fix input logic (pull-up) + add GPIO notes

LED Blink Input Output/How-to-write-GPIO-notes.md

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+# INPUT OUTPUT (DDRx Register)
+
+To configure a pin as INPUT or OUTPUT, we use DDRx register.
+
+- DDRx bit = 1 → OUTPUT  
+- DDRx bit = 0 → INPUT  
+
+---
+
+## Problem
+
+Using direct assignment:
+
+DDRB = 0b00000001;
+
+→ PB0 = OUTPUT  
+→ PB1–PB7 = INPUT  
+
+This is not good because:
+- it overwrites all bits  
+- existing configuration is lost  
+
+---
+
+## 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  
+
+---
+
+### 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  
+
+---
+
+## 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.
+
+---
+
+## 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).
+
+---
+
+## INPUT (Important concept)
+
+To use pin as INPUT:
+
+DDRB &= ~(1<<PB0);
+
+Now PB0 is INPUT.
+
+---
+
+## 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  
+
+---
+
+## Reading input (PINx)
+
+PINx register reads actual pin state.
+
+Example:
+
+if (!(PINB & (1<<PB0))) {
+
+}
+
+---
+
+## 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  
+
+---
+
+## Final understanding
+
+- DDRx → direction  
+- PORTx → output / pull-up  
+- PINx → input reading  
+
+---
+
+## Key point
+
+Pull-up makes:
+- Not pressed → HIGH  
+- Pressed → LOW  
+
+So logic is inverted:
+- LOW = pressed  
+- HIGH = not pressed