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170 lines
9.4 KiB
HTML
170 lines
9.4 KiB
HTML
15 years ago
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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>
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<p class='vspace'></p><h2>Bitwise AND (&), Bitwise OR (|), Bitwise XOR (^)</h2>
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<h4>Bitwise AND (&)</h4>
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<p>The bitwise operators perform their calculations at the bit level of variables. They help solve a wide range of common programming problems. Much of the material below is from an excellent tutorial on bitwise math wihch may be found <a class='urllink' href='http://www.arduino.cc/playground/Code/BitMath' rel='nofollow'>here.</a>
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</p>
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<p class='vspace'></p><h4>Description and Syntax</h4>
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<p>Below are descriptions and syntax for all of the operators. Further details may be found in the referenced tutorial.
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</p>
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<p class='vspace'></p><h4>Bitwise AND (&)</h4>
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<p>The bitwise AND operator in C++ is a single ampersand, &, used between two other integer expressions. Bitwise AND operates on each bit position of the surrounding expressions independently, according to this rule: if both input bits are 1, the resulting output is 1, otherwise the output is 0. Another way of expressing this is:
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</p>
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<p class='vspace'></p><pre> 0 0 1 1 operand1
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0 1 0 1 operand2
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----------
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0 0 0 1 (operand1 & operand2) - returned result
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</pre>
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<p class='vspace'></p><p>In Arduino, the type int is a 16-bit value, so using & between two int expressions causes 16 simultaneous AND operations to occur. In a code fragment like:
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</p>
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<p class='vspace'></p><pre> int a = 92; // in binary: 0000000001011100
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int b = 101; // in binary: 0000000001100101
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int c = a & b; // result: 0000000001000100, or 68 in decimal.
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</pre>
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<p class='vspace'></p><p>Each of the 16 bits in a and b are processed by using the bitwise AND, and all 16 resulting bits are stored in c, resulting in the value 01000100 in binary, which is 68 in decimal.
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</p>
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<p class='vspace'></p><p>One of the most common uses of bitwise AND is to select a particular bit (or bits) from an integer value, often called masking. See below for an example
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</p>
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<p class='vspace'></p><h4>Bitwise OR (|)</h4>
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<p>The bitwise OR operator in C++ is the vertical bar symbol, |. Like the & operator, | operates independently each bit in its two surrounding integer expressions, but what it does is different (of course). The bitwise OR of two bits is 1 if either or both of the input bits is 1, otherwise it is 0. In other words:
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</p>
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<p class='vspace'></p><pre> 0 0 1 1 operand1
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0 1 0 1 operand2
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----------
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0 1 1 1 (operand1 | operand2) - returned result
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</pre>
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<p class='vspace'></p><p>Here is an example of the bitwise OR used in a snippet of C++ code:
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</p>
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<p class='vspace'></p><pre> int a = 92; // in binary: 0000000001011100
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int b = 101; // in binary: 0000000001100101
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int c = a | b; // result: 0000000001111101, or 125 in decimal.
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</pre>
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<p class='vspace'></p><h4>Example Program</h4>
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<p>A common job for the bitwise AND and OR operators is what programmers call Read-Modify-Write on a port. On microcontrollers, a port is an 8 bit number that represents something about the condition of the pins. Writing to a port controls all of the pins at once.
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</p>
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<p class='vspace'></p><p>PORTD is a built-in constant that refers to the output states of digital pins 0,1,2,3,4,5,6,7. If there is 1 in an bit position, then that pin is HIGH. (The pins already need to be set to outputs with the pinMode() command.) So if we write <code>PORTD = B00110001;</code> we have made pins 2,3 & 7 HIGH.
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One slight hitch here is that we <em>may</em> also have changeed the state of Pins 0 & 1, which are used by the Arduino for serial communications so we may have interfered with serial communication.
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</p>
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<p class='vspace'></p><pre> Our algorithm for the program is:
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</pre><ul><li>Get PORTD and clear out only the bits corresponding to the pins we wish to control (with bitwise AND).
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</li><li>Combine the modified PORTD value with the new value for the pins under control (with biwise OR).
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</li></ul><p class='vspace'></p><pre>int i; // counter variable
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int j;
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void setup(){
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DDRD = DDRD | B11111100; // set direction bits for pins 2 to 7, leave 0 and 1 untouched (xx | 00 == xx)
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// same as pinMode(pin, OUTPUT) for pins 2 to 7
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Serial.begin(9600);
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}
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void loop(){
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for (i=0; i<64; i++){
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PORTD = PORTD & B00000011; // clear out bits 2 - 7, leave pins 0 and 1 untouched (xx & 11 == xx)
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j = (i << 2); // shift variable up to pins 2 - 7 - to avoid pins 0 and 1
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PORTD = PORTD | j; // combine the port information with the new information for LED pins
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Serial.println(PORTD, BIN); // debug to show masking
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delay(100);
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}
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}
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</pre>
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<p class='vspace'></p><h4>Bitwise XOR (^)</h4>
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<p>There is a somewhat unusual operator in C++ called bitwise EXCLUSIVE OR, also known as bitwise XOR. (In English this is usually pronounced "eks-or".) The bitwise XOR operator is written using the caret symbol ^. This operator is very similar to the bitwise OR operator |, only it evaluates to 0 for a given bit position when both of the input bits for that position are 1:
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</p>
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<p class='vspace'></p><pre> 0 0 1 1 operand1
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0 1 0 1 operand2
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----------
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0 1 1 0 (operand1 ^ operand2) - returned result
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</pre>
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<p class='vspace'></p><p>Another way to look at bitwise XOR is that each bit in the result is a 1 if the input bits are different, or 0 if they are the same.
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</p>
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<p class='vspace'></p><p>Here is a simple code example:
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</p>
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<p class='vspace'></p><pre> int x = 12; // binary: 1100
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int y = 10; // binary: 1010
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int z = x ^ y; // binary: 0110, or decimal 6
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</pre>
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<p class='vspace'></p><p>The ^ operator is often used to toggle (i.e. change from 0 to 1, or 1 to 0) some of the bits in an integer expression. In a bitwise OR operation if there is a 1 in the mask bit, that bit is inverted; if there is a 0, the bit is not inverted and stays the same. Below is a program to blink digital pin 5.
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</p>
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<p class='vspace'></p><pre>
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// Blink_Pin_5
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// demo for Exclusive OR
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void setup(){
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DDRD = DDRD | B00100000; // set digital pin five as OUTPUT
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Serial.begin(9600);
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}
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void loop(){
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PORTD = PORTD ^ B00100000; // invert bit 5 (digital pin 5), leave others untouched
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delay(100);
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}
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</pre>
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<p class='vspace'></p><p>See Also
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</p><ul><li><a class='wikilink' href='Boolean.html'>&&</a>(Boolean AND)
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</li><li><a class='wikilink' href='Boolean.html'>||</a>(Boolean OR)
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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>
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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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