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digistump-avr/cores/tiny/wiring_pulse.c

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+/*
+  wiring_pulse.c - pulseIn() function
+  Part of Arduino - http://www.arduino.cc/
+
+  Copyright (c) 2005-2006 David A. Mellis
+
+  This library is free software; you can redistribute it and/or
+  modify it under the terms of the GNU Lesser General Public
+  License as published by the Free Software Foundation; either
+  version 2.1 of the License, or (at your option) any later version.
+
+  This library is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General
+  Public License along with this library; if not, write to the
+  Free Software Foundation, Inc., 59 Temple Place, Suite 330,
+  Boston, MA  02111-1307  USA
+
+  $Id: wiring.c 248 2007-02-03 15:36:30Z mellis $
+*/
+
+#include "wiring_private.h"
+#include "pins_arduino.h"
+
+/* 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. */
+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
+	// 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) / 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++;
+	}
+
+	// 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); 
+}