เขียน ISR ที่ปลอดภัย

An ISR, or interrupt service routine, is the function that runs when an interrupt fires. It has rules that ordinary functions do not.

The golden rule: make your ISR as short as possible. The main program is paused while it runs, so finish fast and get out.

Inside the ISR, just set a flag or bump a counter. Let the loop do the slow work like printing or math once it sees the flag.

An ISR and your loop both touch the same variable. The compiler may not expect it to change behind the loop's back, which causes subtle bugs. 🐞

Mark any variable shared with an ISR as volatile. It tells the compiler the value can change at any moment, so always reread it.

Never call delay in an ISR. The timing that delay needs is itself paused during an interrupt, so it simply will not work.

Avoid Serial.print inside an ISR. It relies on interrupts that are blocked while your handler runs, so output can stall or garble.

Inside an ISR, millis stops advancing because its background counting is paused. Read elapsed time in the loop, not the handler.

Back in the loop, check the flag, act on it, then clear it. This is where the real response safely takes place.

When the loop reads a multi-byte volatile value an ISR updates, briefly disable interrupts so you get a clean, unsplit number.

Follow these rules and your ISR stays fast and predictable. Break them and you get freezes, lost data, or values that never seem to update.

Why must a variable shared between an ISR and the loop be marked volatile?

You learned to keep an ISR short, mark shared data volatile, skip delay and Serial, and do the real work back in the loop. 🎉