Functional overview STM32L100x6/8/B-A
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.
• Master clock source : three different clock sources can be used to drive the master clock:
– 1-24 MHz high-speed external crystal (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 (65.5 kHz, 131 kHz, 262 kHz, 524 kHz, 1.05 MHz, 2.1 MHz, 4.2 MHz) with a consumption proportional to speed, down to 750 nA typical. 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 LCD controller and 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 and LCD clock sources: the LSI, LSE or HSE sources can be chosen to clock the RTC and the LCD, whatever the system clock.
• USB clock source: the embedded PLL has a dedicated 48 MHz clock output to supply the USB interface.
• 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 a HSE clock failure occurs, the master clock is automatically switched to HSI and a software interrupt 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, the high-speed APB
(APB2) and the low-speed APB (APB1) 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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Functional overview
Figure 2. Clock tree
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