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Since the main problem has been solved, I've created another question here related to the problem of the counter incrementation.

Since the main problem has been solved, I've created another question here related to the problem of the counter incrementation.

I would like to set up a timer in order to call a function 100 times per second. I'm using the Arduino Mega and Timer3 with a prescaler of 256. To choose the prescaler factor I've considered the following steps:

• CPU freq: 16MHz
• Timer resolution: 65536 (16 bits)
• Divide CPU freq by the chosen prescaler: 16x10^6/256=62500
• Divide the rest through the desired freq 62500/100=625
• Put the result in OCR3 register.

volatile int cont = 0;
unsigned long aCont = 0;
void setup()
{
 [...]
  // initialize Timer3
  cli();          // disable global interrupts
  TCCR3A = 0;     // set entire TCCR3A register to 0
  TCCR3B = 0;     // same for TCCR3B
 
  // set compare match register to desired timer count:
  OCR3A = 625; // for 100Hz
  // turn on CTC mode:
  TCCR3B |= (1 << WGM12);
  // Set CS30 and CS32 bits for 256 prescaler:
  TCCR3B |= (1 << CS32);
  // enable timer compare interrupt:
  TIMSK3 |= (1 << OCIE3A);
  // enable global interrupts:
  sei();
}

void loop()
{
 // Print every second the number of ISR invoked -> should be 100
 if ( millis() % 1000 == 0)
 {
   Serial.println();
   Serial.print(" tick: ");
   Serial.println(contatore);
   contatore = 0;
 }
}

[...]

// This is the 457-th line
ISR(TIMER3_COMPA_vect)
{
    accRoutine();
    contatore++;
}

void accRoutine()
{
  // reads analog values
}

EDIT

The counter it's supposed to increment its value 800 times per second but this is what I get:

Setup completed
 tick: 0
 tick: 0
 tick: 0
 tick: 1
 tick: 0
 tick: 0
 tick: 0
 tick: 0

It is a problem related to variable cont?

I would like to set up a timer in order to call a function 800 times per second. I'm using the Arduino Mega and Timer3 with a prescaler of 1024. To choose the prescaler factor I've considered the following steps:

• CPU freq: 16MHz
• Timer resolution: 65536 (16 bits)
• Divide CPU freq by the chosen prescaler: 16x10^6/1024=15625
• Divide the rest through the desired freq 62500/800=19.
• Put the result + 1 in OCR3 register.

volatile int cont = 0;
unsigned long aCont = 0;
void setup()
{
 [...]
  // initialize Timer3
  cli();          // disable global interrupts
  TCCR3A = 0;     // set entire TCCR3A register to 0
  TCCR3B = 0;     // same for TCCR3B
 
  // set compare match register to desired timer count: 800 Hz
  OCR3A = 20;
  // turn on CTC mode:
  TCCR3B |= (1 << WGM12);
  // Set CS10 and CS12 bits for 1024 prescaler:
  TCCR3B |= (1 << CS30) | (1 << CS32);
  // enable timer compare interrupt:
  TIMSK3 |= (1 << OCIE3A);
  // enable global interrupts:
  sei();
}

void loop()
{
 // Print every second the number of ISR invoked -> should be 100
 if ( millis() % 1000 == 0)
 {
   Serial.println();
   Serial.print(" tick: ");
   Serial.println(contatore);
   contatore = 0;
 }
}

[...]

// This is the 457-th line
ISR(TIMER3_COMPA_vect)
{
    accRoutine();
    contatore++;
}

void accRoutine()
{
  // reads analog values
}

SOLUTION

Since the main problem has been solved, I've created another question here related to the problem of the counter incrementation.