work.c

/* enable exti0 interrupt */

    EXTI->IMR |= EXTI_IMR_MR0;

    /* nvic enable exti0 irq */

    NVIC_EnableIRQ(EXTI0_IRQn);

    /* setup SysTick timer */

    SysTick_Config(16000);

/* hardware delay and switch */

void hard_delay(void) {

    static unsigned int counter;

    counter++;

    /* wait a half second */

    if (counter > 500) {

        /* switch leds */

        LED_PORT->ODR ^= ALL_LEDS;

        counter = 0;

    }

    /* if exit0 interrupt disable */

    if (!(EXTI->IMR & EXTI_IMR_MR0)) {

        /* enable exti0 interrupt */

        EXTI->IMR |= EXTI_IMR_MR0;

    }

}