Lesson 1: Digital output

Pin 13 on the UNO R3 is bit 5 of the PORTB register¹. This lesson blinks it twice: once with Arduino calls, then again by writing the register yourself.

What you need

Arduino UNO R3
USB cable
Nothing else: the board has an LED on pin 13

Wire it, then check it

The UNO R3 has a built-⁠in LED on pin 13⁵, so this lesson needs no wiring.

Before you run it on a real board, check each connection The USB cable is plugged into the board and the computer The Arduino IDE has “Arduino Uno” and the right port selected

Not working on a real board? Start with the wiring checklist.

Arduino version

l1-blink-arduino.ino

// Lesson 1, Arduino version.
// Blinks the UNO R3's LED on pin 13: 1 s on, 1 s off.
void setup() {
  pinMode(13, OUTPUT);
}

void loop() {
  digitalWrite(13, HIGH);
  delay(1000);
  digitalWrite(13, LOW);
  delay(1000);
}

Once pinMode(13, OUTPUT) makes pin 13 an output, digitalWrite(13, HIGH) sets it to 5 V and LOW sets it to 0 V³. delay(1000) waits one second and blocks the rest of the sketch while it waits⁴.

Register version

l1-blink-registers.ino

// Lesson 1, register version: the same blink, written
// directly to the port registers.
// Pin 13 is PORTB bit 5 (PB5) on the UNO R3.
void setup() {
  DDRB |= (1 << DDB5);       // pin 13 is an output
}

void loop() {
  PORTB |= (1 << PORTB5);    // pin 13 HIGH
  delay(1000);
  PORTB &= ~(1 << PORTB5);   // pin 13 LOW
  delay(1000);
}

A 1 in a DDRB bit makes that pin an output². PORTB sets the output level, and PINB reads the pins⁶. |= with (1 << PORTB5) sets bit 5 and leaves the others alone; &= ~ with the same mask clears bit 5 only.

PORTB bits 7 to 0, with the port pin, Arduino pin and chip pin each one drives
PORTB bit	7	6	5	4	3	2	1	0
Port pin	PB7	PB6	PB5	PB4	PB3	PB2	PB1	PB0
Arduino pin	none	none	13	12	11	10	9	8
Chip pin	10	9	19	18	17	16	15	14

PORTB bits 7 to 0, with the port pin, Arduino pin and chip pin each one drives
PORTB bit	Port pin	Arduino pin	Chip pin
7	PB7	none	10
6	PB6	none	9
5	PB5	13	19
4	PB4	12	18
3	PB3	11	17
2	PB2	10	16
1	PB1	9	15
0	PB0	8	14

PORTB bit 5 is port pin PB5, which the UNO’s pin map calls pin 13¹. On the 28-⁠pin chip it is pin 19². Bits 6 and 7 go to chip pins 9 and 10, XTAL1 and XTAL2², and have no Arduino pin number¹.

Registers in this lesson (ATmega328P data-⁠space addresses)²

Register	Address	Bit 7	Bit 6	Bit 5	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0
DDRB	0x24	DDB7	DDB6	DDB5	DDB4	DDB3	DDB2	DDB1	DDB0
PORTB	0x25	PORTB7	PORTB6	PORTB5	PORTB4	PORTB3	PORTB2	PORTB1	PORTB0
PINB	0x23	PINB7	PINB6	PINB5	PINB4	PINB3	PINB2	PINB1	PINB0

Run it in the lab

Switch between the two programs. The register panel shows the same PORTB5 bit turning on and off in both.

Drill

Which register bit is pin 13 on an UNO R3?
Show the answer
PORTB bit 5 (PB5). The UNO’s pin map puts digital pin 13 on PB5. ¹
What does DDRB |= (1 << DDB5); do?
Show the answer
It makes pin 13 an output, like pinMode(13, OUTPUT). A 1 in a pin’s DDxn bit makes that pin an output. ²
Why does PORTB &= ~(1 << PORTB5); leave pins 8 to 12 alone?
Show the answer
The AND with an inverted mask clears bit 5 only. Every other bit of the mask is 1, so ANDing keeps those bits as they were.

Sources

ArduinoCore-⁠avr, variants/standard/pins_arduino.h: LED_BUILTIN is 13; pin 13 is PB5
Microchip ATmega328P datasheet DS40002061A: Pin configurations (p12); I/O ports; PORTB and DDRB at p100
Arduino reference: digitalWrite
Arduino reference: delay
Arduino UNO R3 technical specifications: Built-⁠in LED Pin: 13
Arduino: Port Manipulation (retired page, written for the ATmega8/168): DDRx, PORTx and PINx

Next: Lesson 2