Interruptions

The state of a microcontroller's output signals is known, because they are generated by the program and then stored on the output ports. On the other hand, the state of a signal at the microcontroller's input is only known when it is sampled, i.e. when the program reads the signal value at the relevant input port. Consequently, if the processor needs to react quickly to a change in the state of an input signal, it should repeatedly read the corresponding input port (continuous scanning). However, in a real-time system, it is forbidden to waste processor computing time in active waiting loops.

In order to act on an external event without requiring continuous polling, processors have an interrupt mechanism. This allows the program to be modified...