LSE bypass looks to be unsupported by mbed.
@@ -161,7 +161,7 @@
#endif /* LSE_VALUE */
#if !defined (LSE_STARTUP_TIMEOUT)
- #define LSE_STARTUP_TIMEOUT ((uint32_t)5000U) /*!< Time out for LSE start up, in ms */
+ #define LSE_STARTUP_TIMEOUT ((uint32_t)200U) /*!< Time out for LSE start up, in ms */
#endif /* HSE_STARTUP_TIMEOUT */
/**