STM32F0xx system clock

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STM32F0xx system clock

The STM32F0xx microcontroller family has various clock sources that can be used to drive

the system clock:



An 8-MHz High-speed internal (HSI) RC oscillator clock



A 4- to 32-MHz High-speed external (HSE) oscillator clock



A Phase-locked loop (PLL) that is clocked by HSI or HSE or HSI48 oscillators



A 48-MHz High-speed internal (HSI48) RC oscillator clock

Typically, the High-speed internal (HSI) RC oscillator has a frequency of 8 MHz or 48 MHz

(HSI48) and consumes 80 µA (HSI) or 312 µA (HSI48).

The internal RC oscillator (HSI) has the advantage of providing a low-cost clock source (no

external components required). It also has a faster startup time and a lower power

consumption than the external oscillator. The HSI oscillator can be calibrated to improve its

accuracy. But even with calibration, the internal RC oscillator frequency is less accurate

than the frequency of an external crystal oscillator or a ceramic resonator (tens of ppm).

Note:

The internal 48 MHz RC oscillator is mainly used to provide a high precision clock to the

USB peripheral by means of a special Clock Recovery System (CRS) circuitry, which uses

the USB SOF signal or the LSE or an external signal to automatically adjust the oscillator

frequency on-the-fly, through a very small steps.

Figure 1. Simplified clock tree

The STM32F0xx devices also have three secondary clock sources (that cannot be used as

system clock sources):



A 40 kHz Low-Speed Internal (LSI) RC which is designed to drive the independent

watchdog and optionally the Real time clock (RTC). The LSI oscillator cannot be

calibrated, but can be measured to evaluate frequency deviations (due to temperature

and voltage changes).



A 32.768 kHz Low-speed external crystal (LSE crystal) which optionally drives the Real

time clock (RTC)



A 14 MHz high speed internal RC (HSI14) dedicated to ADC.