Datasheet

DS10668 Rev 6 23/126
STM32L031x4/6 Functional overview
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3.5 Clock management
The clock controller distributes the clocks coming from different oscillators to the core and
the peripherals. It also manages clock gating for low-power modes and ensures clock
robustness. It features:
• Clock prescaler
To get the best trade-off between speed and current consumption, the clock frequency
to the CPU and peripherals can be adjusted by a programmable prescaler.
• Safe clock switching
Clock sources can be changed safely on the fly in Run mode through a configuration
register.
• Clock management
To reduce power consumption, the clock controller can stop the clock to the core,
individual peripherals or memory.
• System clock source
Three different clock sources can be used to drive the master clock SYSCLK:
– 1-25 MHz high-speed external (HSE), that can supply a PLL
– 16 MHz high-speed internal RC oscillator (HSI), trimmable by software, that can
supply a PLL
– Multispeed internal RC oscillator (MSI), trimmable by software, able to generate 7
frequencies (65 kHz, 131 kHz, 262 kHz, 524 kHz, 1.05 MHz, 2.1 MHz, 4.2 MHz).
When a 32.768 kHz clock source is available in the system (LSE), the MSI
frequency can be trimmed by software down to a ±0.5% accuracy.
• Auxiliary clock source
Two ultra-low-power clock sources that can be used to drive the real-time clock:
– 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 sources
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 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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