ISR on NANO is not started

Projects Programming
1

I’m trying to get the demo ISR program to work on a NANO board. The code compiles and downloads OK. I have verified that the code runs by turning on the LED. However, it appears that the interrupt service routine is not activated.

bool ledOn = false;
const unsigned long TIME_TICKS = 2000; // for 100 ms interrupts at 48 MHz
const bool PRESCALER = B00000011; // Prescaler of 64
const bool ENABLE_ISR = B00000010; // Enable the ISR to generate an interrupt

ISR (TIMER1_COMPA_vect)
{
    ledOn = !ledOn;   // toggle led
    if (ledOn) {
      digitalWrite(LED_BUILTIN, HIGH);
    } else {
      digitalWrite(LED_BUILTIN, LOW);      
    }
} // End of ISR 

void setup()
{
//  flash the LED at the interrupt rate
  pinMode(LED_BUILTIN, OUTPUT);
  digitalWrite(LED_BUILTIN, LOW);
  ledOn = false;

  TCCR1A = 0;           // Init Timer1A
  TCCR1B = 0;           // Init Timer1B
  TCCR1B |= PRESCALER;  // Set Prescaler
  OCR1A = TIME_TICKS;   // Timer Compare1A Register
  TIMSK1 |= ENABLE_ISR; // Enable Timer COMPA Interrupt
}

void loop()
{
  // set up main function here
}
2

Change

bool ledOn = false;

to

volatile bool ledOn = false;

Change

const bool PRESCALER = B00000011; // Prescaler of 64
const bool ENABLE_ISR = B00000010; // Enable the ISR to generate an interrupt

to

const uint8_t PRESCALER = B00000011; // Prescaler of 64
const uint8_t ENABLE_ISR = B00000010; // Enable the ISR to generate an interrupt
3

Thanks for your help! That fixed the problem. I'll pay more attention to the constants and their types in the future.

4

Probably not necessary, since there isn't a ISR vs main path conflict.

This however is quite important. Assigning any non-zero value to a "bool" will result in a value of 1 (true), so you'll have the wrong bit pattern.

Also not that you probably don't want "normal" mode (TCCR1A = 1B = 0), since that won't reset the count when you hit the match; the counter will keep going until it overflows. (OTOH, a count of 2000 won't be visible to the human eye.)

5

Thanks for the clarification. My actual interrupt will be set for 10ms.

6

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