STM32L496XX Datasheet, PDF (36/263 Pages) STMicroelectronics – 25 nA Shutdown mode

Functional overview
STM32L496xx
3.12
Clocks and startup
The clock controller (see Figure 4) 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: four different clock sources can be used to drive the master
clock SYSCLK:
– 4-48 MHz high-speed external crystal or ceramic resonator (HSE), that can supply
a PLL. The HSE can also be configured in bypass mode for an external clock.
– 16 MHz high-speed internal RC oscillator (HSI16), trimmable by software, that can
supply a PLL
– Multispeed internal RC oscillator (MSI), trimmable by software, able to generate
12 frequencies from 100 kHz to 48 MHz. When a 32.768 kHz clock source is
available in the system (LSE), the MSI frequency can be automatically trimmed by
hardware to reach better than ±0.25% accuracy. In this mode the MSI can feed the
USB device, saving the need of an external high-speed crystal (HSE). The MSI
can supply a PLL.
– System PLL which can be fed by HSE, HSI16 or MSI, with a maximum frequency
at 80 MHz.
• RC48 with clock recovery system (HSI48): internal 48 MHz clock source (HSI48)can be
used to drive the USB, the SDMMC or the RNG peripherals. This clock can be output
on the MCO.
• Auxiliary clock source: two ultralow-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), supporting four drive capability
modes. The LSE can also be configured in bypass mode for an external clock.
– 32 kHz low-speed internal RC (LSI), also used to drive the independent watchdog.
The LSI clock accuracy is ±5% accuracy.
• Peripheral clock sources: Several peripherals (USB, SDMMC, RNG, SAI, USARTs,
I2Cs, LPTimers, ADC, SWPMI) have their own independent clock whatever the system
clock. Three PLLs, each having three independent outputs allowing the highest
flexibility, can generate independent clocks for the ADC, the USB/SDMMC/RNG and
the two SAIs.
• Startup clock: after reset, the microcontroller restarts by default with an internal 4 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 HSI16 and a software
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