void configure_exti() {
     RCC_AHB1ENRbits.GPIOAEN = 1; //enable PA clock
     RCC_APB2ENRbits.SYSCFGEN = 1; //enable syscfg periph
     GPIOA_MODERbits.MODER8 = 0; //set PA0 as input
     GPIOA_PUPDRbits.PUPDR8 = 0x1; //pullup

     SYSCFG_EXTICR3bits.EXTI8 = 0x0; //set PA8 to be EXTI pin
     EXTI_IMRbits.MR8 = 1; //unmask bit
     EXTI_RTSRbits.TR8 = 1; //detect falling edge

     NVIC_IntEnable(IVT_INT_EXTI9_5);
     EnableInterrupts();
}