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Bit masks are used to access specific bits in a byte of data. This is often useful as a method of iteration, for example when sending a byte of data serially out a single pin. In this example the pin needs to change it's state from high to low for each bit in the byte to be transmitted. This is accomplished using what are known as bitwise operations and a bit mask.
Bitwise operations perform logical functions that take affect on the bit level. Standard bitwise operations include AND (&) OR (|) Left Shift (<<) and Right Shift (>>).
The AND (&) operator will result in a 1 at each bit position where both input values were 1. For example:
x: 10001101
y: 01010111
x & y: 00000101
The OR (|) operator (also known as Inclusive Or) will result in a 1 at each bit position where either input values were 1. For example:
x: 10001101
y: 01010111
x | y: 11011111
The Left Shift (<<) operator will shift a value to the left the specified number of times. For example:
y = 1010
x = y << 1
yields: x = 0100
All the bits in the byte get shifted one position to the left and the bit on the left end drops off.
The Right Shift (>>) operator works identically to left shift except that it shifts the value to the right the specified number of times For example:
y = 1010
x = y >> 1
yields: x = 0101
All the bits in the byte get shifted one position to the right and the bit on the right end drops off.
For a practical example, let's take the value 170, binary 10101010. To pulse this value out of pin 7 the code might look as follows:
byte transmit = 7; //define our transmit pin
byte data = 170; //value to transmit, binary 10101010
byte mask = 1; //our bitmask
byte bitDelay = 100;
void setup()
{
pinMode(transmit,OUTPUT);
}
void loop()
{
for (mask = 00000001; mask>0; mask <<= 1) { //iterate through bit mask
if (data & mask){ // if bitwise AND resolves to true
digitalWrite(transmit,HIGH); // send 1
}
else{ //if bitwise and resolves to false
digitalWrite(transmit,LOW); // send 0
}
delayMicroseconds(bitDelay); //delay
}
}
Here we use a FOR loop to iterate through a bit mask value, shifting the value one position left each time through the loop. In this example we use the <<= operator which is exactly like the << operator except that it compacts the statement mask = mask << 1 into a shorter line.
We then perform a bitwise AND operation on the value and the bitmask. This way as the bitmask shifts left through each position in the byte it will be compared against each bit in the byte we are sending sequentially and can then be used to set our output pin either high or low accordingly.
So in this example, first time through the loop the mask = 00000001 and the value = 10101010 so our operation looks like:
00000001 & 10101010 ________ 00000000
And our output pin gets set to 0. Second time throught he loop the mask = 00000010, so our operation looks like:
00000010 & 10101010 ________ 00000010
And our output pin gets set to 1. The loop will continue to iterate through each bit in the mask until the 1 gets shifted left off the end of the 8 bits and our mask =0. Then all 8 bits have been sent and our loop exits.