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TCA9555: Power-on glitch

Roy Hsu
Genius 9350 points
Part Number: TCA9555
Other Parts Discussed in Thread: TCA9554

Hi Team,

I'd like to know about the power-on-glitch of the TCA9554 and TCA9555. The TCA9554 states that "No Glitch at Power Up" but similar description does not appear in the TCA9555 datasheet. Is that because we added some "glitch-suppressing" circuitry inside TCA9554 but not in the TCA9555?

How does the power-up glitch look like for the TCA9555? And is there any way to suppress the glitch?

Thanks!

Roy

  • 0
    TI__Guru* 77651 points

    Hey Roy,

    The two devices should actually have the same IP, the 9555 is just the 9554 with 8 additional p-ports.

    The glitch this is talking about is a power on reset glitch which we have seen in our PCA family of devices. I'm not sure why this was pointed out on the 9554 and not on the 9555, my guess is one of the technical owners before me modified the datasheet for 9554 but not for the rest of the IO expanders.

    All of our I2C slave devices do have 'glitch' filters on the inputs of SDA/SCL but that is a requirement set by the I2C standard.

    The glitch that was present in the PCA line up was, at very slow (or very fast) ramp rates the device may start up as outputs (random p-ports) rather than inputs which is why the first bullet says that in the TCA line up they power up as inputs.

    -Bobby

  • 0 in reply to BOBBY
    TI__Genius 9350 points

    Hi Bobby,

    Can you elaborate on the difference between "powering up as inputs" and "powering up as outputs"? What are the pros and cons?

    Thanks!

    Roy

  • 0 in reply to Roy Hsu
    TI__Guru* 77651 points

    Roy,

    If you power up as an output when you expect the device to be an input, you could damage whatever is connected to the 'i/o' pin or the i/o pin since you were expecting it to be high impedance but instead it pulls to GND or Vcc.

    If the I/O pin was originally expected to be pulled up externally or internally with a weak pull up resistor but instead powers up as an output LOW and it was connected to an enable pin of another device, this could affect the end equipment upon power up.

    There is no advantage to powering up I/O pins being output highs or lows at random. The TCA devices were meant to correct bad PoR power ups if the device was powered up outside of its intended power supply recommendation.

    -Bobby