EEPROM - Arduino Sketches: Tools and Techniques for Programming Wizardry - page 107

Now, time to try that again with RED . The value of CYAN is 110, and RED is 001.
The i rst two bits are 1 in CYAN and 0 in RED . They return 0. The third bit is 0 in
CYAN and 1 in RED . The logical AND process returns 000. There is no RED in CYAN
because CYAN AND RED returns 0.
To read boolean data, read the byte containing the data from EEPROM and
then perform a logical AND with the reference value. To create boolean data, you
must take an empty variable (initialized as 0) and then perform logical OR opera-
tions with reference values. What happens if you want to update an existing
value? You already know how to set a bit, using a logical OR , but to clear a bit,
you must use a logical NOT AND . NOT inverts a status; if it was previously TRUE ,
it will become FALSE . By inverting the reference, you keep every bit that is set
except the one you want to clear. To toggle a bit, simply use the logical XOR to
invert its status. XOR , short for Exclusive OR , will be true if and only if one of the
inputs is TRUE ; if they are both TRUE , then the result will be FALSE .
Figure 6-3 shows a table of logical operators, showing the effect of each.

A B A | B A & B A ^ B ~A

00 0 0 0 1

10 1 0 1 0

01 1 0 1 1

11 1 1 0 0

AND
Figure 6-3: Logical operators

OR XOR NOT

Following is a short example of how to perform bitwise operations. A bitwise
OR is performed using the | symbol:

value |= RED; // Bitwise OR. Sets the BLUE bit

To perform a bitwise AND , use the & symbol:

vavalue &= ~GREEN; // Bitwise AND. Clears the RED bit (AND NOT RED)

And i nally, to perform an exclusive OR , use the ^ symbol:

value ^= BLUE; // Bitwise XOR. Toggles the GREEN bit

Reading and Writing Strings
Strings are generally an array of char values and as such can be easily stored
and recalled. In Arduino, it's possible to use a char array as a string, or you
can use the String data type for more robust data manipulation, at the cost of
program size. With character arrays, you can recall the entire allocated memory
and print it out as required.