Electronic – ATtiny13A – Can’t generate software PWM with CTC mode


I'm trying to make a remote control RGB LED light using an ATtiny13A.

I know the ATtiny85 is better suited for this purpose, and I know I might not eventually be able to fit the whole code, but for now my main concern is to generate a software PWM using interrupts in CTC mode.

I cannot operate in any other mode (except for fast PWM with OCR0A as TOP which is basically the same thing) because the IR receiver code I am using needs a 38 kHz frequency which it generates using CTC and OCR0A=122.

So I'm trying to (and I've seen people mention this on the Internet) use the Output Compare A and Output Compare B interrupts to generate a software PWM.

OCR0A, which is also used by the IR code, determines the frequency, which I don't care about. And OCR0B, determines the duty cycle of the PWM which I'll be using for changing the LED colors.

I'm expecting to be able to get a PWM with 0-100% duty cycle by changing the OCR0B value from 0 to OCR0A. This is my understanding of what should happen:

The wave form

But what actually is happening is this (this is from Proteus ISIS simulation):

As you can see below, I'm able to get about 25%-75% duty cycle but for ~0-25% and ~75-100% the wave form is just stuck and doesn't change.

YELLOW line: Hardware PWM

RED line: Software PWM with fixed duty cycle

GREEN line: Software PWM with varying duty cycle

Oscilloscope results

And here is my code:

#ifndef        F_CPU
    #define        F_CPU        (9600000UL) // 9.6 MHz

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

int main(void)

    TCCR0A = 0x00;                        // Init to zero
    TCCR0B = 0x00;

    TCCR0A |= (1<<WGM01);                 // CTC mode
    TCCR0A |= (1<<COM0A0);                // Toggle OC0A on compare match (50% PWM on PINB0)
                                          // => YELLOW line on oscilloscope

    TIMSK0 |= (1<<OCIE0A) | (1<<OCIE0B);  // Compare match A and compare match B interrupt enabled

    TCCR0B |= (1<<CS00);                  // Prescalar 1


    DDRB = 0xFF;                          // All ports output

    while (1)
        OCR0A = 122;                      // This is the value I'll be using in my main program
        for(int i=0; i<OCR0A; i++)
            OCR0B = i;                    // Should change the duty cycle

    PORTB ^= (1<<PINB3);                  // Toggle PINB3 on compare match (50% <SOFTWARE> PWM on PINB3)
                                          // =>RED line on oscilloscope
    PORTB &= ~(1<<PINB4);                 // PINB4 LOW
                                          // =>GREEN line on oscilloscope

    PORTB |= (1<<PINB4);                  // PINB4 HIGH

Best Answer

A minimal software PWM could look like this:

volatile uint16_t dutyCycle;

uint8_t currentPwmCount;

  const uint8_t cnt = currentPwmCount + 1; // will overflow from 255 to 0
  currentPwmCount = cnt;
  if ( cnt <= dutyCyle ) {
    // Output 0 to pin
  } else {
    // Output 1 to pin

Your program sets dutyCycle to the desired value and the ISR outputs the corresponding PWM signal. dutyCycle is a uint16_t to allow for values between 0 and 256 inclusive; 256 is bigger than any possible value of currentPwmCount and thus provides full 100% duty cycle.

If you don't need 0% (or 100%) you can shave off some cycles by using a uint8_t so that either 0 results in a duty cycle of 1/256 and 255 is 100% or 0 is 0% and 255 is a duty cycle of 255/256.

You still don't have much time in a 38kHz ISR; using a little inline assembler you can probably cut the cycle count of the ISR by 1/3 to 1/2. Alternative: Run your PWM code only every other timer overflow, halving the PWM frequency.

If you have multiple PWM channels and the pins you're PMW-ing are all on the same PORT you can also collect all pins' states in a variable and finally output them to the port in one step which then only needs the read-from-port, and-with-mask, or-with-new-state, write-to-port once instead of once per pin/channel.


volatile uint8_t dutyCycleRed;
volatile uint8_t dutyCycleGreen;
volatile uint8_t dutyCycleBlue;

#define PIN_RED (0) // Example: Red on Pin 0
#define PIN_GREEN (4) // Green on pin 4
#define PIN_BLUE (7) // Blue on pin 7

#define BIT_RED (1<<PIN_RED)
#define BIT_GREEN (1<<PIN_GREEN)
#define BIT_BLUE (1<<PIN_BLUE)

#define RGB_PORT_MASK ((uint8_t)(~(BIT_RED | BIT_GREEN | BIT_BLUE)))

uint8_t currentPwmCount;

  uint8_t cnt = currentPwmCount + 1;
  if ( cnt > 254 ) {
    /* Let the counter overflow from 254 -> 0, so that 255 is never reached
       -> duty cycle 255 = 100% */
    cnt = 0;
  currentPwmCount = cnt;
  uint8_t output = 0;
  if ( cnt < dutyCycleRed ) {
    output |= BIT_RED;
  if ( cnt < dutyCycleGreen ) {
    output |= BIT_GREEN;
  if ( cnt < dutyCycleBlue ) {
    output |= BIT_BLUE;

  PORTx = (PORTx & RGB_PORT_MASK) | output;

This code maps the duty cycle to a logical 1 output on the pins; if your LEDs have 'negative logic' (LED on when pin is low), you can invert the polarity of the PWM signal by simply changing if (cnt < dutyCycle...) to if (cnt >= dutyCycle...).