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PCA9534A: PCA9534A holding SDA low at power up

Amy Domino
Prodigy 60 points
Part Number: PCA9534A
Other Parts Discussed in Thread: PCA9554A, TCA9534A, TCA9554A, TCA9534

Hello,

I am using 3x PCA9534A IO expanders and sometimes the device will hold the SDA line low at power up. All that is on the bus is a micro controller and 3x PCA9534As. The micro is modular so I removed that and still see SDA line pulled low during power up. The VCC 3.3V rail looks clean  (no glitches) and rises in about 20ms. SDA starts to rise with VCC and once VCC hits about 1V, the SDA line get's pulled low. This happens maybe 30% of the time when I power up. When VCC hits Vpor, the outputs should go to their default state but what about SDA? Is this a know issue with PCA9534A?

I was using the NXP version of PCA9554A w/o this issue, but switched to this part due to supply chain issues. All my IO have external pull downs, so the internal pull ups are not needed.

  • 0
    Prodigy 60 points

    the pink signal is SDA and blue is VCC. It is 1V/div so SDA actually only gets to about 0.5V

  • 0 in reply to Amy Domino
    TI__Guru 67630 points

    Hi Amy,

    Is it possible for you to ramp up Vcc from GND? It looks like the blue signal is hanging around 200mV during the power up process. Our PCA family has had issues with power ups when they are done above GND. The other option is to use our TCA family which has a more robust power up circuit. The TCA9534A/TCA9554A should be pin to pin and be a functional replacement.

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 60 points

    Hey Bobby,

    Yeah - I have a lot of capacitance on the 3v3 rail and it takes a long while to fall when powered down. If I allow the rail to drop below 200mV before powering up again, the part can come up properly. I added a bleed resistor from 3v3 to gnd to drop the rail faster (~300ms) and that helped. I would rather not have to have that resistor, but looks like the TCA9534 does not come in the QFN package. I'll keep that in mind next time I rev the board though. 

    Also I am curious. Is the Vporf value in the table below invalid then? If not, what is the difference between the 0.2V voltage threshold and the 0.767V Vporf threshold?

  • 0 in reply to Amy Domino
    TI__Guru 67630 points

    Hi Amy,

    This device was one where we shut down the FAB site for and instead replaced the device with newer dies made in a newer process technology. We updated the datasheet when this occurred and sent out PCNs during that time. The newer dies have a most robust PoR circuit similar to the TCA devices in which they have both a rising and falling PoR. 

    If you'd like, you can send me a message offline at duynguyen@ti.com and I can send you the older datasheet which would have spec's that are inline with the die you have.

    -Bobby