STM32 RNG Clocking Error

I'm beginner in embedded developent, and I'm using board STM32F417VGx

There are troubles with getting RNG to work
I tried to do it either with wrighting values directly to registers and with CMSIS Peripheral Libraty fuctions.

So, my code:

int main() {
    SystemInit();              /* Initialize clocks */
    Init_GPIO();               /* Initialize LED's pins */
    Init_RNG();                /* Initialize RNG */

    RCC_ClocksTypeDef clocks;  
    RCC_GetClocksFreq(&clocks); /* Check clocks */
                                /* All clocks are 16 MHz (F42400) */

    while(1) {
        unsigned rdiode = randomRange(0, 3);
        toggleDiode(GPIOD, DIODES[rdiode]);
        soft_delay(1000000);
    }

}

InitRNG():

void Init_RNG() {
    RCC_AHB2PeriphClockCmd(RCC_AHB2Periph_RNG, ENABLE);
    RNG_Cmd(ENABLE);
}

randomRange(uint, uint):

uint32_t randomRange(uint32_t low, uint32_t int high){
    while(RNG_GetFlagStatus(RNG_FLAG_DRDY) == RESET);
    return RNG_GetRandomNumber() % (high-low) + low;
}

And it hangs on while(RNG_GetFlagStatus(RNG_FLAG_DRDY) == RESET);
CEIS in RNG_SR is 0x1, so there is clocking error.

I'd looked everywhere in google, in every tutorial RNG works out-of-box, nobody even cares about checking SEIS and CEIS flags (?_?)
So I know that RNG needs at least 48MHz clock, but I can't find out what clocking source it uses and how to ajust it's clock :C

In stm32f4xx.h I have those clocking values (was recommended in one tutorial), but HSE is external (not on-board) clocking source, isn't it?

#define HSE_VALUE    ((uint32_t)8000000)