The ATmega328 has a total of three input/output ports, port B, port C, and port D. Two of these ports, namely port B and port D, are associated with the 14 digital input/output pins on the Arduino Uno. Port C is associated with the analog input pins which can also be used as digital input/output pins if needed. The following table summarizes the mapping between the Arduino Uno digital pins and the ATmega328 port pins.

Arduino Digital Pin	Port Pin
0	PD0
1	PD1
2	PD2
3	PD3
4	PD4
5	PD5
6	PD6
7	PD7
8	PB0
9	PB1
10	PB2
11	PB3
12	PB4
13	PB5

 

To turn on or off a Arduino digital pin, its mode is first set using pinMode() and then its state is set using digitalWrite(). For example, to turn on digital pin 5 for one second then turn if off for one second repeatedly, the following code could be used.

int myPin = 5;
void setup() {
   pinMode(myPin,OUTPUT);
}
void loop() {
   digitalWrite(myPin,HIGH);
   delay(1000);
   digitalWrite(myPin,LOW);
   delay(1000);
}

It is also possible to control the digital pins by accessing the port registers directly. For example, the following achieves the same result as the above code. It is explained below.

void setup() {
   DDRD = DDRD | B00100000; 
}
void loop() {
   PORTD = PORTD | B00100000;
   delay(1000);
   PORTD = PORTD & B11011111;
   delay(1000);
}

Each port has a one byte register defining the direction of the associated pins. This register is called DDRD for port D and DDRB for port B. From zero to seven, each bit in this register corresponds to the comparably numbered pin of that port. For example, the fifth bit of DDRD controls port pin D5 or Arduino digital pin 5. If this bit is set to zero, the pin is an input. If this bit is set to one, the pin is an output pin. Thus, the line of code in the setup function above sets Arduino digital pin 5 to an output pin using a bitwise or operator. Each port also has a one byte register defining the state of the associated pins. This register is called PORTD for port D and PORTB for port B. Just like the DDRx registers, each bit in these registers from zero to seven corresponds to the comparably numbered pin of that port. For example, the fifth bit of PORTD controls the state of pin D5 or Arduino digital pin 5. If this bit is set to zero, the pin is set to 0 V. If this bit is set to one, the pin is set to 5 V (on the Arduino Uno). Thus the lines of code in the loop function above first turns on Arduino digital pin 5 using a bitwise or and then turns this pin off using a bitwise and.

Controlling digital i/o pins at the register level is incredibly powerful. For example, you could turn all 8 pins of port D using a single line of code if you wanted to. However, with great power comes great responsibility. Manipulating registers can be tricky and may yield unexpected results. For example, buried in the ATmega328 data sheet is a warning that one can not change a pin from the high impedance input off state to the output on state in one step. You must first go to an intermediate state of output low or input high before going to output high.