ST STM32U3 Application Note page 17

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, from 3 to 96 MHz
• HSE: high-speed external crystal or clock, from 4 to 50 MHz
• Accurate MSIS in PLL-mode from 3 to 96 MHz with input from 32.768 kHz, 16 MHz, or 32 MHz crystal
oscillator.
The MSIS is used as system clock source after start-up from reset, configured at 12 MHz.
The devices have the following additional clock sources:
• MSIK: multi-speed internal RC oscillator clock used for peripheral kernel clocks, from 3 to 96 MHz
• LSI: 32 kHz/250 Hz low-speed internal RC that drives the independent watchdog and optionally the RTC
used for auto-wake-up 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 USB, the SDMMC and the RNG
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 96 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 start-up stabilization time. The loading capacitance values must be adjusted
according to the selected oscillator.
Clock source
External clock
Crystal/ceramic resonators
4.1.1
AN6011 - Rev 1
Table 3. HSE/LSE clock sources
Hardware configuration
OSC_IN
External souce
OSC_IN
C
L1
CL1 and CL2 values depend on the quartz. Refer to document
OSC_OUT
GPIO
OSC_OUT
C
L2
Load
capacitors
[3]
AN6011
Clocks
for more details.
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