int pos = A0;
void setup()
{
  PORTD = 0b11111111;
}
void loop() {
  uint16_t pos = map(analogRead(0), 0,1023,1000,2000);  // map pot to servo us
  PORTD |= 0b100;          // Servo pulse positive
  delayMicroseconds(pos);  // pulse of 1 to 2ms determines servo position.
  PORTD &= ~0b100;         // Servo pulse off
  delay(20);               // 20ms till the next pulse.
}