I read the datasheet again and found that I confused about IOCON and GPPU. I now know that IOCON ODR is used to select interrupt output as push pull or open drain, and GPPU is used to enable which interrupt pin has a weak pull up.
I always thought a pull up resistor is used for setting input pins, to make sure that if the input is floating, the pull up will set the input to high.
Now I need to think again. If an output pin is push pull, does it means that there should NOT be any pull up (or down)? In other words, it is only for open drain output that I need to consider setting a pull up resistor. So if I know the output side has already an external pull up resistor, then I need not add an extra pull up (though no harm done)?
.END
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