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PCA9554 Questions

Jim CHEN
Expert 4795 points
Other Parts Discussed in Thread: PCA9554

Hi Sir,

We tested some of the noise in the PCA9554 clock signal(SCL) .(as Figure

And these noise happens to occur shock near the VIH.

I would like to ask some question~

1. What would happen when the swing happen on the cross point? (VIH / VIL)?

2. What measurement data is available to determine if this noise will cause an erroneous trigger occur?

Thank for your help.

  • 0
    TI__Genius 16450 points
    Jim,

    Noise on the rising edge like this points to a few things:
    1) There are fluctuations on the supply that powers the I2C bus.
    2) There is some ringing occuring from significant amount of parasitic inductance.


    The cross point of this device is not exactly 70%, but in reality closer to 60% (we provide some margin on the datasheet values). It also has a 50 ns de-glitch filter for noise issues like this. But if the width of this pulse is greater than 50 ns, then you can run into slave/master clock miss-sync issue. And you will have communication problems.
  • 0 in reply to Jonathan Valdez
    TI__Genius 16450 points
    Jim,

    As a follow up to this question as well, I just realized that you are seeing this on the SCL line. The source of the noise is not the PCA9554, because the PCA9554 does not have any form of internal pull-down (or pull-up) on the SCL line. For this reason, the PCA9554 cannot interfere with the SCL line.