Timer3 Arduino maximum period

Question

At the moment I'm trying to learn microcontrollers with Arduino, and have a problem with the timer.

Arduino works at 16MHz clock, and the best prescaler is 1024, which means the timer increments the value every 64 us. Timer 3 is a 16 bit timer, which gives us a maximum time of 64*65535= 4194240, ca. 4s

But what if I want to do a job every 8 seconds? Any idea?

Here's my code:

TCCR3A = 0;
TCCR3B = 0;  
TCCR3B |= (1<<WGM32); //CTC
TCCR3B |= (1 << CS32);
TCCR3B |= (1 << CS30);// 1024 prescaler 
TIMSK3 |= (1<<OCIE3A); 
sei();

Answer 1

You can't - not directly. Instead you do a job at a higher frequency which just counts. When it has counted enough you do your other job.

For instance, you might trigger your interrupt once a second. That interrupt just adds 1 to a count and returns. Unless that count just hit 8, when you reset the count to 0 and run your 8 second routine.

Answer 2

You know that the inbuilt function millis() would let you achieve that goal?

For example:

unsigned long startTime;

void loop ()
  {
  if (millis () - startTime >= 8000)
    {
    startTime = millis ();  // reset start time to now
    // do something that needs doing every 8 seconds
    }

  // do other stuff

  }  // end of loop

Answer 3

If the 5% error (~4.2 sec vs. 4.0 sec) isn't a problem, 2 counter overflows will give 8 seconds with the same 5% error, or ~8.4 sec. If you need higher accuracy, either start the timer with a slightly smaller initial counter value, (62500 * 64 = 4000000 uSec), choose a divider value that naturally overflow at convenient sub-multiples of the intervals your program will need, and again count N overlows of the timer before your code responds.