PWM with ATMega164PA

Question

I am trying to increase the brightness of an LED using PWM with Timer0 on the ATMega164PA. After running my code below the LED just stays emitted and doesn't vary its brightness.

Please take a look at my code and tell me if there is something I am doing wrong:

#include <avr/io.h>
#include <util/delay.h>
#include <avr/interrupt.h>

int dutycycle = 0;  // Variable for dutycycle 

/********************************************** MAIN ****************************************************/
int main(void)
{
    DDRB |= (1 << PB3); // Make pins output and OC0A pin for PWM 

    TCCR0A |= (1 << COM0A1) | (1<<WGM01) | (1<<WGM00);  // Clear OC0A on comare match and set OC0A at BOTTOM

    TIMSK0 |= (1<<TOIE0);   // Overflow Interrupt Enabled 

    TCNT0 = 0;  // Set Counter Value Register for comparison with OCR0A

    OCR0A = (dutycycle / 100) * 255;    // Set duty cycle ON period

    sei();      // Enable global interrupts 

    TCCR0B |= (1 << CS00);  // Prescale of 1 - start timer 

    while (1)
    {
        _delay_ms(500);

        dutycycle += 10;        // increase duty cycle by 10% every 500ms 

        if (dutycycle > 100)    // if duty cycle is greater than 100% set to 0
        {
            dutycycle = 0; 
        }
    }
}

ISR(TIMER0_OVF_vect)
{
    OCR0A = (dutycycle / 100) * 255;    // Set duty cycle ON period
}

score 3 · Answer 1 · answered Feb 16 '17 at 14:49

3

I'm not sure of the logic of your approach, but I can see an obvious problem that is causing you difficulties.

Integer division does not produce a fraction. Instead, it rounds the result down to the nearest integer. This means that dutycycle / 100 will almost always be 0, since you ensure that dutycycle <= 100. So OCR0A is almost always 0. The one exception is when dutycycle is exactly 100, which sets OCR0A to 255.

One way around this is to use OCR0A = dutycycle * 255 / 100; instead. I don't know if this will fix all the problems, just the first one I see.

answered Feb 16 '17 at 14:49

UncleO

8,299
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29

I am trying to increment the duty cycle by 10% until it reaches 100%. When this happens I want to reset the duty cycle value back to 0 and repeat. I only get an output when I change the integer value at the top of my code, it seems the calculation inside the main is being ignored and OCR0A is just being assigned to the duty cycle integer. I made the change you suggested but still no luck :( – LoneCoder Feb 20 '17 at 14:29

PWM with ATMega164PA

1 Answers1