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PCA9555: I2C not working

alpesh kavia1
Prodigy 210 points
Part Number: PCA9555

The PCA9555PWR has the following address pins;

A0 = HIGH

A1 = LOW

A2 = LOW

The first write access results in a NACK. We have tried all address options from the microcontroller and it still gives a NACK.

The start and address look correct. See the attached images. 

We tried standard and fast I2Csetting in the micro controller - both fail.

Standard had 10k pull-ups and fast had 3k3 pull-ups.

The power supply ramp time appears to be acceptable and there are no glitches.

I have got screen shots from the oscilloscope.

This is holding up the project.

Below screenshot is I2C using 10k pullup, standard setting in the DSP.

Below screenshot is I2C using 3k3 pullup, fast setting in the DSP.

Below screenshot is the power supply ramp period.

Here is the schematic, only the DSP, power supply and I2C bus. Note, the second I2C device (LTC2991) is working fine.

I2C_copy.pdf

  • +1
    TI__Guru 72871 points

    Hi Alpesh,

    In your schematic the SDA net is tied to the SCL pin of our device and the SCL net is tied to the SDA pin. 

    The power up ramp rate looks a little rough (non monotonic) as well, our PCA devices can have issues with non-default values being populated in the registers if the ramp isn't good. Issues can be resolved by writing to all the registers after power up as a workaround. 

    -Bobby