Grumpy_Mike:
If I use an ISR in mode CHANGE, I won't even care which way it flipped.
I think you are missing something. Some hall effect sensors will give a high when they see say a north pole, they continue to give high when the field is removed and only give a low with a south pole.
Oh, I'm certainly missing a lot of things. :~ But I think I do understand this. I did notice, when browsing the sensors, that a variety of modes is possible. If I use the US1881 -- a latch -- that dc42 suggested, and do an even number of magnets, alternating poles, then a change ISR would trip on both poles, alternating H/L. I won't care if it's going hi-low, or low-hi. If I'm off here, I do hope for enlightenment.