The inputs on the NXP PCAL6524 i/o expander can be configured with internal pull-up or pull-down resistors.

The datasheet states, "External resistors are required for inputs... that may float. If a driver to an input will never let the input float, a resistor is not needed with internal pull-up or pull-down resistor option."

I thought the whole point of a pull-up or pull-down resistor was to allow the input to float, so I don't understand what the datasheet is saying. Why would you need or want a pull-up or pull-down resistor if the input is not floating?


1 Answer 1


What the datasheet is telling you is that if you DON'T use the internal pull-up or pull-down option, the thing driving that pin must always pull up or down or else you must use an external resistor to prevent floating. A resistor is NOT needed with internal pull-up or pull-down resistor option enabled.

The pull up/down resistors are to prevent floating, not allow it. Floating inputs can cause problems from wasting power, to oscillating, and even other troubles. Digital inputs should always be either high or low, not allowed to float around willy-nilly.

  • 1
    \$\begingroup\$ Thanks for your reply. That's what I thought, but the datasheet as I read it seems to say the opposite, hence my confusion. Maybe it's just not very clearly worded. \$\endgroup\$
    – Slothrop
    Apr 5, 2023 at 14:21
  • 2
    \$\begingroup\$ I agree, it is not worded as clearly as it should be. If you read it with what I have wrote in mind, I think you can see that is what they are getting at, however (and this is typical for these types of devices). \$\endgroup\$ Apr 5, 2023 at 14:22

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