# INPUT OUTPUT (DDRx Register)

To configure a pin as INPUT or OUTPUT, we use DDRx register.

- DDRx bit = 1 → OUTPUT  
- DDRx bit = 0 → INPUT  

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## Problem

Using direct assignment:

DDRB = 0b00000001;

→ PB0 = OUTPUT  
→ PB1–PB7 = INPUT  

This is not good because:
- it overwrites all bits  
- existing configuration is lost  

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## Correct approach (safe and professional)

### Set PB0 as OUTPUT

DDRB |= (1 << PB0);

Explanation:

Assume DDRB = 11111110  

(1 << PB0) = 00000001  

11111110  
00000001   (OR)  
--------  
11111111  

→ Only PB0 becomes OUTPUT  
→ other pins unchanged  

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### Set PB0 as INPUT

DDRB &= ~(1 << PB0);

Explanation:

Assume DDRB = 11111111  

(1 << PB0) = 00000001  
~(1 << PB0) = 11111110  

11111111  
11111110   (AND)  
--------  
11111110  

→ PB0 becomes INPUT  
→ other pins unchanged  

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## Final

- OUTPUT → DDRx |= (1 << Px)  
- INPUT  → DDRx &= ~(1 << Px)  

This method:
- does not disturb other pins  
- is scalable  
- is standard embedded practice  

# PORTx and PINx (Output and Input Control)

0xFF can be used in hex for 8-bit and 0b11111111 in binary.  
Both represent all bits = 1.

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## OUTPUT (Using PORTx)

After setting direction using DDRx, we use PORTx to control output.

Set pin HIGH:

PORTB |= (1<<PB0);

→ PB0 = HIGH (5V)

Set pin LOW:

PORTB &= ~(1<<PB0);

→ PB0 = LOW (0V)

This follows same bit logic (OR to set, AND with NOT to clear).

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## INPUT (Important concept)

To use pin as INPUT:

DDRB &= ~(1<<PB0);

Now PB0 is INPUT.

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## Pull-up requirement

If input is left floating:
- Noise (EMF, static) can cause false HIGH/LOW

So we enable internal pull-up:

PORTB |= (1<<PB0);

Now:
- Default = HIGH  
- Pressed (connected to GND) = LOW  

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## Reading input (PINx)

PINx register reads actual pin state.

Example:

if (!(PINB & (1<<PB0))) {

}

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## Logic explanation

Case 1: Button NOT pressed

PINB bit = 1  

PINB & (1<<PB0) = 1  

!1 = 0 → FALSE  

→ goes to else  

---

Case 2: Button pressed

PINB bit = 0  

PINB & (1<<PB0) = 0  

!0 = 1 → TRUE  

→ enters if  

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## Final understanding

- DDRx → direction  
- PORTx → output / pull-up  
- PINx → input reading  

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## Key point

Pull-up makes:
- Not pressed → HIGH  
- Pressed → LOW  

So logic is inverted:
- LOW = pressed  
- HIGH = not pressed