diff --git a/src/system.c b/src/system.c
--- a/src/system.c
+++ b/src/system.c
@@ -32,11 +32,12 @@ void sysclk_init(void)
     RCC_ClkInitTypeDef RCC_ClkInitStruct;
     RCC_PeriphCLKInitTypeDef PeriphClkInit;
 
-    RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSI|RCC_OSCILLATORTYPE_HSI14;
+    RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSI|RCC_OSCILLATORTYPE_HSI14|RCC_OSCILLATORTYPE_LSI;
     RCC_OscInitStruct.HSIState = RCC_HSI_ON;
     RCC_OscInitStruct.HSI14State = RCC_HSI14_ON;
     RCC_OscInitStruct.HSICalibrationValue = 16;
     RCC_OscInitStruct.HSI14CalibrationValue = 16;
+    RCC_OscInitStruct.LSIState = RCC_LSI_ON;
     RCC_OscInitStruct.PLL.PLLState = RCC_PLL_NONE;
     HAL_RCC_OscConfig(&RCC_OscInitStruct);
 
@@ -50,6 +51,7 @@ void sysclk_init(void)
     PeriphClkInit.PeriphClockSelection = RCC_PERIPHCLK_USART1|RCC_PERIPHCLK_I2C1;
     PeriphClkInit.Usart1ClockSelection = RCC_USART1CLKSOURCE_SYSCLK; //RCC_USART1CLKSOURCE_PCLK1;
     PeriphClkInit.I2c1ClockSelection = RCC_I2C1CLKSOURCE_SYSCLK;
+    PeriphClkInit.RTCClockSelection = RCC_RTCCLKSOURCE_LSI;
     HAL_RCCEx_PeriphCLKConfig(&PeriphClkInit);
 
     HAL_SYSTICK_Config(HAL_RCC_GetHCLKFreq()/1000);