2C host, SMBUSz should be pulled low.  Please be aware that the hub will not connect as a hub in SMBUS mode until the I2C host has set the cfgactive bit in the hub register to exit configuration mode.

    Regards,

    JMMN