embedded hardware_error ai_generated partial

RTC: LSE oscillator not ready, switching to LSI backup clock

ID: embedded/rtc-oscillator-failure

Also available as: JSON · Markdown · 中文
80%Fix Rate
85%Confidence
1Evidence
2024-02-15First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
STM32Cube_FW_H7_V1.11.0 active — — —
STM32Cube_FW_F4_V1.27.1 active — — —
GCC ARM Embedded 12.2.Rel1 active — — —

Root Cause

Low-speed external (LSE) crystal oscillator failed to start or stabilize, usually due to incorrect load capacitors, crystal damage, or PCB parasitics.

generic

中文

低速外部晶振未能启动或稳定，通常由于负载电容错误、晶振损坏或PCB寄生效应引起。

Official Documentation

https://www.st.com/resource/en/application_note/dm00231744-oscillator-design-guide-for-stm32-mcus-and-mpus-stmicroelectronics.pdf

Workarounds

  1. 75% success Verify LSE load capacitors: for a 12.5 pF crystal, use two 22 pF caps in series (effective 11 pF). Adjust via RCC_BDCR register in firmware if needed. 
    Verify LSE load capacitors: for a 12.5 pF crystal, use two 22 pF caps in series (effective 11 pF). Adjust via RCC_BDCR register in firmware if needed.
  2. 95% success Use an external 32.768 kHz square wave oscillator module (e.g., SiTime SiT1532) in bypass mode to bypass crystal issues. 
    Use an external 32.768 kHz square wave oscillator module (e.g., SiTime SiT1532) in bypass mode to bypass crystal issues.

中文步骤

  1. Verify LSE load capacitors: for a 12.5 pF crystal, use two 22 pF caps in series (effective 11 pF). Adjust via RCC_BDCR register in firmware if needed.
  2. Use an external 32.768 kHz square wave oscillator module (e.g., SiTime SiT1532) in bypass mode to bypass crystal issues.

Dead Ends

Common approaches that don't work:

  1. 85% fail

    The MCU RTC peripheral is designed for 32.768 kHz; a higher frequency won't oscillate correctly.

  2. 70% fail

    Overdriving can damage the crystal or cause spurious oscillations; proper load capacitance matching is required.

  3. 90% fail

    LSI is less accurate (typically ±5% vs ±20 ppm for LSE) and may cause RTC drift in time-sensitive applications.