Posted on May 17, 2011 at 15:11Take a look at
http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/ERRATA_SHEET/CD00237242.pdf
section 1.2.7 :
LSE and HSE in bypass mode cannot be used as RTC clockDescription Low speed external (LSE) and high speed external (LSE) oscillators can be configured in bypass mode if an external oscillator is used. In that case, the LSE/HSE oscillator is bypassed and the external clock signal is directly transmitted to the MCU.
If the LSE bypass or HSE bypass is used as RTC clock, the bypass configuration is lost during a reset and cannot be set again by software after reset. The external clock signal is no longer transmitted to the MCU and only a power-on reset can allow a recovery from this state.
Workaround None.
LSE bypass mode and HSE bypass mode (external clocks) must not be used as RTC clock.
