The DDRx Register

• The DDRD register sets the direction of Port D. Each bit of the DDRD register sets the corresponding Port D pin to be either an input or an output. A 1 makes the corresponding pin an output, and a 0 makes the corresponding pin an input. To set the first pin of Port D to be an output pin, you could use the sbi(reg,bit) function, which sets a bit (makes it high or binary 1) in a register:

  sbi(DDRD, 0);   //these two statements are equivalent
  sbi(DDRD, PD0); //both set the first pin of port D to be an output
                  //by setting the 0 bit of the DDRD register.
  

• To set the second pin to be an input, you could use the cbi(reg,bit) function which clears a bit (makes it low or binary 0) in a register:

  cbi(DDRD, 1);   //these two statements are equivalent
  cbi(DDRD, PD1); //both set the second pin of port D to be an output
                  //by setting the 1 bit of the DDRD register.
  

• Note that in C, the bit indexing begins with 0. The bits in a register go from 0 to 7. This may be a source of confusion, because when we talk about the "first pin" or "pin 1" of a port, we are referring to the 0 bit or P0 in C. The AVRmini boards label the first pin as pin 1.

• You can also set the value of all the bits in the DDRx register (or any register) using the outb(reg,byte) command. It writes a byte (8 bits) to a register. For example, if you wanted to set pins 1-4 of port B to output and pins 5-8 to input, you could use:

  outb(DDRB, 0x0F); //Set the low 4 pins of Port B to output
                    //and the high 4 pins to input