Short pulse generation with Arduino Uno Part 1: digitalWrite() pulse width

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):

digitalWrite(outputPin, HIGH);
digitalWrite(outputPin, LOW);

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: