I just verified that I'm able to trigger interrups very cleanly off of the falling edge of the SQW pin on this RTC at the desired 1Hz frequency.
I made use of the internal pull up only.
So I connected the SQW to pin 5 for example and I did the following.
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pinMode ( 5, INPUT_PULLUP );
attachInterrupt ( 5, sqw_isr, FALLING);
And my isr routine is getting called at the 1Hz rate I wanted.
My setup is:
Adafruit Bluefruit LE Feather M0
Adafruit DS3231 RTC FeatherWing
Just adding here for information only.
I wasn't sure whether the internal pull-up would be sufficient until I had proven it out for myself.