/* p6_1.c using User Switch B1 interrupt
 *
 * User Switch B1 is used to generate interrupt through PC13.
 * The user button is connected to PC13. It has a pull-up resitor
 * so PC13 stays high when the button is not pressed.
 * When the button is pressed, PC13 becomes low.
 * The falling-edge of PC13 (when switch is pressed) triggers an
 * interrupt from External Interrupt Controller (EXTI).
 * In the interrupt handler, the user LD2 is blinked twice.
 * The green LED (LD2) is connected to PA5.
 *
 * This program was tested with Keil uVision v5.23 with DFP v2.0.0.
 */

#include "stm32f0xx.h"

void delayMs(int n);

int main(void)
{
    __disable_irq();                /* global disable IRQs */

    RCC->AHBENR |= 0x00080000;      /* enable GPIOC clock */
    RCC->AHBENR |= 0x00020000;      /* enable GPIOA clock */
    RCC->APB2ENR |= 0x00000001;     /* enable SYSCFG clock */

    /* configure PA5 for LED */
    GPIOA->MODER &= ~0x00000C00;    /* clear pin mode */
    GPIOA->MODER |=  0x00000400;    /* set pin to output mode */

    /* configure PC13 for push button interrupt */
    GPIOC->MODER &= ~0x0C000000;    /* clear pin mode to input mode */

    SYSCFG->EXTICR[3] &= ~0x00F0;   /* clear port selection for EXTI13 */
    SYSCFG->EXTICR[3] |= 0x0020;    /* select port C for EXTI13 */

    EXTI->IMR |= 0x2000;            /* unmask EXTI13 */
    EXTI->FTSR |= 0x2000;           /* select falling edge trigger */

    NVIC_EnableIRQ(EXTI4_15_IRQn);

    __enable_irq();                 /* global enable IRQs */

    while (1) {
    }
}

void EXTI4_15_IRQHandler(void)
{
    GPIOA->BSRR = 0x00000020;       /* turn on green LED */
    delayMs(250);
    GPIOA->BRR = 0x00000020;        /* turn off green LED */
    delayMs(250);
    GPIOA->BSRR = 0x00000020;       /* turn on green LED */
    delayMs(250);
    GPIOA->BRR = 0x00000020;        /* turn off green LED */
    delayMs(250);

    EXTI->PR = (1 << 13);           /* clear interrupt pending flag */
}

/* 8 MHz SYSCLK */
void delayMs(int n) {
    int i;
    for (; n > 0; n--)
        for (i = 0; i < 1142; i++) ;
}