clock_divider.ino

#define LED 13
#define NDIVIDERS sizeof(divider_outputs)
uint8_t divider_outputs[] = {
  12, 11, 10, 9,
  8, 7, 6, 5
};

void setup(void) {
  for (uint8_t i = 0; i < NDIVIDERS; i++) {
    pinMode(divider_outputs[i], OUTPUT);
    digitalWrite(divider_outputs[i], LOW);
  }
  pinMode(LED, OUTPUT);
  digitalWrite(LED, LOW);
  attachInterrupt(0, clock_rising, RISING);
}

uint8_t do_clock = 0;
uint16_t ticks = 0;
uint16_t led_off_at = 0;
uint16_t last_clock_at = 0;
uint16_t interval = 0;

void loop(void) {
  if (do_clock) {
    ticks++;
    digitalWrite(LED, HIGH);
    for (uint8_t i = 0; i < NDIVIDERS; i++)
      digitalWrite(divider_outputs[i], ( ticks % (i + 1)  ) == 0);
    while(millis() < led_off_at)
      ;
    digitalWrite(LED, LOW);
    do_clock = 0;
  }
}

void clock_rising(void) {
  uint16_t last = millis();
  do_clock = 1;
  led_off_at = last + 50;
  interval = last - last_clock_at;
  last_clock_at = last;
}