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TCA9555 - Abut default of I/O input

Guru 21045 points
Other Parts Discussed in Thread: TCA9555, TCA9535, PCA9555

Hi Team,

 

We have two questions.

I’d greatly appreciate your verification.

 

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[Q1]

According to the datasheet of TCA9555, “When an I/O is configured as an input, FETs Q1 and Q2 are off, creating a high-impedance input”.

 

 

However, according to “SIMPLIFIED SCHEMATIC OF P-PORT I/Os”, the pull-up resistor(100kohm) is connected between VCC pin and I/O pin.

So, when an I/O is configured as an input, is I/O pin High??

 

 

Which mention is right?

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[Q2]

Our customer would like to use TCA9555 at the following condition.

 

 

I’m understanding that the I/O(Pn) pin is the high impedance when the power on.

Because, the default of I/O is the input which is the high impedance.

 

 

Is my understanding correct?

 

If yes, I’m understanding that the LED isn’t lighted if they connect the pull-down resistor between the Pn pin and GND.

Could you please let us know if you have any concern for this usage?

 

If no, could you please let us know if you have recommended device?

And, I guess that the default of I/O of TPS9535 is the input which is the high impedance.

Is my understanding correct?

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Regards,

Kanemaru

  • Hideto-san,

    Your understanding appears to be correct.
    [Q1]: When an IO is configured as an input, the OUTPUT DRIVER is in high impedance state. The internal pull up is there to make sure the inputs do not float, and provide for a weak pull up. You are correct that the IO will be high in the input mode IF the pin is left floating. The pull up is weak enough that a pull resistor can be used to pull the line low.

    [Q2]: I don't understand why you are wanting to use a pull down resistor when it looks like you will be using the IO pin as an output to drive a MOSFET. Are you concerned that when the device first powers on, the LED could turn on briefly until the MCU can write the proper values to the registers? You can use a pull down to protect against such a case. You would have to make sure that the pull down resistor is strong enough to prevent the MOSFET from turning on, but also not so strong that you could violate IOH limit of the device (10mA)
  • Hi Jonathan-san,

     

    Thank you for the information.

    I have one more question.

    Does TCA9535 not have the internal pull-up resistor(100kohm)?

     

    Regards,

    Kanemaru

  • Kanemaru-san,

    That is correct. The TCA9535 does NOT have the internal 100k pull up. This pull up is a feature unique to the TCA/PCA9555.

  • Hi Jonathan-san,

     

    Thank you very much for your help.

    I understood. thank you very much.

     

    Regards,

    Kanemaru

  • Hi Jonathan,

    May I know that how will this device recognise a high level signal if the input port is already internally pulled up?
    Or you will suggest customer to use TCA9535?

    Thanks and best regards,
    Victor
  • Victor,

    The internal pull-up resistor is weak. 100k. This is a typical value used for buttons/keypads. It is easy to pull the input low.

    If the device is push-pull, then it is ok to to transmit a high as long as the voltage is the same as the VCC.

    If all inputs will use push-pull sources, then the TCA9535 will be a better device, since they do not require internal pull-up resistors.
  • Hi Jonathan,

    Thanks for your quick response.

    "If the device is push-pull, then it is ok to to transmit a high as long as the voltage is the same as the VCC."
    -Do you mean that though TCA9555 is internally pulled up, it still receive high signal when its port is configured as input?

    - The second question is in data sheet page 26, figure 31. As there is already pulled up to VCC internally, why it is still needed to add a 100k in parallel with the LED?

    Thanks and best regards,
    Victor
  • Victor,

    1) Yes, it will receive high signals when configured as an input. It's a 100k pull-up resistor. The 100k pull-up is useful for open-drain type inputs, but will work with push-pull as well, as long as the sinking capability is able to sink the current of Vcc/100k

    2) This graphic was reused from the TCA9535, which does not have an internal pull-up resistor. You are correct, you do not need to have an external 100k pullup because there already is one internally. I am adding this to our datasheet error backlog.
  • Hi Jonathan,

    I understand now. I will feedback it to my customer.
    Many thanks.

    Best regards,
    Victor