pulseIn: modify function to use ASM implementation

Author: facchinm

hardware/arduino/avr/cores/arduino/wiring_private.h

@@ -43,6 +43,8 @@ extern "C"{
 #define sbi(sfr, bit) (_SFR_BYTE(sfr) |= _BV(bit))
 #endif
 
+uint32_t countPulseASM(volatile uint8_t *port, uint8_t bit, uint8_t stateMask, unsigned long maxloops);
+
 #define EXTERNAL_INT_0 0
 #define EXTERNAL_INT_1 1
 #define EXTERNAL_INT_2 2

hardware/arduino/avr/cores/arduino/wiring_pulse.c

@@ -28,7 +28,10 @@
 /* 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 2-3 microseconds
  * to 3 minutes in length, but must be called at least a few dozen microseconds
- * before the start of the pulse. */
+ * before the start of the pulse.
+ *
+ * This function performs better with short pulses in noInterrupt() context
+ */
 unsigned long pulseIn(uint8_t pin, uint8_t state, unsigned long timeout)
 {
 	// cache the port and bit of the pin in order to speed up the
@@ -38,32 +41,12 @@ unsigned long pulseIn(uint8_t pin, uint8_t state, unsigned long timeout)
 	uint8_t port = digitalPinToPort(pin);
 	uint8_t stateMask = (state ? bit : 0);
 	unsigned long width = 0; // keep initialization out of time critical area
-	
+
 	// convert the timeout from microseconds to a number of times through
-	// the initial loop; it takes 16 clock cycles per iteration.
+	// the initial loop; it takes approximately 16 clock cycles per iteration
 	unsigned long numloops = 0;
-	unsigned long maxloops = microsecondsToClockCycles(timeout) / 16;
-	
-	// wait for any previous pulse to end
-	while ((*portInputRegister(port) & bit) == stateMask)
-		if (numloops++ == maxloops)
-			return 0;
-	
-	// wait for the pulse to start
-	while ((*portInputRegister(port) & bit) != stateMask)
-		if (numloops++ == maxloops)
-			return 0;
-	
-	// wait for the pulse to stop
-	while ((*portInputRegister(port) & bit) == stateMask) {
-		if (numloops++ == maxloops)
-			return 0;
-		width++;
-	}
+	unsigned long maxloops = microsecondsToClockCycles(timeout)/16;
 
-	// convert the reading to microseconds. The loop has been determined
-	// to be 20 clock cycles long and have about 16 clocks between the edge
-	// and the start of the loop. There will be some error introduced by
-	// the interrupt handlers.
-	return clockCyclesToMicroseconds(width * 21 + 16); 
-}
+	width = countPulseASM(portInputRegister(port), bit, stateMask, maxloops);
+	return clockCyclesToMicroseconds(width * 16 + 16);
+}