GPIO

The Arduino Uno provides input/output (I/O) ports that enable the microcontroller to interact with external devices such as sensors, motors, and LEDs. These ports are divided into digital pins and analog pins, grouped into internal ports for efficient control.

• PORTB: Controls digital pins D8 to D13.

  Pin Number	Port Bit
  D8	PB0
  D9	PB1
  D10	PB2
  D11	PB3
  D12	PB4
  D13	PB5

• PORTC: Controls analog pins A0 to A5.

  Pin Number	Port Bit
  A0	PC0
  A1	PC1
  A2	PC2
  A3	PC3
  A4	PC4
  A5	PC5

• PORTD: Controls digital pins D0 to D7.

  Pin Number	Port Bit
  D0	PD0
  D1	PD1
  D2	PD2
  D3	PD3
  D4	PD4
  D5	PD5
  D6	PD6
  D7	PD7

Registers for I/O Control

Each port has three registers associated with it:

• DDR (Data Direction Register): DDR controls the data direction (input or output). Set the corresponding bit to 1 for output and 0 for input.
• PORT Register: PORT controls the output state. Set the corresponding bit to 1 to set the pin HIGH and 0 to set it LOW. When configured as input, the PORT register can be used to enable pull-up resistors. To enable pull-up resistors, set the corresponding bit to 1.
• PIN Register: PIN reads the current state of the input pins.

For example, if you would like to configure pin D8 (PB0) as an output and set it HIGH, you can use the following code:

DDRB |= (1 << PB0); // Set D8 (PB0) as output
PORTB |= (1 << PB0); // Set D8 (PB0) HIGH

Bitwise operators

The bitwise operators |, ^, and << are used to manipulate individual bits in the registers. If you are not familiar with them, read the bitwise operators section.

Blinking an LED

A more elaborated example is to blink an LED connected to pin D5. Before diving into the code, let’s first understand the registers involved:

• DDRB: Data Direction Register for PORTB. Setting bit 5 (PB5) to 1 configures pin D5 as an output.
• PORTB: Port Register for PORTB. Setting bit 5 (PB5) to 1 turns the LED on, while clearing it (setting to 0) turns the LED off.

Circuit Setup

Connect an LED to pin D5 with a current-limiting resistor. Assuming that the diode forward voltage is 2V and the current is 20mA, you can use a 220Ω resistor or higher.

Code

This code snippet configures pin D5 as an output and toggles the pin state to blink the LED. The _delay_ms function imported from utils/delay.h is used to introduce a delay between turning the LED on and off. avr/io.h is included to access the I/O registers and macros.

#include <avr/io.h>
#include <util/delay.h>

#define BLINK_DELAY_MS 1000

int main(void) {
  /* set pin 5 of PORTB for output */
  DDRB |= (1 << DDB5);

  while (1) {
    /* toggle pin 5 to turn LED on/off */
    PORTB ^= (1 << PORTB5);
    _delay_ms(BLINK_DELAY_MS);
  }
}