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TCA9555: I2C Timing Requirement Criterion

Shu-Cheng LIN
Guru 11785 points
Part Number: TCA9555
Other Parts Discussed in Thread: PCA9555

Hi Sirs,

Sorry to bother you.

We have a project use TCA9555 and run I2C frequency at 100kHz, we use oscilloscope to measure the SCL rise time that is 844ns and meets I2C 100kHz criterion.

However, BMC chipset always feedback I2C device detection error of TCA9555 until we fine-tune timing requirement to meet 400kHz criterion (rise time < 300 ns)

We think we are under the IC spec(<1000ns) but BMC chipset always feedback I2C device detection error .

Is possible to explain it why have this phenomenon?

By the way, NXP PCA9555 no issue on the same condition.

Thanks!!

  • 0
    TI__Guru 72751 points
    Hey Shu,

    Can you post scopeshots of an I2C transaction (SCL and SDA) of both the TI device and the NXP? I want to compare the two and see if there is any noticeable difference, the only thing I could think of where there would be a difference is when the two devices ACK so I would like to focus on that.

    Thanks,
    -Bobby
  • 0 in reply to BOBBY
    Guru 11785 points

    Hi Sirs,

    The fail phenomenon is our BMC will show Error Message (0x25, 7-bit à 0x4A, 8-bit) at TCA9555, but NXP PCA9555 didn't.

    We think datasheet of both are the same ( I2C input rise time max is 1000ns)

    TCA9555

    1.

    2. 

    3.

    4.

    5.

    PCA9555

    1.

    2.

    3.

    4.

  • 0 in reply to Shu-Cheng LIN
    TI__Guru 72751 points
    Hey Shu,

    I do not see anything that would indicate error with our device. If you increase the pull up resistor using the PCA9555 so that the rise time is 866 ns or greater, does the error come up?

    To me, our device is working and the issue is somehow related to the system's coding and how it checks the I2C line for error.

    It seems like if you want to avoid this issue without changing the code, you could use a stronger pull up resistor.

    Thanks,
    -Bobby
  • 0 in reply to BOBBY
    TI__Guru 62920 points
    Hi Shu-Cheng,

    Have you had a chance to review Bobby's comments above?

    I know there has been some offline discussion via email as well, so if you prefer to continue the discussion there that is OK too (in which case we could close this thread).

    Regards,
    Max