When you use the following code to generate digital pulses with the Arduino Uno (cli() and sei() and to disable and enalbe interrupts in order to ensure consistent pulse width):
cli(); digitalWrite(outputPin, HIGH); digitalWrite(outputPin, LOW); sei();
the time it takes to digitalWrite() dominates. Based on the 16 MHz clock frequency of the ATMega328p, this generates pulses exactly 4 microseconds in length:
