1 /* 2 wiring_pulse.c - pulseIn() function 3 Part of Arduino - http://www.arduino.cc/ 4 5 Copyright (c) 2005-2006 David A. Mellis 6 7 This library is free software; you can redistribute it and/or 8 modify it under the terms of the GNU Lesser General Public 9 License as published by the Free Software Foundation; either 10 version 2.1 of the License, or (at your option) any later version. 11 12 This library is distributed in the hope that it will be useful, 13 but WITHOUT ANY WARRANTY; without even the implied warranty of 14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU 15 Lesser General Public License for more details. 16 17 You should have received a copy of the GNU Lesser General 18 Public License along with this library; if not, write to the 19 Free Software Foundation, Inc., 59 Temple Place, Suite 330, 20 Boston, MA 02111-1307 USA 21 22 $Id: wiring.c 248 2007-02-03 15:36:30Z mellis $ 23 */ 24 25 #include "wiring_private.h" 26 #include "pins_arduino.h" 27 28 /* Measures the length (in microseconds) of a pulse on the pin; state is HIGH 29 * or LOW, the type of pulse to measure. Works on pulses from 2-3 microseconds 30 * to 3 minutes in length, but must be called at least a few dozen microseconds 31 * before the start of the pulse. */ 32 unsigned long pulseIn(uint8_t pin, uint8_t state, unsigned long timeout) 33 { 34 // cache the port and bit of the pin in order to speed up the 35 // pulse width measuring loop and achieve finer resolution. calling 36 // digitalRead() instead yields much coarser resolution. 37 uint8_t bit = digitalPinToBitMask(pin); 38 uint8_t port = digitalPinToPort(pin); 39 uint8_t stateMask = (state ? bit : 0); 40 unsigned long width = 0; // keep initialization out of time critical area 41 42 // convert the timeout from microseconds to a number of times through 43 // the initial loop; it takes 16 clock cycles per iteration. 44 unsigned long numloops = 0; 45 unsigned long maxloops = microsecondsToClockCycles(timeout) / 16; 46 47 // wait for any previous pulse to end 48 while ((*portInputRegister(port) & bit) == stateMask) 49 if (numloops++ == maxloops) 50 return 0; 51 52 // wait for the pulse to start 53 while ((*portInputRegister(port) & bit) != stateMask) 54 if (numloops++ == maxloops) 55 return 0; 56 57 // wait for the pulse to stop 58 while ((*portInputRegister(port) & bit) == stateMask) { 59 if (numloops++ == maxloops) 60 return 0; 61 width++; 62 } 63 64 // convert the reading to microseconds. The loop has been determined 65 // to be 20 clock cycles long and have about 16 clocks between the edge 66 // and the start of the loop. There will be some error introduced by 67 // the interrupt handlers. 68 return clockCyclesToMicroseconds(width * 21 + 16); 69 } 70
