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LM48560TLEVAL: I2C software mode SDA line permanently pulled to GND

Ena Peh
Prodigy 210 points
Part Number: LM48560TLEVAL
Other Parts Discussed in Thread: LM48560

Tool/software:

Hi TI Experts,

I'm currently evaluating the LM48560 using its evaluation board and suspect that I may have inadvertently damaged the IC. I'm configuring the output via software mode, and the I²C signals are coming from a separate evaluation board. However, I've observed that the SDA line appears to be shorted to ground on my oscilloscope—even after isolating all other connections. I'm unable to determine the cause of this persistent short.

Additionally, I’d like to clarify the recommended power-up sequence for this board. In my setup, the pull-up voltage for the SDA line is applied before powering on the LM48560 evaluation board. Should the board be powered up before any I²C communication (SDA/SCL signals) is initiated? Is there a way to verify whether this sequence is critical or if it may have contributed to the issue?

Appreciate any guidance you can provide!

Best 

Ena

  • 0
    TI__Guru** 109395 points

    Hi Ena,

    The EVM has SDA and SCL pull-up resistors, you can use these instead of other pull-ups on the host side to prevent power reaching the device before powering up the EVM.
    It is recommended that external signals are only applied when the device is powered up.

    You may check the pins impedance to GND using a multimeter. You can compare against a new IC to know if the device has been damaged.

    Best regards,
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to Ivan Salazar
    Prodigy 210 points

    Hi Ivan,

    Thanks for the response! Appreciate this. I'm curious—why is it recommended to apply the external signal only after the LM48560 device has been powered on?

    I measured the impedance between the SDA line and ground using a multimeter, both before and after powering up the device. Initially, the impedance was very high, but once the power supply was turned on, the line appeared to be shorted to ground with a resistance of approximately 50 Ohms.

    I also tested a spare LM48560 evaluation board, and it exhibited the same behavior but with the impedance of approximately 100Ohms.

    This makes me wonder if the issue is related to the power-up sequence. Specifically, should the board's power supply be activated before introducing I²C communication and analog signals? Or is it because the current around the pullup is "killing" the device?

    Best Regards

    Ena

  • 0 in reply to Ena Peh
    TI__Guru** 109395 points

    Hi Ena,

    In general, the input voltage specs are set in reference to VDD supply, so if VDD is GND the reference is 0V itself. There are some diodes and other protection but in general it is best not to drive the device pins when not powered.

    Regarding he I2C pins, if you have pull-up resistors on the EVM and also on the host side, the resulting pull-up resistor may be too low, however I would expect I2C fail to ACK before any damage to the device occurs. As I mentioned before, if you have double pair of pull-up resistors you can remove one and just keep the ones on the EVM.

    Best regards,
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to Ivan Salazar
    Prodigy 210 points

    Hi Ivan,

    Thanks for the response! The pullup has actually been removed over at the host side. The ACK did not show up on the oscilloscope and the SDA line is just shorted to ground with 50Ohm impedance permanently.

    So to say this , can I verify that the short may have been because of the pullup resistor used and the power up sequence?

    Best Regards

    Ena

  • 0 in reply to Ena Peh
    TI__Guru** 109395 points

    HI Ena,

    That is a possible reason.

    Best regards,
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to Ivan Salazar
    Prodigy 210 points

    Hi Ivan,

    Thanks.

    What other steps can I take to verify the cause of the short?

    I just want to know the reason behind the short.

    Best 

    Ena

  • 0 in reply to Ena Peh
    TI__Guru** 109395 points

    Hi Ena,

    If the short condition is not repeatable the root cause analysis is complicated, not sure I can help much more in this case specially remotely.

    Best regards,
    -Ivan Salazar
    Applications Engineer