void clock_init(void)
{
    // enable HSI
    RCC->CR |= RCC_CR_HSION;
    while ((RCC->CR & RCC_CR_HSIRDY) ==0);

    RCC->CFGR &=~RCC_CFGR_SW;
    RCC->CFGR |= RCC_CFGR_SW_HSI;
    while((RCC->CFGR & RCC_CFGR_SWS_HSI) != RCC_CFGR_SWS_HSI);

    // ==========================================


}