void SYSCTL_Init (void){
    // get current value
    uint32_t rcc_buf = SYSCTL->RCC;

    // bypass the PLL and system clock divider
    rcc_buf |= SYSCTL_RCC_BYPASS;
    rcc_buf &= ~SYSCTL_RCC_USESYSDIV;
    // write new value
    SYSCTL->RCC = rcc_buf;

    // enable main osc
    rcc_buf &= ~SYSCTL_RCC_MOSCDIS;
    // write new value
    SYSCTL->RCC = rcc_buf;
    SysCtlDelay(524288);

    // select the crystal value and oscillator source
    rcc_buf &= ~(SYSCTL_RCC_XTAL_M | SYSCTL_RCC_OSCSRC_M);
    rcc_buf |= SYSCTL_RCC_XTAL_6MHZ;
    rcc_buf |= SYSCTL_RCC_OSCSRC_MAIN;
    // write new value
    SYSCTL->RCC = rcc_buf;

    SysCtlDelay(16);

    // Clear the PLL lock interrupt.
    SYSCTL->MISC |= SYSCTL_MISC_PLLLMIS;

    // power-down PLL desable
    rcc_buf &= ~SYSCTL_RCC_PWRDN;
    // select the desired system divider
    rcc_buf &= ~SYSCTL_RCC_SYSDIV_M;
    rcc_buf |= SYSCTL_RCC_SYSDIV_4;
    // enable use of the system clock divider
    rcc_buf |= SYSCTL_RCC_USESYSDIV;
    // write new value
    SYSCTL->RCC = rcc_buf;

    while (bit_is_clear(SYSCTL->RIS,SYSCTL_RIS_PLLLRIS)){}
    SYSCTL->MISC |= SYSCTL_MISC_PLLLMIS;

    // enable use of the PLL
    rcc_buf &= ~SYSCTL_RCC_BYPASS;
    // write new value
    SYSCTL->RCC = rcc_buf;

    SysCtlDelay(16);
}