Arduino interrupt returning unreliable values - arduino

Question

I have a simple app to count water flux using a sensor that is equipped with a reed switch.
So the app should only count the number of times the switch closes.
My first code was:
const int sensorPin = 2;
volatile int counter = 0;
void setup() {
Serial.begin(115200);
pinMode(sensorPin, INPUT_PULLUP);
attachInterrupt(digitalPinToInterrupt(sensorPin), sensorISR, FALLING);
}
void loop() {
Serial.print("Counter: ");
Serial.println(counter);
}
void sensorISR() {
counter++;
}
And once a bottle of 20 liters was full the counter would show something like 120.
Then I changed the code as follows:
const int sensorPin = 2;
volatile int counter = 0;
void setup() {
Serial.begin(115200);
pinMode(sensorPin, INPUT_PULLUP);
attachInterrupt(digitalPinToInterrupt(sensorPin), sensorISR, FALLING);
}
void loop() { }
void sensorISR() {
counter++;
Serial.print("Counter: ");
Serial.println(counter);
}
And counter went down to 40 (using the same 20 liters bottle).
The count should be 20L but that is not my problem as it results from bouncing of the reed switch (I will address that latter).
As the project will have 3 sensors and 3 ISRoutines I wonder why putting the Serial.print() command into the main loop can result in such strange results.
Thanks
Paulo

Answer 1

Serial print statements rely on interrupts that are disabled during your ISR. So Serial.print statements don't belong in an ISR.
The reason your count went down is that now your ISR takes longer to execute and it covers up some of the bounce. There are innumerable tutorials on how to debounce something with an Arduino. You can surely find one.
The two easiest are to use a capacitor between the pin and ground for a hardware debounce or to just use millis or micros to note the time that an interrupt occurs and ignore any interrupts that occur within some small time of that.