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Why I can't output a PWM signal using an ATTiny84

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Why I can't output a PWM signal using an ATTiny84

Postby Fankun on Tue Apr 10, 2018 3:00 am

I've recently been moving from Arduino environments to AVR, and I'm programming a Tiny84 with the Sparkfun USBTiny programmer and Atmel Studio 7. For the life of me, I cannot figure out why I can't output a PWM signal.

I've tried so many different example codes, scoured the datasheet and set my own TCCR0A and TCCR0B bits to set the modes, but every program so far has either a) not lit the LED at all or b) lit the LED at full brightness, ie does not correlate to OCR0A at all. The LED MOSFET is on PB2 (OCR0A, right?) and works fine with a simple blink function. The code I'm trying currently: (this one outputs full brightness all the time. I've even tested with a normal 5V led on the output to make sure it wasn't the MOSFET having an extremely slow recovery time)
Code: Select all
#ifndef F_CPU
#define F_CPU 8000000UL // 8 MHz clock speed

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

int main(void)
    DDRB |= 1 << PB2;
    TCCR0A |= (1<<WGM00)|(1<<COM0A1);
    TCCR0B |= (1<<WGM02)|(1<<CS00);

        for (uint8_t i = 0; i < 255; i++) {
            OCR0A = i;

        for (uint8_t i = 255; i >= 0; i--) {
            OCR0A = i;

As I understand, this should be phase correct PWM, clear on OCR0A match, and no pre-scaler

And the (full) schematic:

I'm attempting to put PWM on the White control MOSFET, which is driving a small section of 12V LED strip. MOSFETs are NVR4501NT1G n-channel

This is on a custom-made board which had the problem of a broken ground plane where I haphazardly placed mousebites, so I have two bodge wires to fix that issue.

Any help would be greatly appreciated; even just an example of what should work would let me know whether this is a software issue or another problem with my board.
Posts: 1
Joined: Mon Apr 09, 2018 7:27 am

Why I can't output a PWM signal using an ATTiny84



Re: Why I can't output a PWM signal using an ATTiny84

Postby FipS on Tue Apr 10, 2018 9:02 pm

Sorry, I've never played with PWM modes on ATtiny, so I can't help much. However, after a bit of research, I've come across this page:

ATtiny2313 PWM – Pulse Width Modulation

It shows how to implement LED fading on ATtiny2313 (which I believe is similar to ATtiny84) using PWM.

BTW, I don't think it's related to the problem, but in the second for cycle, you will end up in an infinite loop due to the unsigned integer wrap-around. It should be rewritten, for example like this:

Code: Select all
for (uint8_t i = 255; i-- > 0;) {
    OCR0A = i;
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