Lab 4: Software Debouncing

Real push buttons are noisy. They bounce when pressed/released, causing multiple ON/OFF transitions. Learn software debouncing to eliminate this noise.

Learning Objectives

By the end of this lab, you will:

• 🎯 Understand switch bouncing and why it happens
• 🎯 Implement software debouncing using delays
• 🎯 Detect button press confirmation after bouncing stops
• 🎯 Wait for button release before next action
• 🎯 Build reliable input sensing systems

Prerequisites

• Complete Lab 3 (GPIO input reading)
• Understand delays and timing
• Familiar with software loops

Hardware Required

Component	Details
Microcontroller	STM32F407VG
Button	PA0 user button
LED	PD12 red LED

Theory: Why Buttons Bounce

The Problem

Physical push buttons don't close cleanly. Instead:

1. You press button
2. Contacts touch, but bounce (ON/OFF/ON/OFF...)
3. Contacts settle after ~20-50ms
4. You release button
5. Contacts bounce again (OFF/ON/OFF/ON...)
6. Contacts fully separated after ~20-50ms

Without debouncing:
├─ t=0ms:   Low  (appears as button press)
├─ t=5ms:   High (bounce - false release)
├─ t=10ms:  Low  (bounce - false press)
├─ t=15ms:  High (bounce - false release)
├─ t=20ms:  Low  (settles - true stable state)
└─ Multiple false transitions!

Bounce Timing

• Bounce duration: typically 20-50ms
• Bounce frequency: multiple transitions per ms
• Solution: Wait 20-50ms after detecting press, then check again

Demo

Smooth, reliable button control without false triggers

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)

void led_delay(void) {
    for (volatile int i = 0; i < 300000; i++);
}

int main(void) {
    // Enable clocks
    RCC_AHB1ENR |= (1U << 0);  // GPIOA
    RCC_AHB1ENR |= (1U << 3);  // GPIOD
    
    // PA0 as input
    GPIOA_MODER &= ~(3U << 0);
    
    // PD12 as output
    GPIOD_MODER &= ~(3U << 24);
    GPIOD_MODER |= (1U << 24);
    
    while (1) {
        // Detect button press (HIGH)
        if (GPIOA_IDR & (1 << 0)) {
            led_delay();  // Wait for bouncing to settle (~300ms)
            
            // Confirm press still detected
            if (GPIOA_IDR & (1 << 0)) {
                GPIOD_ODR ^= (1U << 12);  // Toggle LED
                
                // Wait for button release
                while (GPIOA_IDR & (1 << 0));
                
                led_delay();  // Debounce release
            }
        }
    }
    
    return 0;
}

Algorithm Explained

Step 1: Wait for button press
├─ Loop until PA0 = HIGH

Step 2: Wait for bouncing to settle
├─ Delay ~20-50ms (we use 300ms to be safe)

Step 3: Confirm press (not a false bounce)
├─ Check PA0 again
├─ If still HIGH: Button really pressed
├─ If LOW: Was false bounce, go back to Step 1

Step 4: Perform action (toggle LED)
├─ GPIOD_ODR ^= LED

Step 5: Wait for release
├─ Loop while PA0 = HIGH

Step 6: Debounce release
├─ Delay ~20-50ms to let contacts settle

Step 7: Go back to Step 1

Expected Output

User Action:
├─ Press button (once)    → Detect press → Debounce → LED toggles (once)
├─ Release button         → Debounce
├─ Press button again     → LED toggles again
└─ Clean, one-press = one-toggle behavior

Difference from Lab 3: LED only toggles once per press, not continuously while held.

Common Mistakes

Issue	Solution
LED toggles multiple times per press	Debounce delay too short (increase to 300000)
LED responds to noise/glitches	Confirm button state twice
Button lag	Delay too long - reduce iterations
Won't detect releases	Check while loop condition

Key Takeaways

✨ Remember:

1. Real buttons bounce - they're not ideal switches
2. Wait after detecting press to let bouncing settle
3. Confirm state again after waiting
4. Wait for release before accepting another press
5. 20-50ms debounce is typical (we use longer to be safe)

Challenge Exercises

Challenge 1: Debounce Both Edges

Debounce both the press AND release edges separately.

Challenge 2: Count Button Presses

Count how many times button was pressed and display count using LEDs.

Challenge 3: Long Press Detection

Detect if button held for 1 second, perform different action.

Next Steps

🚀 Ready for Lab 5? Learn to display binary numbers on LED arrays using bit manipulation!

Prerequisites for Lab 5: GPIO output control for multiple pins, Bitwise operations