1

I had performance problems with the data acquisition and after some reading on the web, someone told to define a timer with the frequency I need (50hz):

// TIMER 1 for interrupt frequency 50 Hz:  
cli(); // Stop interrupts.

TCCR1A = 0; // Set entire TCCR1A register to 0.
TCCR1B = 0; // Same for TCCR1B.
TCNT1  = 0; // Initialize counter value to 0.

// Set compare match register for 50 Hz increments.
OCR1A = 39999; // = 16000000 / (8 * 50) - 1 (must be <65536)
// Turn on CTC mode.
TCCR1B |= (1 << WGM12);
// Set CS12, CS11 and CS10 bits for 64 prescaler.
TCCR1B |= (0 << CS12) | (1 << CS11) | (1 << CS10);
// Enable timer compare interrupt.
TIMSK1 |= (1 << OCIE1A);

sei(); // Allow interrupts.

But with that, I was unable to achieve good performance; I was expecting 50 reading / seconds. Luckily, I've changed for 8 bits prescaler instead of 64 and I've start to get better performance:

// Set CS12, CS11 and CS10 bits for 8 prescaler.
TCCR1B |= (0 << CS12) | (1 << CS11) | (0 << CS10);

But, honestly, I'm a newbie and I don't understand why!

  • 1
    This is definitely no newbie-level stuff. If yo have enough patience, study gammon.com.au/timers and you will have all your answers. – Edgar Bonet Jan 25 '17 at 21:42
  • @EdgarBonet thanks for the reference. I see Atmega2560 code. I start from Atmega328 samples to implement timer. – Jonathan Anctil Jan 25 '17 at 21:51
  • 1
    The Timer 1 of the ATmega2560 and the ATmega328P are essentially identical. You can study either and apply what you learn to the other. – Edgar Bonet Jan 26 '17 at 8:18
2

// = 16000000 / (8 * 50) - 1 (must be <65536) that 8 in the formula refers to the prescaler. So this formula expect you to have a /8 prescaler. But four lines below you used a /64 prescaler. So your timer runs 6.25 Hz.

When you changed it to /8, you got the correct frequency.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.