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I have a custom ATTiny85 PCB with LEDs connected to GPIOs (PB0-4) through current-limiting resistors. All ports fade LEDs smoothly except PB2, which only switches on/off with no intermediate states. The circuit is identical for all LEDs, ruling out hardware issues. I suspect this is related to PB2's specific capabilities.

#define pin PB1

void setup() {
  pinMode(pin, OUTPUT);
}

void loop() {
  for(int i = 0; i < 256; i++) {
    analogWrite(pin, i);
    delay(10);
  }
  for(int i = 255; i >= 0; i--) {
    analogWrite(pin, i);
    delay(10);
  }
}
  • MCU: ATTiny85
  • IDE: Arduino
  • All LEDs use identical circuits
  • Other ports (PB0,1,3,4) fade normally

Is there something specific about PB2 that requires different configuration for PWM functionality? How can I enable proper fading on this pin?

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1 Answer 1

Reset to default
1

Have a look at the datasheet of the ATtiny85. Figure 11-5 shows how the Timers work when in Output Compare mode (which is the one used for PWM).

Figure 11-5 showing that the wave generator connects to OCnx pins

The wave generator on the bottom connects to the OCnx pins. So only pins marked this way can be used for hardware PWM. Now you can look at the pinout diagram at the start (Figure 1-1):

Pinout diagram of the ATtiny85, PB2 not having the OCnx marking

You see, PB2 doesn't have the OCnx marking. All other pins (besides Vcc, ground and reset) have it. So you cannot use this pin with hardware PWM.

If you really need PWM on that pin, you need to bit-bang it via software, so writing the pin directly in your code. If this fits for you depends on your code and what you want to do with it.

Also note, that you cannot generate hardware PWM signals with different duty cycles on PB3 and PB4, as one is the inverted signal of the other. May be relevant to you, if you wanted to use both these pins for PWM.

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  • It's funny you mention PB3 - I just noticed it's also behaving in a binary fashion. PB4 is fine though. Thank you for your comment. I'll likely leave things as is. Commented Nov 15, 2024 at 16:53
  • PB3 has inverted output (/OC1B) of PB4 (OC1B), you can get the signal there, but not by using arduino's analogWrite and it'll be just inverted PB4 (that's why it's skipped in analogWrite) Commented Nov 15, 2024 at 22:07

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