Markus Kammersberger wrote:
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If i compile and link the code, i get the warning

"MULTIPLE Function call PCF8574_routine CALLER 1:Startup
CALLER 2:EXT_INTERRUPT".

I have tried to use the IRQ only to generate a flag, that a key was pressed and handled the reading from PCF8574 in my main loop. Then it is working.
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This means that you are using the PCF8574 code at irq time and in the main tree.

To be save you sould code extra I2C functions (with extra vars) for the IRQ.

Using the IRQ only as a flag would be also a good idea.

Thomas