External interrupts can be used to make a voltage change on a pin (the pin going from LOW to HIGH, or vice-versa) to cause a function to be called. This can be used to avoid checking for changes on a pin “manually” by waiting in a loop until the pin changes.
External interrupts are often used to detect when events happen outside of the Maple. Example events include when a sensor has data ready to be read, or when a button has been pushed. When such an event happens, an interrupt is raised, and the Maple stops whatever it was doing (it is “interrupted”), and reacts to the event by calling a function (called an interrupt handler) which you specify using attachInterrupt().
Any pin can be used for external interrupts, but there are some restrictions. At most 16 different external interrupts can be used at one time. Further, you can’t just pick any 16 pins to use. This is because every pin on the Maple connects to what is called an EXTI line, and only one pin per EXTI line can be used for external interrupts at a time .
The EXTI Line Pin Map for your board lists which pins connect to which EXTI lines:
|||The underlying reason for this restriction is that the external interrupt lines on the STM32 are shared between GPIO ports. There can be only one external interrupt on each GPIO bit, out of all of the ports. That is, if PA4 has an external interrupt on it, then PB4 can’t have one, too. Since the GPIO bit numbers only go from 0 to 15, there can only be 16 external interrupts at a time.|