Quadrotor

/*

  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); 

}