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103 lines
4.7 KiB
HTML
103 lines
4.7 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<title>Arduino - Volatile </title>
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<p><strong>Reference</strong> <a class='wikilink' href='index.html'>Language</a> (<a class='wikilink' href='Extended.html'>extended</a>) | <a class='wikilink' href='Libraries.html'>Libraries</a> | <a class='wikilink' href='Comparison.html'>Comparison</a> | <a class='wikilink' href='Changes.html'>Changes</a>
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<p class='vspace'></p><h2>volatile keyword</h2>
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<p>volatile is a keyword known as a variable <em>qualifier</em>, it is usually used before the datatype of a variable, to modify the way in which the compiler and subsequent program treats the variable.
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</p>
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<p class='vspace'></p><p>Declaring a variable volatile is a directive to the compiler. The compiler is software which translates your C/C++ code into the machine code, which are the real instructions for the Atmega chip in the Arduino.
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</p>
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<p class='vspace'></p><p>Specifically, it directs the compiler to load the variable from RAM and not from a storage register, which is a temporary memory location where program variables are stored and manipulated. Under certain conditions, the value for a variable stored in registers can be inaccurate.
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</p>
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<p class='vspace'></p><p>A variable should be declared volatile whenever its value can be changed by something beyond the control of the code section in which it appears, such as a concurrently executing thread. In the Arduino, the only place that this is likely to occur is in sections of code associated with interrupts, called an interrupt service routine.
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</p>
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<p class='vspace'></p><h4>Example</h4>
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<pre>
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// toggles LED when interrupt pin changes state
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int pin = 13;
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volatile int state = LOW;
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void setup()
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{
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pinMode(pin, OUTPUT);
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attachInterrupt(0, blink, CHANGE);
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}
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void loop()
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{
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digitalWrite(pin, state);
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}
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void blink()
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{
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state = !state;
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}
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</pre>
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<p class='vspace'></p><h4>See also</h4>
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<ul><li><a class='wikilink' href='AttachInterrupt.html'>AttachInterrupt</a>
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</li></ul><p class='vspace'></p><p><a class='wikilink' href='index.html'>Reference Home</a>
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<p class='vspace'></p><p><em>Corrections, suggestions, and new documentation should be posted to the <a class='urllink' href='http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?board=swbugs' rel='nofollow'>Forum</a>.</em>
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</p>
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<p class='vspace'></p><p>The text of the Arduino reference is licensed under a
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<a class='urllink' href='http://creativecommons.org/licenses/by-sa/3.0/' rel='nofollow'>Creative Commons Attribution-ShareAlike 3.0 License</a>. Code samples in the reference are released into the public domain.
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