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TCA9555: The Power on reset in PCA9555

Part Number: TCA9555

Hi team,

I am confused about the description about the power on reset in the datasheet.

There is a description: After that, Vcc must be lowered to 0V and then back up to the operating voltage for a power-reset cycle.

So for the power-on reset, it means I need to firstly set the Vcc to 0V and then ramp up the Vcc. Is it correct?

I also find a change about the voltage as below:

1.Does this mean by in the previous, we also need to bring the VCC to below 0.2V and then ramp up the VCC to reset the chip?

2. What does this change for? Is there any silicon change or mental change in the chip?

Lacey

Thanks a lot!

  • Hi Lacey,

    Yes, it sounds like your understanding is correct. To make sure the power-on-reset function works properly, it is helpful to make sure that the VCC voltage drops all the way to 0 V. At this point the internal nodes will be fully discharged, which is important for reset functionality.

    The datasheet previously stated 0.2 V for the minimum voltage. This value was determined by testing that forced the VCC level to the minimum voltage. However, in many applications a power cycle actually results in VCC becoming high impedance rather than driven low. This affects the internal circuitry differently - forcing the VCC level low provides a low impedance path to quickly drain any stored charge, while leaving VCC floating provides only a high-impedance (leakage) path to drain charge. Therefore, we recommend that if VCC is made high impedance that users make sure it decays all the way down to 0 V to be sure all the internal nodes have discharged. The datasheet was revised so that this point is more clear, not because of any physical changes to the device.

    Please let me know if you have any further questions regarding this.

    Regards,
    Max
  • Hi Max,

    Thanks a lot for your reply.

    According to your statement, you mean in many application, because the VCC is in high impedance in many application, so force the VCC to 0.2V can't make sure to quickly drain the stored charge. So in order to make sure the power on reset take effect, we adjust the 0.2V to 0V.

    Is it correct?

    Lacey

    Thanks a lot!

  • Lacey,

    Yes, that is correct, although I would make a minor clarification. If VCC is forced (via low impedance) to 0.2 V it should work, but if it is high impedance and allowed to float to 0.2 V then it may not. In that case, you should let it float to 0 V.

    Max