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PCF8574: pull up required when used as an input only

Bertram Brummer
Intellectual 1556 points
Part Number: PCF8574

my customer wants to use the I/o expander just to read 8 Inputs  (no output mode)  Is it mandatory to have the 100k pull-up connected to Vcclike in Figure 18?

If yes why ?  As i understand from the DS if you want to run it as Input you first need to wite a high to the output.

How are the P0-P7 are configured into input mode?

  • 0
    TI__Genius 17225 points

    Hello Bertram,
    Usually you want a pull up or pull down resistor at an input so that it never floats. When an input floats (inputs are generally high z) than it can float to any voltage based on the leakage paths inside and outside the part. That means it can get into a high current consumption state. We do have some IO expanders that internal pull ups if you are looking to save some space.

    Here is a link to an app note that goes into great detail the problem with leafing an cmos input floating.

    www.ti.com/.../scba004d.pdf
    -Francis Houde