I have a 1 Hz square wave connected to INT1, free-running so it is asynchronous WRT the µC. Using the following sketch, the interrupt appears to fire as soon as it's enabled; I consistently see 999 as the first number output. Then the time to the second interrupt varies, and is less than one second.…

External interrupt fires early?

fungus January 25, 2013, 5:03pm 2

Did you try setting the pin as an input and using the pullup resistor?

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