Embedded firmware engineer

It tells the compiler the value can change outside the code it can see, for example in an interrupt or a hardware register, so it must read it every time instead of keeping a copy.

As little as possible: record what happened, set a flag or copy data into a buffer, and return. The main loop does the slow work. Long handlers delay other interrupts.

It blocks: nothing else runs while it waits. Checking millis() against a saved time lets one loop handle many tasks, each at its own rate.

I2C uses two shared wires and addresses for many devices, at modest speed. SPI uses separate select lines and is faster. UART is two wires point to point, with no clock, so both ends agree on a baud rate.

It holds an input at a known high level when nothing drives it, so it does not float. A button then pulls the pin low when pressed.

A mechanical switch bounces for a few milliseconds when pressed. Debouncing ignores changes inside a short window so one press counts once.

Switching a pin on and off fast with a set on-time fraction, the duty cycle. The average power sets LED brightness or motor speed.

OR it with a mask: reg |= (1 << n). Clear it with AND and the inverted mask: reg &= ~(1 << n).

Check power first with a scope (a dip on the supply when a motor starts is common), then look for a watchdog, a stack overflow or a bad pointer. Reproduce it before changing anything.

A microcontroller has its processor, memory and peripherals on one chip and runs one program. A microprocessor needs external memory and usually runs an operating system.