9 Clock sources
9.1 HSE clock (high‑speed external clock)
There are four ways to configure the pins corresponding to the high‑speed external clock (HSE):
• MCO from STLINK-V3EC (default): The MCO output of ST-LINK is used as an input clock of the STM32H5.
The MCO clock frequency cannot be changed. It is fixed at 8 MHz and connected to the PF0/PH0‑OSC_IN
of the STM32H5 series microcontroller. The configuration must be:
– SB49 ON
– SB48 and SB50 OFF
– SB3 and SB4 OFF
• HSE on-board oscillator from X3 crystal (provided): For typical frequencies and its capacitors and resistors,
refer to the STM32H5 series microcontroller datasheet and the application note Oscillator design guide for
STM8AF/AL/S, STM32 MCUs and MPUs (AN2867) for the oscillator design guide. The X3 crystal has the
following characteristics: 25 MHz, 6 pF, and 20 ppm. ST recommends using NX2016SA-25MHz-EXS00A-
CS11321 manufactured by NDK. The configuration must be:
– SB3 and SB4 ON
– C69 and C70 ON with 5.6 pF capacitors
– SB48 and SB50 OFF
– SB49 OFF
• Oscillator from external PF0/PH0: From an external oscillator through pin 29 of the CN11 connector. The
configuration must be:
– SB50 ON
– SB48 and SB49 OFF
– SB3 and SB4 OFF
• HSE not used: PF0/PH0 and PF1/PH1 are used as GPIOs instead of clocks. The configuration must be:
– SB48 and SB50 ON
– SB49 OFF
– SB3 and SB4 OFF
9.2
LSE clock (low‑speed external clock): 32.768 kHz
There are three ways to configure the pins corresponding to the low-speed clock (LSE):
• On-board oscillator (default): X2 crystal. Refer to the application note Oscillator design guide for
STM8AF/AL/S, STM32 MCUs and MPUs (AN2867) for oscillator design guide for STM32H5 series
microcontrollers. ST recommends using NX3215SA-32.768kHZ-EXS00A-MU00525 (32.768 kHz, 9 pf load
capacitance, 20 ppm) from NDK. The configuration must be:
– SB44 and SB45 OFF
– R34 and R35 ON
• Oscillator from external PC14: From an external oscillator through pin 25 of the CN11 connector. The
configuration must be:
– SB45 ON
– SB44 OFF
– R34 and R35 OFF
• LSE not used: PC14 and PC15 are used as GPIOs instead of the low-speed clock.
The configuration must be:
– SB44 and SB45 ON
– R34 and R35 OFF
UM3115
Clock sources
UM3115 - Rev 2
page 19/44
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