======compound bitwise AND (&=)   ======

====Description    ====

The compound bitwise AND operator (&=) is often used with a variable and a constant to force particular bits in a variable to the LOW state (to 0). This is often referred to in programming guides as "clearing" or "resetting" bits.

====Syntax:   ====
<code arduino>
x &= y;   // equivalent to x = x & y; </code>

====Parameters   ====

x: a char, int or long variable\\
y: an integer constant or char, int, or long


====Example:   ====

First, a review of the Bitwise AND (&) operator
<code arduino>
   0  0  1  1    operand1
   0  1  0  1    operand2
   ----------
   0  0  0  1    (operand1 & operand2) - returned result
</code>

Bits that are "bitwise ANDed" with 0 are cleared to 0 so, if myByte is a byte variable,\\

<code arduino>myByte & B00000000 = 0;</code> 

Bits that are "bitwise ANDed" with 1 are unchanged so, \\

<code arduino>myByte & B11111111 = myByte;</code>


Note: because we are dealing with bits in a bitwise operator - it is convenient to use the binary formatter with [[arduino:data types:IntegerConstants|constants.]] The numbers are still the same value in other representations, they are just not as easy to understand. Also, B00000000 is shown for clarity, but zero in any number format is zero (hmmm something philosophical there?)

Consequently - to clear (set to zero) bits 0 & 1 of a variable, while leaving the rest of the variable unchanged, use the compound  bitwise AND operator (&=) with the constant B11111100
<code arduino>
   1  0  1  0  1  0  1  0    variable  
   1  1  1  1  1  1  0  0    mask
   ----------------------
   1  0  1  0  1  0  0  0
   
 variable unchanged
                     bits cleared
</code>  
Here is the same representation with the variable's bits replaced with the symbol x

<code arduino>
   x  x  x  x  x  x  x  x    variable
   1  1  1  1  1  1  0  0    mask
   ----------------------
   x  x  x  x  x  x  0  0

 variable unchanged
                     bits cleared
</code> 
So if:
<code arduino>
myByte =  10101010;

myByte &= B1111100 == B10101000;</code>

See Also
* [[arduino:bitwise operators:BitwiseCompoundOr | |=]] (compound bitwise or)
* [[arduino:bitwise operators:BitwiseAnd | &]] (bitwise AND)
* [[arduino:bitwise operators:BitwiseAnd | |]] (bitwise OR)

Source: arduino.cc