Use more efficient mode for STM32L4XX stop mode (#1097)

Co-authored-by: Frederic Pillon <frederic.pillon@st.com>

Author: angelnu

libraries/SrcWrapper/src/stm32/low_power.c

@@ -223,12 +223,24 @@ void LowPower_stop(serial_t *obj)
   HAL_PWREx_EnableFastWakeUp();
 #endif
 #ifdef __HAL_RCC_WAKEUPSTOP_CLK_CONFIG
-  /* Select HSI as system clock source after Wake Up from Stop mode */
+  /* Select MSI or HSI as system clock source after Wake Up from Stop mode */
+#ifdef RCC_STOP_WAKEUPCLOCK_MSI
+  __HAL_RCC_WAKEUPSTOP_CLK_CONFIG(RCC_STOP_WAKEUPCLOCK_MSI);
+#else
   __HAL_RCC_WAKEUPSTOP_CLK_CONFIG(RCC_STOP_WAKEUPCLOCK_HSI);
+#endif
 #endif
 
   /* Enter Stop mode */
-  HAL_PWR_EnterSTOPMode(PWR_LOWPOWERREGULATOR_ON, PWR_STOPENTRY_WFI);
+#if defined(STM32L4xx)
+  if ((WakeUpUart == NULL) || (WakeUpUart->Instance == (USART_TypeDef *)LPUART1_BASE)) {
+    // STM32L4xx supports STOP2 mode which halves consumption
+    HAL_PWREx_EnterSTOP2Mode(PWR_STOPENTRY_WFI);
+  } else
+#endif
+  {
+    HAL_PWR_EnterSTOPMode(PWR_LOWPOWERREGULATOR_ON, PWR_STOPENTRY_WFI);
+  }
 
   /* Exit Stop mode reset clocks */
   SystemClock_ConfigFromStop();