stmhal/rtc: LSx oscillator is only initialized upon initial power up.

Initial power up also includes VBAT.

If LSE is configured but fails to start, LSI is used until next full power
cycle.  Also handles STM32F7xx variant.

stmhal/rtc.c

@@ -160,9 +160,14 @@ void rtc_init(void) {
 
 STATIC void RTC_CalendarConfig(void);
 
+#if defined(MICROPY_HW_RTC_USE_LSE) && MICROPY_HW_RTC_USE_LSE
+STATIC bool rtc_use_lse = true;
+#else
+STATIC bool rtc_use_lse = false;
+#endif
+
 void rtc_init(void) {
     RTCHandle.Instance = RTC;
-    RTC_DateTypeDef date;
 
     /* Configure RTC prescaler and RTC data registers */
     /* RTC configured as follow:
@@ -179,49 +184,62 @@ void rtc_init(void) {
     RTCHandle.Init.OutPutPolarity = RTC_OUTPUT_POLARITY_HIGH;
     RTCHandle.Init.OutPutType = RTC_OUTPUT_TYPE_OPENDRAIN;
 
-    // if LTE enabled & ready --> no need to (re-)init RTC
     if ((RCC->BDCR & (RCC_BDCR_LSEON | RCC_BDCR_LSERDY)) == (RCC_BDCR_LSEON | RCC_BDCR_LSERDY)) {
+        // LSE is enabled & ready --> no need to (re-)init RTC
         // remove Backup Domain write protection
-        #if defined(MCU_SERIES_F7)
-        PWR->CR1 |= PWR_CR1_DBP;
-        #else
-        PWR->CR |= PWR_CR_DBP;
-        #endif
+        HAL_PWR_EnableBkUpAccess();
         // Clear source Reset Flag
         __HAL_RCC_CLEAR_RESET_FLAGS();
         // provide some status information
         rtc_info |= 0x40000 | (RCC->BDCR & 7) | (RCC->CSR & 3) << 8;
         return;
+    } else if ((RCC->BDCR & RCC_BDCR_RTCSEL) == RCC_BDCR_RTCSEL_1) {
+        // LSI is already active
+        // remove Backup Domain write protection
+        HAL_PWR_EnableBkUpAccess();
+        // Clear source Reset Flag
+        __HAL_RCC_CLEAR_RESET_FLAGS();
+        RCC->CSR |= 1;
+        // provide some status information
+        rtc_info |= 0x80000 | (RCC->BDCR & 7) | (RCC->CSR & 3) << 8;
+        return;
     }
 
     mp_uint_t tick = HAL_GetTick();
 
     if (HAL_RTC_Init(&RTCHandle) != HAL_OK) {
-        // init error
-        rtc_info = 0xffff; // indicate error
-        return;
+        if (rtc_use_lse) {
+            // fall back to LSI...
+            rtc_use_lse = false;
+            PWR->CR |= PWR_CR_DBP;
+            RTCHandle.State = HAL_RTC_STATE_RESET;
+            if (HAL_RTC_Init(&RTCHandle) != HAL_OK) {
+                rtc_info = 0x0100ffff; // indicate error
+                return;
+            }
+        } else {
+            // init error
+            rtc_info = 0xffff; // indicate error
+            return;
+        }
     }
 
     // record how long it took for the RTC to start up
     rtc_info = HAL_GetTick() - tick;
 
-    HAL_RTC_GetDate(&RTCHandle, &date, FORMAT_BIN);
-    if (date.Year == 0 && date.Month ==0 && date.Date == 0) {
-        // fresh reset; configure RTC Calendar
-        RTC_CalendarConfig();
-    } else {
-        // RTC was previously set, so leave it alone
-        if(__HAL_RCC_GET_FLAG(RCC_FLAG_PORRST) != RESET) {
-            // power on reset occurred
-            rtc_info |= 0x10000;
-        }
-        if(__HAL_RCC_GET_FLAG(RCC_FLAG_PINRST) != RESET) {
-            // external reset occurred
-            rtc_info |= 0x20000;
-        }
-        // Clear source Reset Flag
-        __HAL_RCC_CLEAR_RESET_FLAGS();
+    // fresh reset; configure RTC Calendar
+    RTC_CalendarConfig();
+
+    if(__HAL_RCC_GET_FLAG(RCC_FLAG_PORRST) != RESET) {
+        // power on reset occurred
+        rtc_info |= 0x10000;
     }
+    if(__HAL_RCC_GET_FLAG(RCC_FLAG_PINRST) != RESET) {
+        // external reset occurred
+        rtc_info |= 0x20000;
+    }
+    // Clear source Reset Flag
+    __HAL_RCC_CLEAR_RESET_FLAGS();
 }
 
 STATIC void RTC_CalendarConfig(void) {
@@ -275,24 +293,24 @@ void HAL_RTC_MspInit(RTC_HandleTypeDef *hrtc) {
 
     RCC_OscInitStruct.OscillatorType =  RCC_OSCILLATORTYPE_LSI | RCC_OSCILLATORTYPE_LSE;
     RCC_OscInitStruct.PLL.PLLState = RCC_PLL_NONE;
-    #if defined(MICROPY_HW_RTC_USE_LSE) && MICROPY_HW_RTC_USE_LSE
-    RCC_OscInitStruct.LSEState = RCC_LSE_ON;
-    RCC_OscInitStruct.LSIState = RCC_LSI_OFF;
-    #else
-    RCC_OscInitStruct.LSEState = RCC_LSE_OFF;
-    RCC_OscInitStruct.LSIState = RCC_LSI_ON;
-    #endif
+    if (rtc_use_lse) {
+        RCC_OscInitStruct.LSEState = RCC_LSE_ON;
+        RCC_OscInitStruct.LSIState = RCC_LSI_OFF;
+    } else {
+        RCC_OscInitStruct.LSEState = RCC_LSE_OFF;
+        RCC_OscInitStruct.LSIState = RCC_LSI_ON;
+    }
     if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK) {
         //Error_Handler();
         return;
     }
 
     PeriphClkInitStruct.PeriphClockSelection = RCC_PERIPHCLK_RTC;
-    #if defined(MICROPY_HW_RTC_USE_LSE) && MICROPY_HW_RTC_USE_LSE
-    PeriphClkInitStruct.RTCClockSelection = RCC_RTCCLKSOURCE_LSE;
-    #else
-    PeriphClkInitStruct.RTCClockSelection = RCC_RTCCLKSOURCE_LSI;
-    #endif
+    if (rtc_use_lse) {
+        PeriphClkInitStruct.RTCClockSelection = RCC_RTCCLKSOURCE_LSE;
+    } else {
+        PeriphClkInitStruct.RTCClockSelection = RCC_RTCCLKSOURCE_LSI;
+    }
     if (HAL_RCCEx_PeriphCLKConfig(&PeriphClkInitStruct) != HAL_OK) {
         //Error_Handler();
         return;