Incorrect timings on ATmega/ATtiny with internal clocks

[carlosefr]

The delay, micros and millis functions don't work properly when using the internal clocks on ATmega and ATtiny:

• On an ATmega 168/328p running at 8 MHz, the sketch below blinks the LED once a second as expected, but running at 1 MHz it blinks the LED faster (around 8 times a second);
• On an ATtiny 85 running at 8 MHz, the sketch below blinks the LED slower (around once every 8 seconds), but running at 1 MHz blinks the LED about once a second as expected.

I'm using the following configurations, which used to work correctly at least for 1, 8, and 16 MHz (external and internal clocks) the last time I checked (which was a while ago):

• https://github.com/damellis/attiny
• https://github.com/carlosefr/atmega

On top of this, the delayMicroseconds doesn't seem to work for any clock speed. But that's the case for a regular Arduino running at 16 MHz, so I suspect either this is another issue entirely or I'm not doing something right.

void myDelay(unsigned long ms) {
  unsigned long end_ms = micros() + (ms * 1000);
  
  while (1) {
    if (micros() >= end_ms) {
      break;
    }
  }
}

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

void loop() {
  digitalWrite(LED, HIGH);
  delay(1000);
  digitalWrite(LED, LOW);
  myDelay(1000);
}

[carlosefr]

the last time I checked (which was a while ago)

This part was the key. I just checked the fuses and noticed they weren't as I expected. I'd completely forgot that from the Arduino IDE one has to "Burn Bootloader" to update the fuses after selecting a different clock speed from the menu.

Sorry about the noise.