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TCA9554/A : high-z pins ?

Michael Raleigh
Expert 3245 points
Other Parts Discussed in Thread: TCA9554, TCA9534, PCA9534A

Hello,

Wanted to confirm if the only way to enter High-Z on the TCA9554/A is by setting them as inputs?

Also, do you see any issues with powering up this device with +1.8V and pulling up an I/O pin with 100k to +3.3V, as long as we ensure the I/O pin is either an input or a low output? 

Thanks,

  • 0
    TI__Genius 16510 points

    Michael,

    You are correct that the only way to high-z the pins is to set them as an input.

    However, the TCA9554[A] has internal 100k pull-up resistors to VCC, so it would not be a good idea to pull up to 3.3 with an external 100k resistor.

    If you want the same functionality as the TCA9554 but without the internal 100k pull-up resistors on the ports, try looking at the TCA9534[A]. It's the exact same part, but without the internal resistors on the P ports. 

    With the TCA9534[A], it is OK to over drive the P ports above VCC (up to 5.5V). This is shown in the recommended operating conditions on the datasheet. If it were not OK to put the input above VCC (for an ESD diode, for example), it would have the max listed as VCC.

  • 0 in reply to Jonathan Valdez
    TI__Expert 3245 points
    Thanks for the quick reply Jonathan.

    One additional question. When changing the state of a pin from input to output, does the PCA9534A put the pin in “high” output state by default, before we change it to “low”?
  • 0 in reply to Michael Raleigh
    TI__Genius 16510 points
    Michael,

    Please make sure you use the TCA9534 over the PCA part. There is improved voltage range and power on reset performance for the TCA variant. It is the newer part, and is P2P with the PCA, and is always recommended over the PCA parts.


    In regards to your question, when switching from input to output, the state of the output register will determine the device behavior. By default (after a power on reset), the output register is filled with 1s, which corresponds to high on the output.

    You can change the state of the output register, even if the port is set as an input. You just won't see anything on the pin unless you set the configuration of the port to output.
  • 0 in reply to Jonathan Valdez
    TI__Expert 3245 points
    Yes, that was a typo, thanks for catching it.

    Again thanks for the quick response.