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PCA9536: Device getting locked up randomly

Part Number: PCA9536
Other Parts Discussed in Thread: TCA9534A


I am using PCA9536 in my design. I see that the device behavior is not constant. Every time I power up the behavior is different. Sometimes it works fine and sometimes it pulls the line low.

The only way to resolve is to reboot the board.

Can anyone please help me out with this?

  • Hi ayusman,

    The power-on-reset function of this device (which is used to configure the Pn ports initially as inputs) can be sensitive to how fast VCC ramps and how long it is shut off for each power cycle. In your system, is VCC at 0 V for at least 100 ms prior to power-on and is the ramp time shorter than 10 ms? (You can see Section 9.1 of the datasheet for more information on this limitation.)

  • Hi Max,

    When we power on the board, I believe the 100 ms duration does not come to picture as the board is already off. I did measure the  Vcc ramp up  time and its around 4 ms.

    Let me know if I have missed anything in my understanding of the section 9.1.



  • Ayusman,

    It sounds like your ramp behavior is within the datasheet's guidelines - thank you for confirming.

    What is the value stored in register 3 when this issue occurs? I'd like to know if the ports are unexpectedly configured as outputs or if there is some other issue unrelated to the register configuration.

  • While it does sound like your ramp profile is within the recommended parameters, would you also mind providing a scope shot of VCC?

    The issue you are having sounds like a POR errata issue, but it's possible there is a non-monotonic ramp profile causing the device to latch and become unresponsive/hang the bus.

    Out of curiosity, is there a reason you are drawn towards the PCA9536, instead of a more recent device with a much more robust POR circuit such as the TCA9534A? We do not have a 4-bit variant in the TCA flavor, but we do offer 8-bit variants. There are a few advantages of the TCA over our older PCAs, such as:

    * Significantly improved and more robust POR circuit

    * Lower VCC operating support (goes down to 1.65 V instead of 2.3 V of PCA devices)

    * Other general improvements

  • Hi Max,

    I don't know the value of register 3 as I am not to able  to access the device.

    Hi Jonathan,

    I wanted to use a 4-bit IO expander and wasnt aware that TCA are better than PCA devices.

    I am attaching the snapshot of Vcc and IO voltage ramp up. My Vcc and IO voltage (SDA,SCL) are different.



  • Ayusman,

    Your VCC ramp looks pretty much ideal. Your IO voltage behavior is very interesting.

    It looks like your IO pins are left floating? Is this correct (I haven't seen a schematic)? This is very interesting. Do you see this delay of several milliseconds before the bus rises if you remove the PCA9536?

    Yes, we typically recommend TCA devices over PCA. The reason is that the PCA devices were developed a while ago, where the simulation technologies available were not nearly as fast and sophisticated as they are now. As a result, the newer TCA devices are more robust since it is possible to run many more simulations in less time.

  • Jonathan,

    Thanks for the reply. The behavior is same even when the PCA9536 is not present. The IO voltage rail and Vcc rails both are 3.3V but are generated differently. Hence the delay in the, I am also trying to figure out why the ramp time of IO voltage is not smooth as Vcc.

    Any other thoughts on what can we do to resolve this issue.