These programs will use direct register access for clarity of understanding how peripherals are configured at the lowest level. This is sometimes called "bare-metal" programming.

Assumption: Using an STM32F401RE Nucleo board.
- On-board LED: Connected to PA5 (GPIO Port A, Pin 5).
- On-board User Button: Connected to PC13 (GPIO Port C, Pin 13).
- System Clock Frequency: Assuming default 16 MHz HSI (internal RC oscillator) or configured to 84 MHz (common for STM32F4). We'll assume 84 MHz system clock for timer example for a practical number.

Objective: To continuously blink the on-board LED (PA5) with a 500 ms ON/500 ms OFF period using a simple software delay.

// Include the appropriate device header file for STM32F4xx series
#include "stm32f4xx.h"
// Simple software delay function
void delay_ms(unsigned int ms) {
    unsigned long i;
    unsigned long cycles_per_ms = 84000; // For 84 MHz, approx 84000 cycles for 1ms
    for (i = 0; i < ms * cycles_per_ms; i++);
}
int main(void) {
    // Enable Clock for GPIOA
    RCC->AHB1ENR |= RCC_AHB1ENR_GPIOAEN;
    // Configure PA5 as General Purpose Output
    GPIOA->MODER &= ~(GPIO_MODER_MODER5_Msk);
    GPIOA->MODER |= GPIO_MODER_MODER5_0;
    // Turn LED ON (Set PA5 high)
    GPIOA->ODR |= (1U << 5);
    delay_ms(500);
    // Turn LED OFF (Set PA5 low)
    GPIOA->ODR &= ~(1U << 5);
    delay_ms(500);
}

Objective: To read the state of the on-board User Button (PC13) and control the on-board LED (PA5). When the button is pressed, the LED should turn ON; when released, it should turn OFF. (Assuming the button is active-low: connected to GND when pressed, pulled high when released).

#include "stm32f4xx.h"
#define LED_PIN_NUM 5
#define BUTTON_PIN_NUM 13
int main(void) {
    RCC->AHB1ENR |= RCC_AHB1ENR_GPIOAEN; // Enable GPIOA clock
    RCC->AHB1ENR |= RCC_AHB1ENR_GPIOCEN; // Enable GPIOC clock
    GPIOA->MODER &= ~(GPIO_MODER_MODER5_Msk);
    GPIOA->MODER |= GPIO_MODER_MODER5_0;
    GPIOC->MODER &= ~(GPIO_MODER_MODER13_Msk); // Input mode
    GPIOC->PUPDR |= GPIO_PUPDR_PUPDR13_0; // Pull-up
    while (1) {
        if ((GPIOC->IDR & (1U << BUTTON_PIN_NUM)) == 0) {
            GPIOA->ODR |= (1U << LED_PIN_NUM);
        } else {
            GPIOA->ODR &= ~(1U << LED_PIN_NUM);
        }
    }
}

Objective: To generate a precise delay of 500 milliseconds using a general-purpose timer (e.g., TIM2) by polling its update flag, and use this delay to blink the on-board LED (PA5).

#include "stm32f4xx.h"
#define LED_PIN_NUM 5
void timer_delay_ms(unsigned int ms) {
    RCC->APB1ENR |= RCC_APB1ENR_TIM2EN; // Enable TIM2
    TIM2->PSC = 41999; // Set prescaler
    TIM2->ARR = ms - 1; // Set ARR
    TIM2->EGR |= TIM_EGR_UG; // Update event
    while (!(TIM2->SR & TIM_SR_UIF)); // Wait for UIF
    TIM2->SR &= ~TIM_SR_UIF; // Clear UIF
    TIM2->CR1 &= ~TIM_CR1_CEN; // Stop timer
}
int main(void) {
    RCC->AHB1ENR |= RCC_AHB1ENR_GPIOAEN;
    GPIOA->MODER &= ~(GPIO_MODER_MODER5_Msk);
    GPIOA->MODER |= GPIO_MODER_MODER5_0;
    while (1) {
        GPIOA->ODR |= (1U << LED_PIN_NUM);
        timer_delay_ms(500);
        GPIOA->ODR &= ~(1U << LED_PIN_NUM);
        timer_delay_ms(500);
    }
}