Atmega328 PWM duty cicle

[cosminnci]

I try to make a pwm with a 1,563 hz frequency but I have problems changing the duty cicle.

Code: [Select]

        const byte n = 9;
        TCCR2A = bit (WGM20) | bit (WGM21) | bit (COM2B1);
        TCCR2B = bit (WGM22) | bit (CS22) | bit (CS21) | bit (CS20);
        OCR2A =  n;
        OCR2B = ((n + 1) / 2) - 1; // 50% duty cycle
I know OCR2B is 8 bit and 255 values
but according to above I can use only 9 steps of duty cicle.

how can I use the full range of 255 steps and still have the 1,563 hz frequency?

thank you.

[Kalvin]

Have you checked whether PWM-output mode "Toggle OC2A on Compare Match" would help?

[Brutte]

What about formatting your code? 75 posts and no advance so far..

TCCR2A = bit (WGM20) | bit (WGM21) | bit (COM2B1); // fast PWM, clear OC2A on compare

The code does not match the comment, the comment does not match the code.

[cosminnci]

ok, I edited the post, using the code tags.

[obiwanjacobi]

I don't see you setting the clock source / prescaler...?

[cosminnci]

the bit (CS22) | bit (CS21) | bit (CS20) sets the prescaler to 1024

[obiwanjacobi]

Ok setting the WGM20:2 bits will select FastPWM mode (chapter 17.7.3). Looking at the Waveform Generation Mode description (Table 17-8) it looks like it only works on Output Compare A. The full period (cycle) is mainly determined by the prescaler.

So you should be setting the COM2A0:1 flags (not the COM2B) to output the signal. Do not forget to set the appropriate DDR register to set that pin to output - otherwise you will not see it (when pin is set to input).

Not sure what the numbers are to get your desired frequency but the formulas are also in the datasheet.

Hope it helps,
Marc