software PWN on attiny85

Using Arduino Microcontrollers
2

hilukasz:
The code implies it should go all the way down.

Not at all...

This sequence:

    digitalWrite(spin,HIGH);
    delayMicroseconds(sp*freq);
    digitalWrite(spin,LOW);

Doesn't take 0 clock cycles to execute. The LED will be on for the time it takes to do multiply sp*freq, call/return from delayMicroseconds, do the digitalWrite(LOW), etc.

You might also get a CPU interrupt in the middle of it, slowing it down even more and causing bright flashes.

Try this:

void spwm(int freq,int spin,int sp){
  if (sp!=0) {
    //on
    digitalWrite(spin,HIGH);
    delayMicroseconds(sp*freq);
  
    // off
    digitalWrite(spin,LOW);
    delayMicroseconds(sp*(255-freq));
  }
}