What is GPIO

GPIO (General Purpose Input/Output) is a pin on a microcontroller that you can control with software — set it HIGH (3.3V or 5V) or LOW (0V), or read whether an external device is pulling it HIGH or LOW.

Digital I/O Basics

Mode	What It Does	Example
OUTPUT	Pin drives voltage HIGH or LOW	Turn on LED, control relay
INPUT	Pin reads external voltage level	Read button press, detect motion
INPUT_PULLUP	Pin reads, with internal resistor to VCC	Button without external resistor

Arduino Example

const int ledPin = 13;    // OUTPUT
const int buttonPin = 2;  // INPUT_PULLUP

void setup() {
  pinMode(ledPin, OUTPUT);
  pinMode(buttonPin, INPUT_PULLUP);
}

void loop() {
  if (digitalRead(buttonPin) == LOW) { // pressed (pullup inverts)
    digitalWrite(ledPin, HIGH);
  } else {
    digitalWrite(ledPin, LOW);
  }
}

Voltage Levels

Board	HIGH =	LOW =	Max Input	Notes
Arduino Uno	5V	0V	5V	5V tolerant
ESP32	3.3V	0V	3.6V	NOT 5V tolerant
Raspberry Pi	3.3V	0V	3.6V	NOT 5V tolerant
STM32	3.3V	0V	3.6V (some 5V tolerant)	Check datasheet

5V signal into 3.3V pin = dead board

Always check voltage levels. ESP32 and Raspberry Pi GPIOs are NOT 5V tolerant. Use a level shifter or voltage divider when connecting 5V sensors.

Common Pitfalls

Problem	Likely Cause	Fix
Pin always reads HIGH or LOW	No pull-up/down	Use INPUT_PULLUP or add external resistor
Pin does not output expected voltage	Wrong pinMode	Must call pinMode() in setup()
LED on ESP32 is very dim	3.3V logic, LED needs resistor	Use 100-220 ohm resistor for 3.3V
GPIO pin dead	Overvoltage or short	The pin is physically damaged; use another