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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.

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)

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