# Digital INPUT OUTPUT

There are three registers which initialize Digital INPUT and DIGITAL OUTPUT port:

1. DDRB  
2. DDRC  
3. DDRD  

Each pin is 1 bit of DDRx.

Here P means Port  
Example: PC → Port C  

Microcontroller pins are grouped as:

1. PORTB → PB0–PB7  
2. PORTC → PC0–PC6  
3. PORTD → PD0–PD7  

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## Important

- In Arduino Uno, PC7 does not exist  
  → PORTC is only PC0–PC6  

- PB6 and PB7 are used by 16MHz crystal  
  → DO NOT USE  

- PC6 is RESET pin  
  → DO NOT USE as GPIO  
  → can reset system  

- PD0 and PD1 are Serial pins (RX TX)  
  → used for USB upload and debugging  
  → DO NOT USE during development  

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## SPI (just remember)

- PB3 → MOSI  
- PB4 → MISO  
- PB5 → SCK  

→ used for SPI and programming  
→ can be used as GPIO but avoid if SPI used  
→ PB5 has onboard LED  

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## I2C (just remember)

- PC4 → SDA  
- PC5 → SCL  

→ used for I2C communication  

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## Safe pins for Digital I/O

- PORTD → PD2–PD7  
- PORTB → PB0–PB2 (PB3–PB5 usable but SPI)  
- PORTC → PC0–PC5 (PC4 PC5 shared with I2C)

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## Use with care

- PB3 PB4 PB5 → SPI pins  
- PC4 PC5 → I2C pins  

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## Final

Best pins to use:

PD2–PD7  
PB0–PB2  
PC0–PC5  

Avoid:

PB6 PB7 → crystal  
PC6 → reset  
PD0 PD1 → serial