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PCA9539: min spec of I2C input fall time ticf and output fall time tocf

Koji Ikeda
Expert 6295 points
Part Number: PCA9539

Hello support team,

Customer inquires why PCA9539 has ticf and tocf spec, could you support? They have never seen this spec in other I2C product and their usage condition is much faster than the spec, so they would like to know what happens if the time does not meet the spec.

Thanks,

Koji Ikeda 

  • 0
    Guru 316690 points

    These values are copied from the I²C specification. Their purpose is to prevent ringing on the signal lines. (If this happens on the clock line, you increase the risk of transmission errors.)

    If your outputs are faster than the minimum, then they are using more drive strength than necessary. Configure them for lower drive strength, or add a small series resistor.

  • 0
    TI__Mastermind 47188 points

    Hi Koji, 

    Ticf and Tocf has a minimum and maximum spec 20+0.1Cb < Ticf or Tocf < 300ns, based off I2C standard spec. 

    I agree with what Clemens' has stated. These specs are here in order to keep ringing effects under control. If signals are rise too fast, ringing effects become more prevalent and could hurt signal integrity. Similarly, too slow of a signal might not rise or fall in time for the controller/target device to read an accurate signal. 

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    TI__Expert 6295 points

    Hello Tyler-san, Clemens-san,

    Thank you for your support. They would like to clarify, even if the min spec can't be satisfied with the datasheet spec, if the waveform is very clean without ringing and communication can work properly, do you see any other issue or no issue?

    Thanks,
    Koji Ikeda

  • 0 in reply to Koji Ikeda
    Guru 316690 points

    If the waveform is clean (at the input of every device), then there is no problem.

  • +1 in reply to Koji Ikeda
    TI__Mastermind 47188 points

    Hi Koji,

    I agree with Clemens. If your input waveforms are generally clean on the I2C bus and you are faster than the minimum rise/fall time specs, you should be good to go.

    Regards,

    Tyler