4 Clocks
The following clock sources can be used to drive the system clock (SYSCLK):
• HSI16: high-speed internal 16 MHz RC oscillator clock
• MSIS: multi-speed internal RC oscillator clock
• HSE: high-speed external crystal or clock, from 4 to 50 MHz
• PLL1 clock
The MSIS is used as system clock source after startup from reset, configured at 4 MHz.
The devices have the following additional clock sources:
• MSIK: multi-speed internal RC oscillator clock used for peripherals kernel clocks
• LSI: 32 kHz low-speed internal RC that drives the independent watchdog and optionally the RTC used for
auto-wakeup from Stop and Standby modes
• LSE: 32.768 kHz low-speed external crystal or clock that optionally drives the real-time clock (rtc_ck)
• HSI48: internal 48 MHz RC that potentially drives the OTG FS, the SDMMC and the RNG
• SHSI: secure high-speed internal RC that drives the secure AES (SAES).
• PLL2 and PLL3 clocks
Each clock source can be switched on or off independently when it is not used, to optimize power consumption.
Several pre-scalers can be used to configure the AHB and the APB frequencies domains with a maximum
frequency of 160 MHz.
4.1 HSE clock
The high-speed external clock signal (HSE) can be generated from the following clock sources:
• HSE external crystal/ceramic resonator
• HSE user external clock that feeds OSC_IN pin
The resonator and the load capacitors must be placed as close as possible to the oscillator pins in order
to minimize output distortion and startup stabilization time. The loading capacitance values must be adjusted
according to the selected oscillator.
AN5373
Clocks
AN5373 - Rev 1
page 17/37
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