Gerhard Fiedler wrote:
> This can work. But you only get an interrupt every other zero
> transition.

There are some easy ways to deal with this.  First, the power line frequency
is quite accurate, so a timer will tell you very well where the second zero
crossing is if you know the first.  Second, it's only a few instructions to
change the edge polarity of the interrupt every interrupt.  You only have to
do this ever 8.3mS.


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