Uno interruptions not working similarly

Hi guys,

I'm working on a project where I have a DC motor rotating approximately 180 degrees back-and-forth. It should change direction every time it hits an end-switch.

I'm using interruptions, with both pin 2 and pin 3 having FALLING-interruptions. So, every time the switch is pushed down, an interruption is triggered which changes the direction of the motor.

This works perfectly with the switch connected to pin 2. However, the switch on pin 3 is giving me a headache. Every time it is pushed down, the motor completely stops, and continues moving only after the switch is released. This behaviour is obviously not suitable for my purpose. The problem seems not to be on the switch, since I've used both on both pins, and the problem seems to be pin-related.

I'm also tracking the rotation direction with an enumeration, to filter out some error interruptions.

Here's my code:

enum Direction{FORWARDS, BACKWARDS};
enum Direction dir;

void setup() {
  // put your setup code here, to run once:
  pinMode(12, OUTPUT); // motor A direction
  pinMode(9, OUTPUT);  // motor A break
  pinMode(2, INPUT_PULLUP); // switch 1 input with pull-up
  pinMode(3, INPUT_PULLUP); // switch 2 input with pull-up
  dir = FORWARDS;
  attachInterrupt(digitalPinToInterrupt(3), dir_change2, FALLING);
  attachInterrupt(digitalPinToInterrupt(2), dir_change1, FALLING);

void loop() {

void dir_change1() {
  if (dir == FORWARDS) {
    digitalWrite(9, HIGH); // engage break for channel A
    digitalWrite(12, LOW); // backward direction channel A
    digitalWrite(9, LOW); // disengage break for channel A
    dir = BACKWARDS;

void dir_change2() {
  if (dir == BACKWARDS) {
    digitalWrite(9, HIGH); // engage break for channel A
    digitalWrite(12, HIGH); // forward direction channel A
    digitalWrite(9, LOW); // disengage break for channel A
    dir = FORWARDS;

I'm using the Arduino Motor Shield V3 (with Arduino Uno).

If any of you have some tips how to solve this issue, I would be very thankful.

The variable direction should be declared as volatile since it is updated in an ISR. There may be other problems.

Edit. Why is the trigger "FALLING" ? are the switches push to break ? Anyway, since your code appears to be protected against switch bounce by an interlock, you could try "CHANGE" instead.

Hi, and thanks for the reply. The purpose of FALLING is to notice when the switch is pressed, pulling the pin to the ground. Therefore, no interruption is triggered, when it jumps back up as the motor rotates away.

Right now I'm pretty sure the problem is that the Motor Shield actually uses the D3 pin for PWM speed control. Therefore, when the pin is pulled down, the speed becomes zero, and thus the motor is not actually moving.

I think I'll just do this without interruptions.

In the case of a switch like that, the distinction between rising and falling is almost irrelevant anyway because of contact bounce and you can't assume it will be clean.

An interrupt is anyway unnecessary in this application and polling the switches and watching for status changes, as it looks like you are now intending to do, is the better solution.

Thanks, lessons learned. I got it working now.