LIBRARYInterrupts.
Instead of polling a pin over and over, an interrupt lets the hardware call your code the instant something happens. Why the handler must be short, and what can trigger one.
Instead of constantly asking has it happened yet, an interrupt lets the hardware call your code the instant something happens. It is how a board reacts to a button press or a sensor's data-ready line right away, without wasting cycles endlessly checking a pin that has not changed.
Polling versus interrupts
Polling means looping and reading a pin over and over, waiting for it to change. You catch the event only as often as the loop comes back around, and between checks you burn the CPU on nothing useful. An interrupt flips that around: the hardware watches for the event, and the moment it fires, it jumps the CPU straight into a handler. No wasted checking, and a near-instant response.
The ISR: short and fast
The handler is an interrupt service routine, an ISR. It runs immediately, cutting in on whatever the CPU was doing, so it has to be short. Set a flag, grab one reading, nudge a counter, then return and let the main code do the slow work when it gets to it. A long ISR blocks everything else while it runs, including other interrupts, which is how a board starts dropping events or stuttering.
What can interrupt
The common sources are a GPIO edge (a button, or a sensor's data-ready line), a timer reaching its target count (the last lesson), or a peripheral finishing a transfer. On the ESP32 you attach an interrupt to a GPIO and name the function to run when that pin sees a rising or a falling edge, and the chip handles the rest.
▸Deep dive· Debouncing: keep the messy decision out of the ISR
A mechanical button does not make one clean edge when you press it; its contacts bounce, throwing several fast edges from a single press. A raw GPIO interrupt will fire on each one, so one press looks like five. The standard fix is to have the ISR do almost nothing: just record a flag or a timestamp and return. The main loop then debounces it, ignoring repeat triggers that land within a few milliseconds of the first. The button still interrupts instantly, but the timing judgement lives in slow, ordinary code where it belongs, and the ISR stays short.
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One Thousand Drones engineering team · verified 2026-07
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