DocID026156 Rev 2

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AN4467

Clocks

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2.4

Clock security system on HSE (CSSHSE)

The clock security system can be activated by software. In this case, the clock detector is

enabled after the HSE oscillator startup delay, and disabled when this oscillator is stopped.

If a failure is detected on the HSE oscillator clock, this oscillator is automatically disabled

and an interrupt is generated to inform the software about the failure (clock security system

on HSE interrupt, CSSHSEI), allowing the MCU to perform rescue operations. The

If the HSE oscillator is used directly or indirectly as the system clock (indirectly means: it is

used as the PLL input clock, and the PLL clock is used as the system clock), a detected

failure causes the system clock to switch to the MSI oscillator and the external HSE

oscillator to be disabled. If the HSE oscillator clock is the clock entry of the PLL used as the

system clock when the failure occurs, the PLL is also disabled.

For details, see the STM32L0xx reference manuals (RM0367, RM0376, RM0377).

2.5

HSI16 clock

The HSI16 clock signal is generated from an internal 16 MHz RC oscillator and can be used

directly as a system clock or can used as PLL input. The HSI16 RC oscillator has the

advantage of providing a clock source at low cost (no external components). It also has a

faster startup time than the HSE crystal oscillator. However, even with calibration, the

frequency is less accurate than an external crystal oscillator or ceramic resonator.

Calibration

RC oscillator frequencies can vary from one chip to another due to manufacturing process

If the application is subject to voltage or temperature variations, the RC oscillator speed will

be impacted. You can trim the HSI16 frequency in the application through the RCC_ICSCR

register. Typically, this uses the HSE or LSE as reference (see RM0367/376/377 for details

on clock measurement with TIM21). For more information refer to AN3300 “How to calibrate

an STM32Lxx internal RC oscillator”.

2.6

LSI clock

The LSI RC acts as an low-power clock source that can be kept running in Stop and

Standby mode for the independent watchdog (IWDG) and RTC. The clock frequency is

around 40 kHz (between 30 and 60 kHz). For more details, refer to the electrical

characteristics section of the datasheets.