root / arduino-1.0 / hardware / arduino / cores / arduino / wiring_pulse.c @ 58d82c77
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1 | 58d82c77 | Tom Mullins | /*
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2 | wiring_pulse.c - pulseIn() function
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3 | Part of Arduino - http://www.arduino.cc/
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4 | |||
5 | Copyright (c) 2005-2006 David A. Mellis
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6 | |||
7 | This library is free software; you can redistribute it and/or
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8 | modify it under the terms of the GNU Lesser General Public
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9 | License as published by the Free Software Foundation; either
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10 | version 2.1 of the License, or (at your option) any later version.
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11 | |||
12 | This library is distributed in the hope that it will be useful,
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13 | but WITHOUT ANY WARRANTY; without even the implied warranty of
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14 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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15 | Lesser General Public License for more details.
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16 | |||
17 | You should have received a copy of the GNU Lesser General
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18 | Public License along with this library; if not, write to the
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19 | Free Software Foundation, Inc., 59 Temple Place, Suite 330,
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20 | Boston, MA 02111-1307 USA
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21 | |||
22 | $Id: wiring.c 248 2007-02-03 15:36:30Z mellis $
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23 | */
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24 | |||
25 | #include "wiring_private.h" |
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26 | #include "pins_arduino.h" |
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27 | |||
28 | /* Measures the length (in microseconds) of a pulse on the pin; state is HIGH
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29 | * or LOW, the type of pulse to measure. Works on pulses from 2-3 microseconds
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30 | * to 3 minutes in length, but must be called at least a few dozen microseconds
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31 | * before the start of the pulse. */
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32 | unsigned long pulseIn(uint8_t pin, uint8_t state, unsigned long timeout) |
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33 | { |
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34 | // cache the port and bit of the pin in order to speed up the
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35 | // pulse width measuring loop and achieve finer resolution. calling
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36 | // digitalRead() instead yields much coarser resolution.
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37 | uint8_t bit = digitalPinToBitMask(pin); |
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38 | uint8_t port = digitalPinToPort(pin); |
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39 | uint8_t stateMask = (state ? bit : 0);
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40 | unsigned long width = 0; // keep initialization out of time critical area |
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41 | |||
42 | // convert the timeout from microseconds to a number of times through
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43 | // the initial loop; it takes 16 clock cycles per iteration.
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44 | unsigned long numloops = 0; |
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45 | unsigned long maxloops = microsecondsToClockCycles(timeout) / 16; |
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46 | |||
47 | // wait for any previous pulse to end
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48 | while ((*portInputRegister(port) & bit) == stateMask)
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49 | if (numloops++ == maxloops)
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50 | return 0; |
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51 | |||
52 | // wait for the pulse to start
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53 | while ((*portInputRegister(port) & bit) != stateMask)
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54 | if (numloops++ == maxloops)
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55 | return 0; |
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56 | |||
57 | // wait for the pulse to stop
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58 | while ((*portInputRegister(port) & bit) == stateMask) {
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59 | if (numloops++ == maxloops)
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60 | return 0; |
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61 | width++; |
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62 | } |
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63 | |||
64 | // convert the reading to microseconds. The loop has been determined
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65 | // to be 20 clock cycles long and have about 16 clocks between the edge
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66 | // and the start of the loop. There will be some error introduced by
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67 | // the interrupt handlers.
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68 | return clockCyclesToMicroseconds(width * 21 + 16); |
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69 | } |