Clock Sources and Types in STM32

Acknowledgements

I was googling, I came across this lecture slide: [](https://www.dmi.unict.it/santoro/teaching/lsm/slides/ClockSTM32.pdf)[^1][^3] by Professor Corrado Santoro (Universitaà di Catania). It basically covers all the necessary topics for understanding STM32 clock.

Clock Sources and Types in Diagram

In STM32F4, there are two categories:

• High Speed vs. Low Speed
• External vs. Internal

High Speed External (HSE)

• It has the advantage of producing a very accurate rate on the main clock.
• HSE can be generated from two possible clock: 1) crystal/ceramic resonator 2) user clock

High Speed Internal (HSI)

• This internal 16MHz RC oscillator clock can be used directly as a system clock (usually it’s STM32F4’s system clock by default).¹
• It provides a clock source at low cost because it requires no external components. It also has a faster startup time than the HSE crystal oscillator.
• However, the frequency is less accurate than an HSE.

Low Speed External (LSE)

• It is generated from the same source as HSE, an external crystal or a ceramic resonator.
• But the clock frequency (32.768 kHz) is significantly slow.²
• Then why is this needed? It is low-powered, but provided highly accurate clock source to the real-time clock peripheral (RTC) for clock/calendar or other timing functions.

Low Speed Internal (LSI)

• It acts as a low-power clock source that can be kept running in Stop and Standby mode for the independent *watchdog (IWDG) and Auto-wakeup unit (AWU).
• *Watchdog: If a software fails, or hangs (not refreshed) within the certain amount of time, then this hardware timer “watchdog” will trigger a system reset to restart the application. Independent Watchdog (IWDG) has its own clock source whereas Window Watchdog (WWDG) is clocked from the main system clock.²

Reference