startup.c

    0,                  /*!%5 RCC                          */                                           

    exti0_irq_handler,  /*!%6 EXTI Line0                   */                       

    0,                  /*!%7 EXTI Line1                   */

/* exti0 irq handler */

void exti0_irq_handler(void) {

    if (EXTI->PR | EXTI_PR_PR0) {

        /* change pwm submode for tim4 ccch2,4 */

        TIM4->CCMR1 ^= TIM_CCMR1_OC2M_0;

        TIM4->CCMR2 ^= TIM_CCMR2_OC4M_0;

        /* clear pending irq */

        EXTI->PR |= EXTI_PR_PR0;

        /* disable exti0 interrupt */

        EXTI->IMR &= ~EXTI_IMR_MR0;

    }

}