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TCA8418 SCL / SDA pullup values??

Tom Smith
Expert 2930 points


The datasheet (pg.3 / pins 22/23) specifically states 'connect to Vcc through a pullup'. The test circuit in the datasheet (pg. 28) identifies a 1K ohm pullup. The eval schematic uses a 10K ohm pullup (R5 + R18) BUT has DNI (Do Not Install). Also on the schematic, the I2C interface between the EEPROM + TUSB3210 uses a 1.5K ohm pull-ups??? It looks like 1.5K would be correct. Please advise.

-Tom-

  • 0
    TI__Intellectual 855 points

    Hi Tom,

    1) The rise time of the SDA, SCL edges depends on the I2C bus capacitance and the value of the pull-up resistor.  The pull-up resistor needs to be strong enough to allow rise times that are fast enough for the I2C bus frequency being used.

    2) The lowest value of the pull-ups is restricted by the drive strength (IOL max) of the SDA and SCL drivers.  If the pull-up resistor is too strong (low value), the SDA, SCL outputs might not be able to establish a VOL that is low enough.

    If your setup is similar to that of the EVAL board, i.e. one master, and one or two peripherals that are relatively close to each other, 1.5k pull-ups should work fine.

    Regards,

    Prasad 

  • 0 in reply to Prasad Dhond
    Expert 2930 points

    Thanks Prasad,

    Based on the eval schematic the 'INT' line uses a 10K pullup. That is accurate correct? Again the datasheet, pg.3 for the INT pin calls out 'connect to VCC through a pullup resistor'. 

    -Tom-

  • 0 in reply to Tom Smith
    TI__Intellectual 855 points

    Hi Tom,

    Yes - that is correct.  INT\ is an open-drain output and must be connected to VCC through a pull-up resistor.

    Regards,

    Prasad