/*
 * NUCLEO-C031C6 BARE-METAL CODE - LED BLINK + BUTTON INTERRUPT
 * 
 * Board: Nucleo-C031C6 (STM32C031C6, Cortex-M0+)
 * LED:   PA5 (User LED LD2/LD4)
 * Button: PC13 (User Button B1 - active low)
 * 
 * Behavior:
 * - LED blinks continuously at 1Hz
 * - Pressing button toggles LED state (interrupt-driven)
 */

#include "stm32c0xx.h"

/* Simple delay function (approximate) */
void delay_ms(uint32_t ms) {
    for (volatile uint32_t i = 0; i < (ms * 4000); i++);
}

/* LED toggle function */
void led_toggle(void) {
    GPIOA->ODR ^= (1 << 5);
}

/* LED ON function */
void led_on(void) {
    GPIOA->ODR |= (1 << 5);
}

/* LED OFF function */
void led_off(void) {
    GPIOA->ODR &= ~(1 << 5);
}

/* EXTI4_15 Interrupt Handler */
void EXTI4_15_IRQHandler(void) {
    /* Check if EXTI line 13 triggered the interrupt */
    if (EXTI->FPR1 & (1 << 13)) {
        /* Clear the pending flag */
        EXTI->FPR1 = (1 << 13);
        
        /* Toggle LED on button press */
        led_toggle();
    }
}

int main(void) {
    /* ============================================
     * STEP 1: ENABLE CLOCKS
     * ============================================ */
    
    /* Enable clock for GPIOA (LED on PA5) */
    RCC->IOPENR |= RCC_IOPENR_GPIOAEN;
    
    /* Enable clock for GPIOC (Button on PC13) */
    RCC->IOPENR |= RCC_IOPENR_GPIOCEN;
    
    /* Enable clock for SYSCFG (for EXTI configuration) */
    RCC->APBENR2 |= RCC_APBENR2_SYSCFGEN;
    
    /* ============================================
     * STEP 2: CONFIGURE LED (PA5) AS OUTPUT
     * ============================================ */
    
    /* Clear mode bits for PA5 (bits 10-11) */
    GPIOA->MODER &= ~(GPIO_MODER_MODE5);
    /* Set as output (01 binary) */
    GPIOA->MODER |= (GPIO_MODER_MODE5_0);
    /* Configure as push-pull (default after reset) */
    GPIOA->OTYPER &= ~GPIO_OTYPER_OT5;
    /* Configure as low speed */
    GPIOA->OSPEEDR &= ~GPIO_OSPEEDR_OSPEED5;
    
    /* ============================================
     * STEP 3: CONFIGURE BUTTON (PC13) AS INPUT
     * ============================================ */
    
    /* Clear mode bits for PC13 (set as input) */
    GPIOC->MODER &= ~(GPIO_MODER_MODE13);
    
    /* Enable internal pull-up on PC13 */
    GPIOC->PUPDR &= ~(GPIO_PUPDR_PUPD13);
    GPIOC->PUPDR |= GPIO_PUPDR_PUPD13_0;  // Pull-up
    
    /* ============================================
     * STEP 4: CONFIGURE EXTI LINE 13 FOR PC13
     * ============================================ */
    
    /* 
     * For STM32C0: EXTI lines are configured through SYSCFG_CFGR1
     * Bits 27:24 control EXTI13 (bits 24-27)
     * Value 0 = PAx, 1 = PBx, 2 = PCx, 3 = PDx, etc.
     */
    
    /* Clear the bits for EXTI13 (bits 27:24 of SYSCFG_CFGR1) */
    SYSCFG->CFGR1 &= ~(0xF << 24);
    /* Set to 2 for PC13 (GPIO Port C) */
    SYSCFG->CFGR1 |= (2 << 24);
    
    /* ============================================
     * STEP 5: CONFIGURE EXTI TRIGGER MODE
     * ============================================ */
    
    /* Clear rising edge trigger for line 13 */
    EXTI->RTSR1 &= ~(1 << 13);
    
    /* Set falling edge trigger (button press: HIGH -> LOW) */
    EXTI->FTSR1 |= (1 << 13);
    
    /* ============================================
     * STEP 6: ENABLE EXTI INTERRUPT
     * ============================================ */
    
    /* Unmask EXTI line 13 (enable interrupt) */
    EXTI->IMR1 |= (1 << 13);
    
    /* ============================================
     * STEP 7: ENABLE INTERRUPT IN NVIC
     * ============================================ */
    
    /* EXTI4_15 interrupt number = 31 */
    NVIC_EnableIRQ(EXTI4_15_IRQn);
    /* Set priority (0 = highest) */
    NVIC_SetPriority(EXTI4_15_IRQn, 0);
    
    /* ============================================
     * STEP 8: MAIN LOOP - BLINK LED
     * ============================================ */
    
    /* Turn LED OFF initially */
    led_off();
    
    while (1) {
        /* Turn LED ON */
        led_on();
        delay_ms(500);
        
        /* Turn LED OFF */
        led_off();
        delay_ms(500);
    }
    
    /* Should never reach here */
    return 0;
}