wiring_pulse.c

/*
  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;
}