pulseIn: add alternative implementation based on micros()

pulseInLong is suitable for long pulses in interrupt context

hardware/arduino/avr/cores/arduino/Arduino.h

@@ -134,6 +134,7 @@ unsigned long micros(void);
 void delay(unsigned long);
 void delayMicroseconds(unsigned int us);
 unsigned long pulseIn(uint8_t pin, uint8_t state, unsigned long timeout);
+unsigned long pulseInLong(uint8_t pin, uint8_t state, unsigned long timeout);
 
 void shiftOut(uint8_t dataPin, uint8_t clockPin, uint8_t bitOrder, uint8_t val);
 uint8_t shiftIn(uint8_t dataPin, uint8_t clockPin, uint8_t bitOrder);
@@ -232,6 +233,7 @@ uint16_t makeWord(byte h, byte l);
 #define word(...) makeWord(__VA_ARGS__)
 
 unsigned long pulseIn(uint8_t pin, uint8_t state, unsigned long timeout = 1000000L);
+unsigned long pulseInLong(uint8_t pin, uint8_t state, unsigned long timeout = 1000000L);
 
 void tone(uint8_t _pin, unsigned int frequency, unsigned long duration = 0);
 void noTone(uint8_t _pin);

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

@@ -49,4 +49,46 @@ unsigned long pulseIn(uint8_t pin, uint8_t state, unsigned long timeout)
 
 	width = countPulseASM(portInputRegister(port), bit, stateMask, maxloops);
 	return clockCyclesToMicroseconds(width * 16 + 16);
-}
+}
+
+/* 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.
+ *
+ * ATTENTION:
+ * this function relies on micros() so cannot be used in noInterrupt() context
+ */
+unsigned long pulseInLong(uint8_t pin, uint8_t state, unsigned long timeout)
+{
+	// cache the port and bit of the pin in order to speed up the
+	// pulse width measuring loop and achieve finer resolution.  calling
+	// digitalRead() instead yields much coarser resolution.
+	uint8_t bit = digitalPinToBitMask(pin);
+	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.
+	unsigned long numloops = 0;
+	unsigned long maxloops = microsecondsToClockCycles(timeout);
+
+	// 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;
+
+    unsigned long start = micros();
+    // wait for the pulse to stop
+    while ((*portInputRegister(port) & bit) == stateMask) {
+        if (numloops++ == maxloops)
+            return 0;
+    }
+    return micros() - start;
+}