Interrupts in Arduino not working (for detecting zero crossing)

[Wrichik Basu]

In my previous thread, I learned about the H11AA1 optocoupler, which can be used instead of the 4N35 to detect zero crossings with Arduino.

Arduino has a specific function for detecting interrupts: attachInterrupt(digitalPinToInterrupt(pin), ISR, mode) This function is supposed to be used rather than the loop() function. Interrupt Service Routines (ISR) have to be short, but the code inside the loop() function can often be long, which will delay the next iteration and an interrupt can be missed.

The function allows the following four modes:

mode: defines when the interrupt should be triggered. Four constants are predefined as valid values:

• LOW to trigger the interrupt whenever the pin is low,
• CHANGE to trigger the interrupt whenever the pin changes value,
• RISING to trigger when the pin goes from low to high,
• FALLING for when the pin goes from high to low.

Code:

const int PIN_INTERRUPT = 7;

void setup() {
  
    Serial.begin(9600);

    pinMode(LED_BUILTIN, OUTPUT);
    pinMode(PIN_INTERRUPT, INPUT);

    digitalWrite(LED_BUILTIN, LOW);

    attachInterrupt(digitalPinToInterrupt(PIN_INTERRUPT), zeroCrossing, FALLING);
}

void loop() {
    Serial.println(analogRead(A0));
}

void zeroCrossing(){
    digitalWrite(LED_BUILTIN, HIGH);
}

The code is quite simple: the inbuilt LED is originally off, but the first time an interrupt is detected, it is turned on, and it stays on thereafter. I do not have an oscilloscope, so I am using the Arduino serial plotter and analogRead() to plot voltage values.

Circuit:

The diagram for the optocoupler follows from the Vishay datasheet.

Problem: Arduino is not detecting the interrupts: the inbuilt LED is not glowing, which implies that the ISR is not being called.

Output of Arduino serial plotter:

The above plot has interesting characteristics — it has a periodic pattern. The spikes almost always go below 300 (< 1.24V, LOW for arduino digital logic) from > 900 (> 4.41V, HIGH for Arduino digital logic), so these are the interrupts I want to see. This implies that the optocoupler is working fine, but Arduino still does not call the ISR.

I have tried many times before; this is the best graph I got. Now I don't know what to do. I cannot keep on using the loop() function because that will later be used for user input.

All help appreciated.

 

[berkeman]

Dumb question, but for most microcontrollers (uCs), you have to "enable" the interrupt(s) before they work. I may have missed it, but I don't see mention of that in your code above...

 

[Wrichik Basu]

Dumb question, but for most microcontrollers (uCs), you have to "enable" the interrupt(s) before they work. I may have missed it, but I don't see mention of that in your code above...

Not a dumb question; I had thought about that before. It's supposed to be enabled by default in Arduinos. I also tried to specifically enable it by calling the interrupts() function, but nothing changed.

 

[Baluncore]

The code is quite simple: the inbuilt LED is originally off, but the first time an interrupt is detected, it is turned on, and it stays on thereafter.

The LED is turned on by every zero crossing ISR, so it appears to stay on.
You are seeing a beat between the mains zero crossings and the loop routine.
Maybe count the ISR calls and copy the sixth bit of that counter to the LED. You should see 1/64'th of the interrupt rate.

 

[Wrichik Basu]

The LED is turned on by every zero crossing ISR, so it appears to stay on.
You are seeing a beat between the mains zero crossings and the loop routine.
Maybe count the ISR calls and copy the sixth bit of that counter to the LED. You should see 1/64'th of the interrupt rate.

Maybe I was a bit unclear: the inbuilt LED is supposed to stay on, but it doesn't turn on at all.

 

[Baluncore]

Test the LED by turning it on and off with delays in the main Loop().
When you know you have the LED control OK, then try the interupt.

 

[Wrichik Basu]

Test the LED by turning it on and off with delays in the main Loop().
When you know you have the LED control OK, then try the interupt.

Sorry, but I don't know if I fully understand what you are saying. If I use delay() in loop() in the above code, I will miss some of those spikes. I can use delayMicroseconds() instead, but I don't see why that is necessary. If you are asking me to test whether the LED is functioning properly, then the answer is yes; digitalWrite(LED_BUILTIN, HIGH) does turn on the LED.

Actually the LED is just a visual indicator that the ISR is being called; inside the ISR, I cannot use Serial.print(), so I am turning on the LED for debugging.

 

[Wrichik Basu]

EUREKA! Found the problem. And the solution.

In Arduino, not all pins can receive interrupts:

Changed the interrupt pin to 2, and now the code works fine.