Lab 2: LED Toggle Using Bitwise Operations

Now that you understand GPIO output control, let's explore toggling pins using the XOR bitwise operator - an elegant technique that doesn't require reading the pin's current state.

Learning Objectives

By the end of this lab, you will:

• 🎯 Understand XOR (^) operation for bit toggling
• 🎯 Master efficient bitwise techniques for GPIO control
• 🎯 Implement toggling without state checking
• 🎯 Control multiple LEDs independently in a loop
• 🎯 Apply elegant bit manipulation patterns

Prerequisites

• ✅ Complete Lab 1 (GPIO fundamentals)
• ✅ Understand bitwise operators (|, &, ~, ^)
• ✅ Familiar with GPIO configuration and ODR register

Hardware Required

Component	Details
Microcontroller	STM32F407VG (STM32F4 Discovery)
LEDs	PD12 (red), PD13 (orange)
Connection	Onboard LEDs at GPIOD pins 12-15
Power Supply	USB or 5V adapter

Theory: XOR Toggling

The Problem With Traditional Toggle

// To toggle, you normally need to read state:
int state = GPIOD_ODR & (1 << 12);  // Read current
if (state)
    GPIOD_ODR &= ~(1 << 12);        // If ON, turn OFF
else
    GPIOD_ODR |= (1 << 12);         // If OFF, turn ON

The XOR Solution

The exclusive OR (XOR) operation has a special property: A ^ 1 always flips the value.

Truth Table:
0 ^ 1 = 1   ← 0 becomes 1
1 ^ 1 = 0   ← 1 becomes 0

So: GPIOD_ODR ^= (1 << 12); toggles the pin in one instruction!

Running Code

// Enable GPIOD clock
RCC_AHB1ENR |= (1 << 3);

// Configure PD12 and PD13 as output
GPIOD_MODER &= ~(3U << 24);
GPIOD_MODER |= (1U << 24);
GPIOD_MODER &= ~(3U << 26);
GPIOD_MODER |= (1U << 26);

// Toggle loop
while (1) {
    GPIOD_ODR ^= (1 << 12);  // Toggle red LED
    GPIOD_ODR ^= (1 << 13);  // Toggle orange LED
    delay();
}

Demo

Red and orange LEDs toggling in unison

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)

void led_delay(void) {
    for (volatile int i = 0; i < 500000; i++);
}

int main(void) {
    // Enable GPIOD clock
    RCC_AHB1ENR |= (1U << 3);
    
    // Configure PD12 and PD13 as outputs
    GPIOD_MODER &= ~(3U << 24);
    GPIOD_MODER |= (1U << 24);
    GPIOD_MODER &= ~(3U << 26);
    GPIOD_MODER |= (1U << 26);

    while (1) {
        // Toggle using XOR
        GPIOD_ODR ^= (1U << 12);
        GPIOD_ODR ^= (1U << 13);
        led_delay();
    }
    
    return 0;
}

Expected Output

Red and orange LEDs blink in synchronization at approximately 0.25 Hz (one complete cycle per 4 seconds).

Common Mistakes

Issue	Solution
LEDs don't respond	Verify MODER bits are 01 for output
Using |- instead of ^=	Use ^= for toggle, |= for set, &= for clear
No visible toggling	Increase delay value
Forgot volatile keyword	Add volatile to register pointers

Key Takeaways

✨ Remember:

1. XOR with 1 always flips a bit
2. No need to read state before toggling
3. Bitwise operations are fundamental in embedded systems
4. Multiple independent toggles in sequence work perfectly

Challenge Exercises

Challenge 1: Four-LED Pattern

Toggle all 4 LEDs (pins 12, 13, 14, 15) in sequence.

Challenge 2: Alternating Pattern

Create: Red ON → Orange OFF → Green ON → Blue OFF

Challenge 3: LED Pulse

Implement a pattern where LEDs light up sequentially, then turn off sequentially.

Next Steps

🚀 Ready for Lab 3? Learn to read GPIO input from a push button and control LED output!

Prerequisites for Lab 3: GPIO output control, Understanding bitwise operations