Introduction to C language bitwise operators and relevant truth tables. in AVR uC's

As we seen in the last tutorial, sometimes bitwise operators are needed, in order to change just a pin, shit a bit, etc..
So lets start by revising quickly the logic truth tables, so we understand a bit better what we on about.

There are logical operators called AND, OR, NOT and XOR among others, that are the ones we will discuss now.
*While in digital systems, the gates work with 2 inputs at least, here we have to work with the 8 bits ! 

Example AND binary number operation (the "&" is used in C programming for AND):


01001011 &
10001101
     =
00001001

Example OR binary number operation ( the character " | " is the OR, used in C programming )

01001011 |
10001101
     =
11001111


Example XOR - Exclusive OR - binary number operation ( the character   "^"  is the OR, used in C programming )

01001011 ^
10001101
equals
11000110

* The NOT operator inverts the sense of the bit, so a 1 becomes a 0, and a 0 becomes a 1.

 |       =     bit OR

 &     =    bit AND

 ~      =    bit NOT

 ^      =    bit EXLUSIVE OR (XOR)

<<    =    bit LEFT SHIFT

>>    =    bit RIGHT SHIFT




These operators work on bits and not logical values. Take two 8 bit bytes, combine with any of these operators, and you will get another 8 bit byte according to the operator's function. These operators work on the individual bits inside the byte.

A truth table helps to explain each operation. In a truth table, a 1 bit stands for true, and a 0 stands for false.

So, we already know what the <avr/io.h> does, but the #include <util/delay.h> hasnt been spoken about; Just like the name implies, the delay.h will provide us with a way to create delays in our program (see http://www.nongnu.org/avr-libc/user-manual/group__util__delay.html for more details). And Just like the name implies, the delay.h will provide us with a way to create delays in our program.

So we know we have DDRD = 0b11111110; 

We need only to change the one bit: the pin 0 bit.If we look at the ways above, we can do this with the OR operator, using a binary mask.

DDRD = DDRD | 0b00000001;

This will "OR" it to a mask. The mask is: 0b00000001. While this looks like the actual binary number, if the previous DDRD was, say, 0b01001010, then doing an OR to that with our mask would be: 0b01001010 | 0b00000001 = 0b01001011. So this result only the pin 0 bit has changed!

This statememt can be further compressed in C++:

DDRD |= 0b00000001;

So why "DDRD |= 1 << PIND0"?

1 << PINB0 is the act of creating the mask. The "1" represents what will be inserted into the mask, the << is a left shift operator. It does EXACTLY what it says, and PIND0 is a number of positions that the "1" will shift left. In essence, PIND0 is just equal to 0. So, you start with a 0b00000000, and you add a "1" to make 0b00000001 and then you shift it left 0 positions. So you are left with 0b00000001, the same number from above. So, what if it was PIND4? The statement would be: 1 << PIND4. The "1" would be left shifted 4 times resulting in: 0b00010000. Remember, we are using a zero index, so there is four 0s after the 1.


We know how to set a specific bit in the binary number, but we don't know how to toggle a bit ( change it to 1 if it is a 0 and vice versa).

#include <avr/io.h>

#include <util/delay.h>

int main(void)

   {

        DDRB |= 1 << PIND0; 

         while (1)

         {

           PORTB |= 1 << PINB0;

            _delay_ms(100);

           PORTB &= ~(1 << PINB0);

           _delay_ms(100);

          }

    }



If the delay is going to be the same for on and off, we could shorten the previous four lines to only two and take advantage of the XOR operation.


The " ^ " Bitwise operator toggles the bit, so that if it is a 1 will then become a 0, and vice versa

















#include <avr/io.h>



#include <util/delay.h>




int main(void)




{




  DDRB |= 1 << PIND0; 




  while (1)


  {

    PORTB ^= 1 << PINB0;  */ The " ^ " Bitwise operator toggles the bit, so that if it is a 1 will then become a 0, and vice versa*/

_delay_ms(100);

  }

}





*(...to be continued)