Commented the pulseIn function.

Author: damellis

Diff for: targets/arduino/wiring.c

@@ -381,21 +381,38 @@ unsigned long pulseIn(int pin, int state)
 }
 */
 
+/* Measures the length (in microseconds) of a pulse on the pin; state is HIGH
+ * or LOW, the type of pulse to measure.  Works on pulses from 10 microseconds
+ * to 3 minutes in length, but must be called at least N microseconds before
+ * the start of the pulse. */
 unsigned long pulseIn(int pin, int state)
 {
-	unsigned long width = 0;
+	// cache the port and bit of the pin in order to speed up the
+	// pulse width measuring loop and achieve finer resolution.  calling
+	// digitalWrite() instead yields much coarser resolution.
 	int r = port_to_input[digitalPinToPort(pin)];
 	int bit = digitalPinToBit(pin);
 	int mask = 1 << bit;
+	unsigned long width = 0;
 
+	// compute the desired bit pattern for the port reading (e.g. set or
+	// clear the bit corresponding to the pin being read).  the !!state
+	// ensures that the function treats any non-zero value of state as HIGH.
 	state = (!!state) << bit;
 
+	// wait for the pulse to start
 	while ((_SFR_IO8(r) & mask) != state)
 		;
-		
+	
+	// wait for the pulse to stop
 	while ((_SFR_IO8(r) & mask) == state)
 		width++;
-		
+	
+	// convert the reading to microseconds.  the slower the CPU speed, the
+	// proportionally fewer iterations of the loop will occur (e.g. a 
+	// 4 MHz clock will yield a width that is one-fourth of that read with
+	// a 16 MHz clock).  each loop was empirically determined to take
+	// approximately 23/20 of a microsecond with a 16 MHz clock.
 	return width * (16000000UL / F_CPU) * 20 / 23;
 }