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PCA9555: Will internal pull-up resistor impact I/O level recognization?

Josh Wei
Intellectual 1397 points
Part Number: PCA9555
Other Parts Discussed in Thread: TCA9535

Tool/software:

Hi team,

If config one of I/O port as input which has pull-up resistor, like below picture shows. Will the pull-up resistor impact the output of I/O and can't recognize the logic correctly?

For example: When P01 is initialized, P00 as an input port will read a high level due to the presence of a pull-up resistor. When the activation level of the input signal is also high, how to distinguish whether it is a high level caused by the pull-up resistor or a high level of the external input?
How to implement Open-load and other diagnostics for the P01 port?

Best,

Josh

  • 0
    TI__Mastermind 35753 points

    Hi Josh,

    Josh Wei said:
    For example: When P01 is initialized, P00 as an input port will read a high level due to the presence of a pull-up resistor. When the activation level of the input signal is also high, how to distinguish whether it is a high level caused by the pull-up resistor or a high level of the external input?
    How to implement Open-load and other diagnostics for the P01 port?

    A "HIGH" is a "HIGH" regardless of whether it is pulled up by 100k resistor or if externally driven HIGH but some other device. When reading the logic through the I2C bus on the IO expander, it will read a HIGH (logic 1). 

    If P00 is externally driven LOW past a VIL, then the pin will read a "LOW" (logic 0) regardless of the pull-up resistor. A VIL is met because the pull-down strength is stronger than the 100k pull-up strength. 

    Removing the internal pull-up resistor is not possible. Consider switching to the TCA9535 which is p2p compatible to this device but without the 100k pull-ups internal. 

    Regards,

    TYler