# LEDs, Buttons and Pull-ups — Arduino & Embedded C

Source: https://www.geekswithgeeks.com/en/arduino-embedded-c/gpio-led-button

> pinMode, digitalWrite, digitalRead.

## Outputs, inputs and floating pins

`pinMode(pin, OUTPUT)` lets `digitalWrite` drive the pin HIGH (near the supply voltage) or LOW (0 V). An LED **always needs a series resistor** to limit current: on a 5 V Uno R3, around 220 Ω to 1 kΩ is common. The ATmega328P datasheet gives 40 mA per pin as an absolute maximum, so design for well under that (about 20 mA or less) and respect the total-package limit; check your board's datasheet. As an `INPUT`, an unconnected pin **floats** and reads random values, so a button needs a **pull-up** or **pull-down** resistor to hold a defined level. `INPUT_PULLUP` enables an internal pull-up: the pin reads HIGH when the button is open and LOW when the button connects it to ground (inverted logic).

## Reading and driving pins

Digital pins are either HIGH or LOW; with the right resistors they read buttons and drive LEDs safely.

![Three ideas: pins, LEDs and buttons; debouncing; non-blocking timing with millis().](assets/figures/arduino-embedded-c/section-2-map.svg) — Figure 2.1 — Input, processing and output on digital pins.

## Button turns on an LED

Wiring (Uno R3): LED anode via 330 Ω resistor to pin 8, cathode to GND; push button between pin 2 and GND.

```cpp
const uint8_t LED_PIN = 8;
const uint8_t BUTTON_PIN = 2;

void setup() {
  pinMode(LED_PIN, OUTPUT);
  pinMode(BUTTON_PIN, INPUT_PULLUP);  // internal pull-up: idle = HIGH
}

void loop() {
  bool pressed = (digitalRead(BUTTON_PIN) == LOW);  // pressed pulls to GND
  digitalWrite(LED_PIN, pressed ? HIGH : LOW);
}
```

## Never short an output

Do not connect an output pin directly to GND or to another output without a resistor. An output driving HIGH into a pin driven LOW can exceed current limits and damage the chip.

**Quiz:** With INPUT_PULLUP and a button wired to GND, what does digitalRead return while the button is pressed?

- [ ] HIGH
- [x] LOW
- [ ] A random value
- [ ] The analog voltage

*Answer:* LOW. The button connects the pin to ground, overriding the weak internal pull-up.