void setup()
{
  DDRD |= 0b111100;
  PORTD ^= 0b111100;
  delay(10);
  PORTD ^= 0b111100;
  Serial.begin(115200);
}

unsigned long last = 0;

byte digits[4] = {
  2, 4, 7, 1};

byte positions[4] = {
  1, 4, 6, 9};


void loop()
{
  while(!(PIND & 0b10000000));

  unsigned long micro = micros();
  unsigned long duration = micro - last;
  last = micro;

  unsigned long pdigit = duration / 10 - duration / 150;
  unsigned int blinkduration = pdigit / 50;

  for(byte c = 0; c < 4; c++)
  {
    while(micros() < micro + positions[c] * pdigit + pdigit / 4);

    PORTD ^= _BV(c + 2);
    delayMicroseconds(blinkduration);
    PORTD ^= _BV(c + 2);
  }


  if(Serial.available() >= 4)
  {
    Serial.readBytes((char*)positions, 4);
    for(byte b = 0; b < 4; b++)
      positions[b] = 10 - (positions[b] - '0') + b;
  }
  while(PIND & 0b10000000);
}