LED Control

1. LED Control Overview

GPIO is the most fundamental peripheral interface of MCUs. Pins can be configured as output mode via software to drive external circuits. This experiment demonstrates the basic usage of three core functions (pinMode, digitalWrite, delay) by blinking two onboard LEDs synchronously at 1Hz.

Typical Application Scenarios:

2. Core Concepts

2.1 Common Anode Configuration

The onboard LEDs use common anode configuration: anode is fixed at 3.3V, cathode connects to GPIO through a current-limiting resistor.

VDD(3.3V) → LED anode → LED cathode → current-limiting resistor → GPIO pin

digitalWrite(pin, LOW) = ON, which is the opposite of the intuition "1=ON". Refer to the table above to understand.

2.2 Core API

pinMode(pin, mode) — Configures GPIO direction.

digitalWrite(pin, value) — Writes GPIO output level. LOW = 0V, HIGH = 3.3V. Writes directly to register, single call < 0.1μs.

delay(ms) — Blocking delay, underlying is FreeRTOS vTaskDelay(): task enters Blocked state and yields CPU, automatically wakes up when expired.

Do not use for loop busy-wait — it will starve IDLE and trigger watchdog.

3. Hardware Dependencies

Both LEDs are onboard, no external components required.

4. Project Architecture and Flow

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setup()                     // Executes only once
  ├─ pinMode(43, OUTPUT)    // GPIO43 push-pull output
  └─ pinMode(44, OUTPUT)    // GPIO44 push-pull output

loop()                      // Infinite loop
  ├─ digitalWrite(43, LOW)  // ON
  ├─ digitalWrite(44, LOW)
  ├─ delay(500)             // Hold for 500ms
  ├─ digitalWrite(43, HIGH) // OFF
  ├─ digitalWrite(44, HIGH)
  └─ delay(500)             // Hold for 500ms → back to start

5. Source Code Details

Full source code: Source Code → Arduino → 1.Basic Peripherals → 1.GPIO_LED → 1.GPIO_LED.ino

5.1 File Header and Macros

Define pin numbers first, then reference all GPIO operations through macro names for better readability and easier modification.

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#define LED0_PIN  43   // Onboard LED0
#define LED1_PIN  44   // Onboard LED1

digitalWrite(LED0_PIN, LOW) is instantly recognizable as operating LED0, compared to digitalWrite(43, 0); if hardware revision moves LED to GPIO48, only one macro line needs changing.

5.2 setup() — Initialization

Configure both GPIOs as output mode so they can drive the LEDs.

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void setup() {
  pinMode(LED0_PIN, OUTPUT);
  pinMode(LED1_PIN, OUTPUT);
}

pinMode(pin, OUTPUT) calls gpio_set_direction() underneath, setting GPIO to push-pull output with ~10mA drive current.

5.3 loop() — Main Loop

ON then OFF, each delay 500ms. The repeated execution of loop() creates 1Hz blinking. In common anode configuration, LOW=ON — when GPIO pulls low, current flows through LED to GND, LED illuminates.

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void loop() {
  // ON — LOW: GPIO pulls to GND, LED has voltage difference
  digitalWrite(LED0_PIN, LOW);
  digitalWrite(LED1_PIN, LOW);
  delay(500);

  // OFF — HIGH: GPIO pulls to 3.3V, LED at equal potential
  digitalWrite(LED0_PIN, HIGH);
  digitalWrite(LED1_PIN, HIGH);
  delay(500);
}

digitalWrite writes directly to GPIO output register, single call < 0.1μs. delay(500) uses vTaskDelay() underneath, task enters Blocked state and yields CPU, automatically wakes up after 500ms, completely different from busy-wait.

6. Operation Flow

6.1 Build and Flash

Arduino flash process: See Arduino → 1. Before Development → 2. Build and Flash.

6.2 Normal Behavior

After power-on, both LEDs blink synchronously at 1Hz: ON 500ms → OFF 500ms → repeat.

6.3 Verification Methods

7. Common Issues