ISR (TIMER1_OVF_vect) only starts running after 3-4 seconds

Using Arduino Programming Questions
#1

Hi all

building a very simple timer that only shows seconds 0-59 and restarts at 0. I choose to use Timer 1 overflow for more precision. Using delay() did not satisfy me.

The timing should startup immediately, but it doesn’t. At startup it displays 00, but it waits 3-4 seconds before proceeding.

Variables:

long preset = 49911; // ------------------------------65535 - (1 / (1024 / 16000000) -1)

Timer initialization:

void timer_init()
{

  TCNT1 = preset;                                // for 1 sec at 16 MHz
  TCCR1A = 0x00;
  TCCR1B = (1 << CS10) | (1 << CS12);            // Timer mode with 1024 prescaler
  TIMSK1 = (1 << TOIE1) ;                        // Enable timer1 overflow interrupt(TOIE1)
  sei();                                         // Enable global interrupts by setting global interrupt enable bit in SREG

}

The actual interrupt routine:

ISR (TIMER1_OVF_vect)    // Timer1 ISR
{

  TCNT1 = preset;
  sec++;
  if (sec > 59) {
    sec = 0;
  }


}

timer_init() is called from within setup, immediately followed by an instruction to display the timer.

void setup()
{

  Serial.begin(57600);
  Serial.println("timer gestart");
  mydisplay.shutdown(0, false);  // turns on display
  mydisplay.setIntensity(0, brt);

  timer_init();
  displayTeller(sec);    // routine to display the seconds, with Serial.print or with 2 digit-7-seg display
  prevSec = sec;
}

I am using a ‘barebones’ Atmega328 at 16MHz.

Running the sketch from an Arduino Uno with Serial.printing or running it from the AtMega328 with 7 segments display does not matter, the problem remains the same: at startup the sketch immediately displays the initial value of 00 and then waits 3-4 seconds to start counting up. It looks like the timer overflow needs more than 1 sec for the first cycle…?
Any ideas what I could do?

Thanks!

Jan

(edit after first comment)

Please note that I do not see any reason why the printing should make it hang.
This is a short version with Serial.print only:

long preset = 49911; // ------------------------------65535 - (1 / (1024 / 16000000) -1)

unsigned long startTijd;
unsigned int brt     = 15, // helderheid, max 15
             sec     =  0,
             prevSec     ;

ISR (TIMER1_OVF_vect)    // Timer1 ISR
{

  TCNT1 = preset;
  sec++;
  if (sec > 59) {
    sec = 0;
  }


}

void timer_init()
{

  TCNT1 = preset;                                // for 1 sec at 16 MHz
  TCCR1A = 0x00;
  TCCR1B = (1 << CS10) | (1 << CS12);            // Timer mode with 1024 prescaler
  TIMSK1 = (1 << TOIE1) ;                        // Enable timer1 overflow interrupt(TOIE1)
  sei();                                         // Enable global interrupts by setting global interrupt enable bit in SREG

}

void displayTeller(int v) {
  Serial.println(sec);
}

void setup()
{

  Serial.begin(57600);
  Serial.println("timer gestart");

  timer_init();
  displayTeller(sec);
  prevSec = sec;
}

void loop()
{

  if (prevSec != sec) {
    displayTeller(sec);
    prevSec = sec;
  }
}

…which still hangs for 3-4 seconds after printing the first 0.
I changed the variable type of preset to int, without any difference. BTW, I have used the same code in other sketches, with preset as a long, but those sketches only needed the ISR after a period of time. That is maybe why I never noticed the hanging.

#2

Not enough information. Please post ALL the code, using code tags.
"preset" should be unsigned int, not long.

but it waits 3-4 seconds before proceeding.

Where in the code? Put in print statements to find out where it is hanging.

#3

I don’t understand the actual reason for this, but in my testing when the setting of
TCNT was not the first timer instruction, the counter worked without delay.

void timer_init()
{

  //TCNT1 = preset;                                // for 1 sec at 16 MHz
  TCCR1A = 0x00;
  TCNT1 = preset; 
  TCCR1B = (1 << CS10) | (1 << CS12);            // Timer mode with 1024 prescaler
  TIMSK1 = (1 << TOIE1) ;                        // Enable timer1 overflow interrupt(TOIE1)
  sei();                                         // Enable global interrupts by setting global interrupt enable bit in SREG

}

unsigned int preset = 49911;
volatile byte sec = 0;
volatile boolean newData = true;

void setup() {
  Serial.begin(115200);
  Serial.println("timer gestart");
  Serial.print("before init() ");
  Serial.println(TCNT1);
  timer_init();
  Serial.print("after init() ");
  Serial.println(TCNT1);
}


void loop() {
  if (newData)
  {
    newData = false;
    Serial.print(sec);
    Serial.print('\t');
    Serial.println(millis());
  }
}

void timer_init()
{
  cli();
  TCNT1 = preset;                                // for 1 sec at 16 MHz
  Serial.println(TCNT1);
  TCCR1A = 0x00;
  TCNT1 = preset;
  Serial.println(TCNT1);
  TCCR1B = (1 << CS10) | (1 << CS12);            // Timer mode with 1024 prescaler
  TIMSK1 = (1 << TOIE1) ;                        // Enable timer1 overflow interrupt(TOIE1) 
  sei();
}

ISR (TIMER1_OVF_vect)    // Timer1 ISR
{
  TCNT1 = preset;
  sec++;
  newData = true;
  if (sec > 59) {
    sec = 0;
  }
}
#4

Did an edit to my original post, with full code and more info.
TIA!

#5

Tried Cattledog’s version, and it works perfectly! Great!

Looking into the sketch and trying out the changes, I found out that there is only 1 reason: the place of

TCNT1 = preset;

This is the right order:

void timer_init()
{

  cli();
  //TCNT1 = preset;                                // for 1 sec at 16 MHz
  Serial.println(TCNT1);
  TCCR1A = 0x00;
  TCNT1 = preset;
  Serial.println(TCNT1);
  TCCR1B = (1 << CS10) | (1 << CS12);            // Timer mode with 1024 prescaler
  TIMSK1 = (1 << TOIE1) ;                        // Enable timer1 overflow interrupt(TOIE1)
  sei();
}

I commented the first, it kept working. When uncommenting the first and commenting the second, it hung for a while. Guess the timer went to it’s maximum, returned to 0 and only then it was limited to preset?

Anyway, thank you, Cattledog!

Regards
Jan

#6