Task 5.1: Binary Counter with Button

Enhance Lab 5 by adding button control. Each button press increments the counter and displays the new value on LEDs.

Learning Objectives

By the end of this task, you will:

• 🎯 Combine button input with LED output
• 🎯 Implement counter increment on button events
• 🎯 Apply debouncing in event-driven code
• 🎯 Manage state transitions in firmware
• 🎯 Build interactive embedded applications

Prerequisites

• ✅ Complete Lab 5 (binary counter)
• ✅ Complete Lab 4 (software debouncing)
• ✅ Understand button press detection

Hardware Required

Component	Details
Microcontroller	STM32F407VG
Button	PA0
LEDs	PD12-PD15 (counter display)

Theory: Event-Driven Updates

Instead of continuous counting, we now:

Loop forever:
  if (button pressed before):
    ├─ Debounce press
    ├─ Confirm real press
    ├─ Increment counter
    ├─ Display new value
    ├─ Wait for release
    ├─ Debounce release
    └─ Ready for next press

  else:
    └─ Display current counter value

Demo

Press button: counter increments and displays new binary value on LEDs

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++);
}

void display_value(int value) {
    // Clear LED bits
    GPIOD_ODR &= ~(0xF << 12);
    // Display value (wraps at 16)
    GPIOD_ODR |= ((value & 0xF) << 12);
}

int main(void) {
    // Enable clocks
    RCC_AHB1ENR |= (1U << 0);  // GPIOA
    RCC_AHB1ENR |= (1U << 3);  // GPIOD
    
    // PA0 as input
    GPIOA_MODER &= ~(3U << 0);
    
    // PD12-PD15 as outputs
    GPIOD_MODER &= ~(0xFF << 24);
    GPIOD_MODER |= (0x55 << 24);
    
    int counter = 1;  // Start at 1
    display_value(counter);
    
    while (1) {
        // Wait for button press
        if (GPIOA_IDR & (1 << 0)) {
            led_delay();  // Debounce press
            
            // Confirm
            if (GPIOA_IDR & (1 << 0)) {
                counter++;  // Increment
                if (counter > 15) counter = 0;  // Wrap around
                
                display_value(counter);
                
                // Wait for release
                while (GPIOA_IDR & (1 << 0));
                led_delay();  // Debounce release
            }
        }
    }
    
    return 0;
}

Algorithm

Initialize: counter = 1, display on LEDs

Loop forever:
  if (button pressed):
    Debounce
    if (still pressed):
      counter = (counter + 1) % 16
      display_value(counter)
      Wait for release
      Debounce release
  else:
    Display current counter

Expected Output

Initial: LEDs show 0001 (decimal 1)

Button Press 1: LEDs show 0010 (decimal 2)
Button Press 2: LEDs show 0011 (decimal 3)
Button Press 3: LEDs show 0100 (decimal 4)
...
Button Press 15: LEDs show 0000 (wraps around)
Button Press 16: LEDs show 0001 (repeats)

Common Mistakes

Issue	Solution
Counter wraps to 0 at 15, then 16	Use modulo: if (counter > 15) counter = 0
Doesn't increment	Check button if condition - maybe inverted
Fast multiple increments	Debounce delay too short - increase to 300000
Wraps to wrong value	Use counter & 0xF to mask to 4 bits

Key Takeaways

✨ Remember:

1. Event-driven code responds to user input
2. Debouncing both edges ensures reliable button control
3. State variables (counter) persist across loop iterations
4. Modulo operator (%) or bit masking wraps values
5. Display updates only when counter changes

Challenge Exercises

Challenge 1: Decrement Mode

Add another button (PA1) to decrement counter instead of increment.

Challenge 2: Display Hex

Show hexadecimal letters (A-F) for values 10-15, not just binary.

Challenge 3: Counter Speed Control

Add button to change delay between increments (fast/slow modes).

Next Steps

🚀 Ready for Lab 6? Learn to generate sound with a buzzer using GPIO timing!

Prerequisites for Lab 6: GPIO output timing, Understanding frequency