Fixed a problem that the STOP2 was falling back to STOP1 because

the LPR was not turned on. Now the deepsleep mode only needs 1.6uA
instead of 10uA.

Author: Helmut Tschemernjak

targets/TARGET_STM/sleep.c

@@ -52,17 +52,20 @@ void hal_deepsleep(void)
 
     // Request to enter STOP mode with regulator in low power mode
 #if TARGET_STM32L4
-    if (__HAL_RCC_PWR_IS_CLK_ENABLED()) {
-        HAL_PWREx_EnableLowPowerRunMode();
-        HAL_PWREx_EnterSTOP2Mode(PWR_STOPENTRY_WFI);
-        HAL_PWREx_DisableLowPowerRunMode();
-    } else {
+    int pwrClockEnabled = __HAL_RCC_PWR_IS_CLK_ENABLED();
+    int lowPowerModeEnabled = PWR->CR1 & PWR_CR1_LPR;
+    
+    if (!pwrClockEnabled)
         __HAL_RCC_PWR_CLK_ENABLE();
-        HAL_PWREx_EnableLowPowerRunMode();
-        HAL_PWREx_EnterSTOP2Mode(PWR_STOPENTRY_WFI);
+    if (lowPowerModeEnabled)
         HAL_PWREx_DisableLowPowerRunMode();
+    
+    HAL_PWREx_EnterSTOP2Mode(PWR_STOPENTRY_WFI);
+    
+    if (lowPowerModeEnabled)
+        HAL_PWREx_EnableLowPowerRunMode();
+    if (!pwrClockEnabled)
         __HAL_RCC_PWR_CLK_DISABLE();
-    }
 #else /* TARGET_STM32L4 */
     HAL_PWR_EnterSTOPMode(PWR_LOWPOWERREGULATOR_ON, PWR_STOPENTRY_WFI);
 #endif /* TARGET_STM32L4 */