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

LED Blink Input Output/GPIO-notes-pins.md

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+# 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  
+
+---
+
+## 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  
+
+---
+
+## 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  
+
+---
+
+## I2C (just remember)
+
+- PC4 → SDA  
+- PC5 → SCL  
+
+→ used for I2C communication  
+
+---
+
+## 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)
+
+---
+
+## Use with care
+
+- PB3 PB4 PB5 → SPI pins  
+- PC4 PC5 → I2C pins  
+
+---
+
+## Final
+
+Best pins to use:
+
+PD2–PD7  
+PB0–PB2  
+PC0–PC5  
+
+Avoid:
+
+PB6 PB7 → crystal  
+PC6 → reset  
+PD0 PD1 → serial