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PCA9555: Strange waveform of PCA9555 while power on...

Part Number: PCA9555
Other Parts Discussed in Thread: TCA9555

Dear Sirs,

While power on, the PCA9555 will have abnormal pull low pulse on SDA to cause I2C device error.

The same design for TCA9555 is no problem.

 Please help on it!

 Below are the waveform and schematics.


  • Hi Peter,

    It is not surprising that the TCA device is preforming better than the PCA counterpart. We see that switching to the TCA version fixes a lot of the problems with the PCA devices.

    Can you explain what the signals in the scope shot are? I assume that the green signal is the SDA line.

    Have you isolated the problem to the PCA9555, are there no other devices connected to the bus?

    Is the device not able to communicate at all after it is powered on?



  • Hi Chris,

    Yes, the green waveform is SDA line.

    As customer design, the system MCU will turn on first after power on, then MCU will enable the PCA9555!

    After the PCA9555 be enabled, the SDA line will be pulled down as you saw in the scope shot, and cause the MCU cannot communicate with other devices.

    Customer also did swap test and issue follow-up with IC.

    If they remove PCA9555, the issue also disappear.

    Please help to advise how to fix it if customer still keeps PCA9555 on the system!


  • Hey Peter, 

    How many PCA units is this happening on?

    How repeatable is the issue?

    What happens if you remove the capacitor on Vcc?


  • Hi Bobby,

    As customer information, there are 6pcs in total 60pcs used now.

    And after remove C1 and C2, there is no any change for the issue.

    May I know what is the purpose to remove the Vcc capacitor?

    Please help to comment any solution that can fix this issue.


  • Hi Bobby,

    Are there any PCN or Errata documents available to share with customer?


  • Hi Peter,

    I was thinking this was an issue related to the power cycle and that the ramp rate of the Vcc is affecting the device. We don't have any errata documents for this behavior. 

    Can you contact me at  

    I think the next step is to confirm the devices are from a verified distributor and then have the failed devices go through our ATE program to see if they pass or fail.