/* $Id: WPulse.cpp 1154 2011-06-07 01:25:23Z bhagman $
||
|| @author         Hernando Barragan <b@wiring.org.co>
|| @url            http://wiring.org.co/
|| @contribution   Brett Hagman <bhagman@wiring.org.co>
||
|| @description
|| | Pulse input/output control methods.
|| | Atmel AVR 8 bit microcontroller series core.
|| |
|| | Wiring Core API
|| #
||
|| @license Please see cores/Common/License.txt.
||
*/

#include <Wiring.h>


/*************************************************************
 * Pulse Input
 *************************************************************/

/* 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); 
}

