Скачать или смотреть Stm32 Clock 1

The Clocks is a very important part of the configuration of your Cortex Mx.
All Cortex Mx have one or more internal oscillator (RC) and one or more external crystals oscillators (Xtal).
s a must to use the eternal clock for RTC and also for high speed serial communications (Ethernet, USART, etc).
Below there is a comparison from internal clock (RC) and external clock (crystal clock).
The top of external crystal oscillator are:
TCXO (Temperature Compensate Xtal Oscillator)
OCXO (Oven Controlled Xtal Oscillator)

For high speed serial communication (Ethernet) is a must use Xtal from 0.1 to max 1 ppm.

Normally the Xtal (for STM32) is in the range from 4 to 26 MHz and for reach the max MCU frequency is a must to use the internal PLL (phase looked loop).
The PLL multiply the Xtal frequency in order to reach the desired max MCU frequency.