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

- **ID:** `embedded/rtc-oscillator-failure`
- **Domain:** embedded
- **Category:** hardware_error
- **Verification:** ai_generated
- **Fix Rate:** 80%

## Root Cause

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

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| STM32Cube_FW_H7_V1.11.0 | active | — | — |
| STM32Cube_FW_F4_V1.27.1 | active | — | — |
| GCC ARM Embedded 12.2.Rel1 | active | — | — |

## Workarounds

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.** (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.
   ```
2. **Use an external 32.768 kHz square wave oscillator module (e.g., SiTime SiT1532) in bypass mode to bypass crystal issues.** (95% success)
   ```
   Use an external 32.768 kHz square wave oscillator module (e.g., SiTime SiT1532) in bypass mode to bypass crystal issues.
   ```

## Dead Ends

- **** — The MCU RTC peripheral is designed for 32.768 kHz; a higher frequency won't oscillate correctly. (85% fail)
- **** — Overdriving can damage the crystal or cause spurious oscillations; proper load capacitance matching is required. (70% fail)
- **** — LSI is less accurate (typically ±5% vs ±20 ppm for LSE) and may cause RTC drift in time-sensitive applications. (90% fail)