Sandbox::https://github.com/functionpointer/ArduinoCore-samd/blob/master/cores/arduino/pulse.c

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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., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA

#include <Arduino.h>

// See pulse_asm.S

extern unsigned long countPulseASM(const volatile uint32_t *port, uint32_t bit, uint32_t stateMask, unsigned long maxloops);

/* 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. */

uint32_t pulseIn(pin_size_t pin, uint8_t state, uint32_t 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.

PinDescription p = g_APinDescription[pin];

uint32_t bit = 1 << p.ulPin;

uint32_t stateMask = state ? bit : 0;

// convert the timeout from microseconds to a number of times through

// the initial loop; it takes (roughly) 13 clock cycles per iteration.

uint32_t maxloops = microsecondsToClockCycles(timeout) / 13;

uint32_t width = countPulseASM(&(PORT->Group[p.ulPort].IN.reg), bit, stateMask, maxloops);

// convert the reading to microseconds. The loop has been determined

// to be 13 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.

if (width)

return clockCyclesToMicroseconds(width * 13 + 16);

else

return 0;

}