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70 lines
2.6 KiB
C
70 lines
2.6 KiB
C
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/*
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wiring_pulse.c - pulseIn() function
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Part of Arduino - http://www.arduino.cc/
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Copyright (c) 2005-2006 David A. Mellis
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This library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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This library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General
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Public License along with this library; if not, write to the
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Free Software Foundation, Inc., 59 Temple Place, Suite 330,
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Boston, MA 02111-1307 USA
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$Id: wiring.c 248 2007-02-03 15:36:30Z mellis $
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*/
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#include "wiring_private.h"
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#include "pins_arduino.h"
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/* Measures the length (in microseconds) of a pulse on the pin; state is HIGH
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* or LOW, the type of pulse to measure. Works on pulses from 2-3 microseconds
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* to 3 minutes in length, but must be called at least a few dozen microseconds
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* before the start of the pulse. */
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unsigned long pulseIn(uint8_t pin, uint8_t state, unsigned long timeout)
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{
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// cache the port and bit of the pin in order to speed up the
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// pulse width measuring loop and achieve finer resolution. calling
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// digitalRead() instead yields much coarser resolution.
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uint8_t bit = digitalPinToBitMask(pin);
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uint8_t port = digitalPinToPort(pin);
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uint8_t stateMask = (state ? bit : 0);
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unsigned long width = 0; // keep initialization out of time critical area
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// convert the timeout from microseconds to a number of times through
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// the initial loop; it takes 16 clock cycles per iteration.
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unsigned long numloops = 0;
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unsigned long maxloops = microsecondsToClockCycles(timeout) / 16;
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// wait for any previous pulse to end
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while ((*portInputRegister(port) & bit) == stateMask)
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if (numloops++ == maxloops)
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return 0;
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// wait for the pulse to start
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while ((*portInputRegister(port) & bit) != stateMask)
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if (numloops++ == maxloops)
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return 0;
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// wait for the pulse to stop
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while ((*portInputRegister(port) & bit) == stateMask) {
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if (numloops++ == maxloops)
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return 0;
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width++;
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}
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// convert the reading to microseconds. The loop has been determined
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// to be 20 clock cycles long and have about 16 clocks between the edge
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// and the start of the loop. There will be some error introduced by
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// the interrupt handlers.
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return clockCyclesToMicroseconds(width * 21 + 16);
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}
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