Interruptions

Notes

• An interrupt is a signal from a physical device that causes the CPU to
  • Save the current state of the computation
  • Transfer control to an interrupt routine to handle the event.
  • Return to the current state
• As such interrupts routines should be very short.
• The nano uses digital pins D2 and D3 for interrupts.
  • On your board, the switch is wired to D2 as well as D4
  • In my demo, the switches are wired to D3 through an OR gate
• To enable an interrupt
  • Set the pin mode to be INPUT_PULLUP
    • pinMode(INTERRUPT_PIN, INPUT_PULLUP);
  • Then attach the pin to a ISR via attachInterrupt
    • attachInterrupt(digitalPinToInterrupt(INTERRUPT_PIN),InputHandler, RISING);
    • There are several modes
      • LOW - trigger when the value is low
      • CHANGE - trigger when the value changes
      • RISING - trigger when the pin goes from low to high
      • FALLING - trigger when the pin goes from high to low.
  • This assumes the function InputHandler is present
• In c++ the volatile qualifier marks a variable as something that should not be cached.
  • We can not pass variables to/from an ISR
  • So we communicate via global variables
  • And we mark them as volatile to make sure the value is not cached.
• An ISR
  • Should be very short
  • Should not call delay, any Serial routines
  • You should not use any other routines that depend on interrupts.
• Did the following
  • Declare global volatile bool BUTTON_HAPPENED{false};

  • void InputHandler(){
        BUTTON_HAPPENED = true;
        digitalWrite(LED_BUILTIN, HIGH);
    }

  • And in my main routine

    
        if(BUTTON_HAPPENED) {
            Serial.println("The button was pushed");
            BUTTON_HAPPENED = false;
            digitalWrite(LED_BUILTIN, LOW);
        } else {
            Serial.println("The button was NOT pushed");
        }

• There is more with interrupts, but this will do for now.