Core resets sometimes using interrupts(?)

Troubleshooting
2

This seems to be a common project nowadays :wink:

Just search zero_cross and you’ll find some more elaborate threads.
Another keyword in connection is SparkIntervalTimer
e.g. Alternative to Delay() within Interrupts?

But the short story - don’t use delays beyond a few microsec in an ISR!

A sidenote: digitalWriteFast() is way faster (but with less sanity checking) than digitalWrite() so it’s the way to do it inside an ISR.