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TUSB8042: SMBUSz pin not described fully in datasheet

WST
Expert 1015 points
Part Number: TUSB8042

I connected up the HUB as described in the data sheet. It wouldn't respond to the upstream host and work.

I had SMBUSz pulled low -AND- the SCL/SMBCLK & SDA/SMBDAT are NCs. According to the datasheet, the HUB should have used the defaults and worked.
I did this so that in the future, if we wanted to program the HUB, we could just connect SDA and SCL with 0 ohm resistors to our CPU. The resistors were no-loads, so the SCL/SDA pins are NCs.
However, upon boot, the HUB just sat there and didn't respond to the upstream host. It was acting like it was in SMBus download mode or something like that.

The datasheet SHOULD state that disconnecting SDA and SCL ISN'T ENOUGH. I also need to FLOAT SMBUSz to turn off the external interface.

Please show me where it tells me that SMBUz can't be tied to ground, it isn't just a mode select.

From my reading of the datasheet, having SDA/SCL should be enough.

When I disconnected SMBUSz from GND, then the HUB responded to the upstream host and all of the external ports turned on.

Warren

P.S. Here are the original connections. I had to remove the 4.64K resistor to get the HUB to respond:

  • 0
    TI__Guru 60870 points

    Hi Warren,

    When SMBUS mode is selected, then the hub won't connect until it has been configured:

    Because the SMBUS could be in use by other devices in a system when the hub exits reset, the levels of SDA/SCL at de-assertion of reset cannot be used to enabled / disable SMBUS mode entry, like it can be for I2C mode entry.  However, the SMBUS obviously won't work without the pullups on SDA and SCL.

    Regards,

    JMMN

  • 0 in reply to JMMN
    Expert 1015 points

    That is fine, but the datasheet doesn't make a distinction between I2C (controller mode) and SMB (peripheral mode) with the de-assertion of reset. It really needs a note that SMBUSz needs to float or be tied high if you want the HUB to power-up and not wait for commands.