# 3. Input/Output (I/O) Programming

On the Arduino Uno (ATmega328P), digital I/O is controlled through **Port B, Port C, and Port D**. Each port is 8-bit, allowing control of up to 8 pins simultaneously.

[![](https://learn.digilabdte.com/uploads/images/gallery/2026-02/scaled-1680-/image-1770526704631.png)](https://learn.digilabdte.com/uploads/images/gallery/2026-02/image-1770526704631.png)

### A. Port to Arduino Pin Mapping

| Port | Bits | Arduino Pin | Notes |
| :--- | :--- | :--- | :--- |
| **Port B** | PB0 - PB5 | Digital Pin 8 - 13 | PB6-PB7 are used for crystal oscillator |
| **Port C** | PC0 - PC5 | Analog Pin A0 - A5 | PC6 is the RESET pin |
| **Port D** | PD0 - PD7 | Digital Pin 0 - 7 | PD0 (RX) and PD1 (TX) for serial communication |

### B. Main I/O Registers

Three main registers control the behavior of each port:

| Register | Full Name | Access | Function |
| :--- | :--- | :--- | :--- |
| **DDRx** | Data Direction Register | Read/Write | Configures pin direction. `0` = Input, `1` = Output |
| **PORTx** | Data Register | Read/Write | **If Output:** Sets logic High (1) or Low (0). **If Input:** Activates internal Pull-up resistor (1) or Tri-state (0) |
| **PINx** | Input Pins Address | Read Only | Reads the physical logic state of the pin (`0` or `1`) |

*(Replace 'x' with Port name, e.g., DDRB, PORTB, PINB)*

### C. Register Bit Configuration Details

#### DDRx - Data Direction Register

| DDRx Bit Value | Pin Direction | Explanation |
| :---: | :--- | :--- |
| **0** | Input | Pin is configured as input (high impedance) |
| **1** | Output | Pin is configured as output (source/sink current) |

#### PORTx - Data Register (Depends on DDRx Configuration)

| DDRx | PORTx | Mode | Pin Condition |
| :---: | :---: | :--- | :--- |
| 0 (Input) | 0 | Tri-state (Hi-Z) | Pin is floating, no pull-up |
| 0 (Input) | 1 | Input Pull-up | Internal pull-up resistor active, pin defaults to HIGH |
| 1 (Output) | 0 | Output Low | Pin outputs 0V (GND) |
| 1 (Output) | 1 | Output High | Pin outputs 5V (VCC) |

#### PINx - Input Pins Register

| PINx Bit Value | Pin Status | Explanation |
| :---: | :--- | :--- |
| **0** | LOW | Pin voltage is below threshold (near 0V) |
| **1** | HIGH | Pin voltage is above threshold (near 5V) |