Lab 3: Switch Input → LED Control

Now you'll learn GPIO input by reading a push button and using it to control an LED. This introduces input reading and conditional logic in embedded systems.

Learning Objectives

By the end of this lab, you will:

• 🎯 Understand GPIO input configuration using MODER register
• 🎯 Read GPIO pins using the IDR (Input Data Register)
• 🎯 Implement conditional LED control based on button state
• 🎯 Use if-else logic for digital input processing
• 🎯 Control output from input states

Prerequisites

• ✅ Complete Labs 1-2 (GPIO output, bitwise operations)
• ✅ Understand GPIO pin configuration (MODER register)
• ✅ Familiar with if-else statements in C

Hardware Required

Component	Details
Microcontroller	STM32F407VG (STM32F4 Discovery)
Button	PA0 (user push button on board)
LED	PD12 (red LED on board)
Connection	Onboard - no external wiring needed

Theory: GPIO Input Reading

Input vs Output

MODER Register (2 bits per pin):
- 00 = Input (HIGH-Z, can read level)
- 01 = General Purpose Output (can write level)
- 10 = Alternate Function
- 11 = Analog

For input: MODER bits = 00 (default/cleared)

Reading the IDR Register

// IDR = Input Data Register at offset 0x10
#define GPIOA_IDR *(volatile unsigned int*)(GPIO_A_BASE + 0x10U)

// Read pin 0 state:
if (GPIOA_IDR & (1 << 0)) {
    // Pin is HIGH (button pressed)
} else {
    // Pin is LOW (button released)
}

Logic Flow

┌─ Read PA0 ?
├─ PA0 = HIGH → Button pressed → Turn ON LED
├─ PA0 = LOW → Button released → Turn OFF LED  
└─ Repeat continuously in while loop

Demo

Push button pressed: LED turns ON immediately. Release button: LED turns OFF

Complete Code

#define RCC_BASE 0x40023800UL
#define RCC_AHB1ENR *(volatile unsigned int*)(RCC_BASE + 0x30U)

#define GPIO_D_BASE 0x40020C00UL  
#define GPIOD_MODER *(volatile unsigned int*)(GPIO_D_BASE + 0x00U)
#define GPIOD_ODR   *(volatile unsigned int*)(GPIO_D_BASE + 0x14U)

#define GPIO_A_BASE 0x40020000UL
#define GPIOA_MODER *(volatile unsigned int*)(GPIO_A_BASE + 0x00U)
#define GPIOA_IDR   *(volatile unsigned int*)(GPIO_A_BASE + 0x10U)

int main(void) {
    // Enable GPIOA and GPIOD clocks
    RCC_AHB1ENR |= (1U << 0);  // GPIOA
    RCC_AHB1ENR |= (1U << 3);  // GPIOD
    
    // PA0 as input (MODER bits = 00, which is default - just verify clear)
    GPIOA_MODER &= ~(3U << 0);
    
    // PD12 as output
    GPIOD_MODER &= ~(3U << 24);
    GPIOD_MODER |= (1U << 24);
    
    while (1) {
        // Read button state from PA0
        if (GPIOA_IDR & (1 << 0)) {
            // Button pressed (HIGH)
            GPIOD_ODR |= (1U << 12);  // LED ON
        } else {
            // Button released (LOW)  
            GPIOD_ODR &= ~(1U << 12); // LED OFF
        }
    }
    
    return 0;
}

Expected Output

User Action:
├─ Press button  → LED turns ON immediately
├─ Hold button   → LED stays ON
├─ Release button → LED turns OFF immediately
└─ Pattern repeats for each button press

Visual: LED directly follows button state with no visible delay.

Common Mistakes

Issue	Solution
Button doesn't work	Verify PA0 is configured as input (MODER = 00)
LED always ON/OFF	Check if condition - maybe inverted logic
No response	Enable both GPIOA and GPIOD clocks
Flickering/noisy	Add debouncing delay (see Lab 4)

Troubleshooting

✓ Verify: Is GPIOA clock enabled in RCC_AHB1ENR?
✓ Verify: Is PA0 cleared in MODER (input = 00)?
✓ Verify: Is PD12 set to output (01)?
✓ Test: Try hardcoding LED ON to verify basic GPIO works

Key Takeaways

✨ Important:

1. Input pins must have MODER bits = 00 (cleared)
2. Read from IDR register to get current pin level
3. Bitwise AND (&) checks if a bit is set
4. If-else logic processes input and controls output
5. No debouncing yet - bouncing may cause flickering

Challenge Exercises

Challenge 1: Inverted Logic

Make the LED turn ON when button is released, OFF when pressed.

Challenge 2: Multiple Buttons

Add PA1 as another button to control PD13 (orange LED) independently.

Challenge 3: Toggle on Press

Make the LED toggle each time the button is pressed (hold doesn't toggle).

Next Steps

🚀 Ready for Lab 4? Learn software debouncing to eliminate button noise and flickering!

Prerequisites for Lab 4: GPIO input reading, Understanding timing/delays