Datasheet

DS10690 Rev 6 23/144
STM32L071xx Functional overview
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– 32.768 kHz low-speed external crystal (LSE)
– 37 kHz low-speed internal RC (LSI), also used to drive the independent watchdog.
The LSI clock can be measured using the high-speed internal RC oscillator for
greater precision.
• RTC clock source
The LSI, LSE or HSE sources can be chosen to clock the RTC, whatever the system
clock.
• Startup clock
After reset, the microcontroller restarts by default with an internal 2.1 MHz clock (MSI).
The prescaler ratio and clock source can be changed by the application program as
soon as the code execution starts.
• Clock security system (CSS)
This feature can be enabled by software. If an HSE clock failure occurs, the master
clock is automatically switched to HSI and a software interrupt is generated if enabled.
Another clock security system can be enabled, in case of failure of the LSE it provides
an interrupt or wakeup event which is generated if enabled.
• Clock-out capability (MCO: microcontroller clock output)
It outputs one of the internal clocks for external use by the application.
Several prescalers allow the configuration of the AHB frequency, each APB (APB1 and
APB2) domains. The maximum frequency of the AHB and the APB domains is 32 MHz. See
Figure 2 for details on the clock tree.
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