# Maple Reference Documentation: External Interrupts

External interrupts can be used to trigger routines to run in response to changes in voltage on a pin. Each GPIO pin on the Maple can be used to detect transitions such as when the voltage goes from low to high, or from high to low. This technique can be used to avoid unnecessary polling of the state of a pin.

- Overview
- Function Reference
- Recommended Reading

## Overview

External interrupts are often used to detect when events happen outside of the microcontroller. These can be used to tell the Maple when events happen, such as 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 can react to it with a preset interrupt handler.

Every GPIO pin on the Maple can be used as an external interrupt, subject to certain constraints; there can be a maximum of 16 different external interrupts set up at a time on the Maple. This is because the external interrupt lines on the STM32 are multiplexed between GPIO ports. In effect, this means that every pin on the Maple maps to a certain EXTI line, and within that EXTI line, only one of the pins that maps to it can be used as an external interrupt at a time.

The following table shows which pins can be used on which lines.

EXTI Line | Maple pins
--- | ---
EXTI0 | 2, 15, 27
EXTI1 | 3, 16, 28
EXTI2 | 1, 17, 25
EXTI3 | 0, 18
EXTI4 | 10, 19
EXTI5 | 4, 13, 20
EXTI6 | 5, 12, 35
EXTI7 | 9, 11, 36
EXTI8 | 6, 14, 37
EXTI9 | 7, 25, 28
EXTI10 | 8, 26, 29
EXTI11 | 30
EXTI12 | 31
EXTI13 | 21, 32
EXTI14 | 22, 33
EXTI15 | 23, 34

Note:You should set the pin mode of your desired pin to an input mode (e.g INPUT or INPUT_FLOATING, INPUT_PULLUP, INPUT_PULLDOWN).

## Function Reference

`attachInterrupt(pin, handler, mode)`
  This command attaches the function `handler` to run when a transition of type `mode` is detected on pin `pin`.

  Mode can be any of the following:

  RISING: To trigger the interrupt when the pin goes from low to high.
  FALLING: To trigger the interrupt when the pin goes from low to high.
  CHANGE: To trigger the interrupt when the pin goes from either low to high or high to low.
`detachInterrupt(pin)`
  Turns off the interrupt handler on pin `pin`.

## Code example

Blink the LED on every transition:

    int pin = 13;
    volatile int state = LOW;

    void setup()
    {
      pinMode(pin, OUTPUT);
      pinMode(0, INPUT_FLOATING);
      attachInterrupt(0, blink, CHANGE);
    }

    void loop()
    {
      digitalWrite(pin, state);
    }

    void blink()
    {
      state = !state;
    }

## Recommended Reading

- STMicro documentation for STM32F103RB microcontroller:
  - All
  - Datasheet (pdf)
  - Reference Manual (pdf)