查看Cores:wiring pulse.c的源代码

<font color="quartz" size ="+2"> '''pulse.c ''' </font>

<pre style="color:royalblue">
/*
  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. There will be some error introduced by
	// the interrupt handlers.

	// Conversion constants are compiler-dependent, different compiler versions
	// have different levels of optimization.
#if __GNUC__==4 && __GNUC_MINOR__==3 && __GNUC_PATCHLEVEL__==2
	// avr-gcc 4.3.2
	return clockCyclesToMicroseconds(width * 21 + 16);
#elif __GNUC__==4 && __GNUC_MINOR__==8 && __GNUC_PATCHLEVEL__==1
	// avr-gcc 4.8.1
	return clockCyclesToMicroseconds(width * 24 + 16);
#elif __GNUC__<=4 && __GNUC_MINOR__<=3
	// avr-gcc <=4.3.x
	#warning "pulseIn() results may not be accurate"
	return clockCyclesToMicroseconds(width * 21 + 16);
#else
	// avr-gcc >4.3.x
	#warning "pulseIn() results may not be accurate"
	return clockCyclesToMicroseconds(width * 24 + 16);
#endif

}

</pre>


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