wiring_pulse.c

Go to the documentation of this file.

/*
  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
*/

#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.
 *
 * 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
        // 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);

        // convert the timeout from microseconds to a number of times through
        // the initial loop; it takes approximately 16 clock cycles per iteration
        unsigned long maxloops = microsecondsToClockCycles(timeout)/16;

        unsigned long width = countPulseASM(portInputRegister(port), bit, stateMask, maxloops);

        // prevent clockCyclesToMicroseconds to return bogus values if countPulseASM timed out
        if (width)
                return clockCyclesToMicroseconds(width * 16 + 16);
        else
                return 0;
}

/* 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 startMicros = micros();

        // wait for any previous pulse to end
        while ((*portInputRegister(port) & bit) == stateMask) {
                if (micros() - startMicros > timeout)
                        return 0;
        }

        // wait for the pulse to start
        while ((*portInputRegister(port) & bit) != stateMask) {
                if (micros() - startMicros > timeout)
                        return 0;
        }

        unsigned long start = micros();
        // wait for the pulse to stop
        while ((*portInputRegister(port) & bit) == stateMask) {
                if (micros() - startMicros > timeout)
                        return 0;
        }
        return micros() - start;
}