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PCA9515A: PCA9515A not good data

shiwei luan
Intellectual 750 points
Part Number: PCA9515A

I designed an I2C interface with PCA9515A show in the attached schematic.

I probed Pin3 and Pin6 when there was I2C communication on the bus, and got the data show in attached picture.

My understanding is Pin 6 (output, pink line) should repeat Pin 3 (input, blue line).

But the positions pointed by yellow arrows, there is a pulse less than 1V and it's inverted at output.

The positions pointed by green arrows, the logic low is not close to the ground.

The good is that I2C can work, but confused by the measurement. Please help to explain this problem.

Thank you,

Shiwei

  • 0
    TI__Guru 62910 points
    Hi Shiwei,

    This is the expected behavior of the device. PCA9515A, along with many other I2C buffer devices, introduces a static voltage offset to the "low" level signals so that when the part drives a low level on one side it does not propagate back to the other side (locking up the device). There is a good description of the behavior in this post: e2e.ti.com/.../2014638. Please let me know if you have any questions, though.

    Regards,
    Max