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August 3, 2026
Question

Min LSE Clock Speed STM32U5

  • August 3, 2026
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The table in the datasheet says that the minimum frequency for LSE in bypass mode is 5kHz (Table 71 of DS13543).
In previous designs (different STM32 MCU) I have successfully used a 125Hz input for the LSE (setting async prescaler to 1 and sync prescaler to 125), however on checking their datasheets they don’t state a minimum frequency.
I have tried running the RTC from a 125Hz source on the STM32U5A5 nucleo board that I have, and it seems to be running fine. Are there any potential problems I might encounter from doing this, as I usually try to avoid going against the datasheet!
I’m just using the LSE for the RTC, not trying to drive any other peripherals.