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Original question:

PCF8574A: Default GPIO Pin State On Power Up Is Unclear In The Datasheet - Is it input, output, what is pull-up value (if...

PCF8574A: How to define P0~P7

Zoe Ko
Expert 3810 points
Part Number: PCF8574A
Other Parts Discussed in Thread: TCA9554A

Hi team,

May I know how to define P0~P7 with input/output/High-Z mode for the PCF8574A?

Is that set by register? And which register?

Thank you!

  • 0
    TI__Mastermind 23176 points
    Hello Zoe,
    This part is a bit unique because it doesn't have registers to configure the port type (input or output), but rather by writing to the port you make it an output and by reading the port you make it into an input. This is covered in the datasheet.

    If you are looking for something that has configurable port setting then maybe look at the TCA9554A. We have several other 8 bit IO expanders you can use that have the registers. See the following app note which is a selection guide.
    www.ti.com/.../slva787.pdf
    Let me know if you have specific questions about when you program the PCF8574A or any other IO expanders you choose.
    -Francis
  • 0 in reply to fhoude
    TI__Mastermind 23176 points
    One more thing, the default state is set to a high as output but with a week pullup current source, which isn't quite an open but still not low impedance to Vcc or ground.