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PCA9518: Datasheet questions

Nitish Gupta
Prodigy 340 points
Part Number: PCA9518

Hello,

Can you please help answer below questions for my customer:

    • If we are not using any I2C line from this and have tied the corresponding enable input to low, do we still need a pull-up on the corresponding I2C lines or can we leave it floating on board if its enable input is tied to GND ?
    • What is the internal pull-up resistor value for Enable input pins ?

Thanks,

Nitish

  • 0
    TI__Guru 73871 points

    Hello Nitish,

    "What is the internal pull-up resistor value for Enable input pins ?"

    Usually with our I2C devices, if they have internal pull up resistors the pull up values are quite weak (at around 80k~100k) in order to keep the supply current lower.

    "If we are not using any I2C line from this and have tied the corresponding enable input to low, do we still need a pull-up on the corresponding I2C lines or can we leave it floating on board if its enable input is tied to GND ?"

    If the channel is disabled and the channel is floating then that should be fine. My recommendation would be to have the pull ups there anyways. (You still need to have pull up resistors on EXP lines

    -Bobby

  • 0 in reply to BOBBY

    "If we are not using any I2C line from this and have tied the corresponding enable input to low, do we still need a pull-up on the corresponding I2C lines or can we leave it floating on board if its enable input is tied to GND ?"

    If the channel is disabled and the channel is floating then that should be fine. My recommendation would be to have the pull ups there anyways. (You still need to have pull up resistors on EXP lines

    For this reply -

    Why are you recommending pull-ups if an I2C is not used ?

  • 0 in reply to Mayank Dixit
    TI__Guru 73871 points

    Hello Mayank,

    The reason I suggest the pull up resistor is actually related to the start up of the device. I believe it would be best to provide a logic high on the unused I2C pins in the case the device wants to see logic highs upon start up.

    Please note that this device is quite old and usually we have access to the design database for the devices to verify something like this however I do not have this access and cannot say with 100% certainty the device will be okay without a pull up resistor. I have seen this device start up badly once before and therefore want to suggest the pull up resistor on the unused I2C lines to increase the likelihood that leaving it floating doesn't accidentally get the device started up in a bad PoR condition. (A strong pull up would be preferred over a weak pull up.)

    -Bobby