diff --git a/system_stm32l1xx.h b/system_stm32l1xx.h
--- a/system_stm32l1xx.h
+++ b/system_stm32l1xx.h
@@ -58,6 +58,8 @@
 
 extern uint32_t SystemCoreClock;          /*!< System Clock Frequency (Core Clock) */
 
+extern uint8_t clock_fail;
+
 /**
   * @}
   */