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PCA9555: PCA9555 input pull up or pull down?

Part Number: PCA9555


The PCA9555 functional block diagram (datasheet figure 18) indicates there is an internal 100K pull up resistor on each I/O, causing an IO when configured as input to be pulled high.

The application diagram figure 29 on page 24 suggests that a switch can be connected using an external pull up resistor to Vcc, which would require that an input is at a logic low when floating, not an internal pull up of 100K to Vcc.

Which one of the two diagrams is correct?

Do the IO's have internal pull up resistors, in that case the external switch topology will not work.

Pls. confirm which one is correct.


  • Hello Jan,

    The functional block diagram is correct, every channel has an internal pull up so as to never leave an input floating.  We do have parts that don't have the internal pullup resistors but that means more parts for your system.  You simply have to set (program) that I/O to an output and at LOW to have it pulled to ground.  Another negative about have internal pullups is that you burn a small amount of current when the set as an output LOW. 

    The application diagram doesn't go into great detail about power up state and defaults.  I think that might be where you are getting confused. 

    Let me know if you have any more questions.  I am happy to help.