Osc Clock Sources; Lse: Osc 32 Khz Clock Supply - ST STM32U5 User Manual

6.7

OSC clock sources

Three clock sources are described below:
• LSE is the 32.768 kHz crystal for the STM32 embedded RTC.
• MCO is the 8 MHz clock from STLINK-V3E MCU for the STM32 microcontroller.
• HSE is the 16 MHz oscillator for the STM32 microcontroller. This clock is available depending on the target
STM32 series microcontroller used on the Nucleo-144 board.
6.7.1

LSE: OSC 32 KHz clock supply

There are three ways to configure the pins corresponding to the low-speed clock (LSE):
LSE on-board oscillator X2 crystal (Default configuration)
Refer to the application note Oscillator design guide for STM8AF/AL/S, STM32 MCUs and MPUs (AN2867).
The X3 crystal has the following characteristics: 32.768 kHz, 9 pF, 20 ppm. It is recommended to use the
NX1610SE‑ 32.768KHZ‑EXS00A‑ MU01499 manufactured by NDK. The following configuration is needed:
• R32 and R33 ON
• SB51 and SB52 OFF
External oscillator connected to PC14 input
From the external oscillator through pin 25 of the ST morpho connector (CN11). The following configuration is
needed:
• R32 and R33 OFF
• SB51 and SB52 ON, for connection from Zio connector CN11 pin 25
LSE not used
PC14 and PC15 are used as GPIOs instead of low‑speed clocks. The following configuration is needed:
• R32 and R33 OFF
• SB51 and SB52 ON
UM2861 - Rev 5
UM2861
OSC clock sources
page 26/49