STM32H7: define RCC_LSE_BYPASS to allow using PC_15 as gpio

Author: facchinm

targets/TARGET_STM/lp_ticker.c

@@ -170,7 +170,11 @@ void lp_ticker_init(void)
 
     /* Enable LSE clock */
     RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_LSE;
+#if MBED_CONF_TARGET_LSE_BYPASS
+    RCC_OscInitStruct.LSEState = RCC_LSE_BYPASS;
+#else
     RCC_OscInitStruct.LSEState = RCC_LSE_ON;
+#endif
     RCC_OscInitStruct.PLL.PLLState = RCC_PLL_NONE;
 
     /* Select the LSE clock as LPTIM peripheral clock */

targets/TARGET_STM/rtc_api.c

@@ -65,7 +65,11 @@ void rtc_init(void)
 #if MBED_CONF_TARGET_LSE_AVAILABLE
     RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_LSE;
     RCC_OscInitStruct.PLL.PLLState   = RCC_PLL_NONE;
+#if MBED_CONF_TARGET_LSE_BYPASS
+    RCC_OscInitStruct.LSEState       = RCC_LSE_BYPASS;
+#else
     RCC_OscInitStruct.LSEState       = RCC_LSE_ON;
+#endif
 
     if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK) {
         error("Cannot initialize RTC with LSE\n");

targets/targets.json

@@ -2709,6 +2709,10 @@
             "usb_speed": {
                 "help": "USE_USB_OTG_FS or USE_USB_OTG_HS or USE_USB_HS_IN_FS",
                 "value": "USE_USB_OTG_HS"
+            },
+            "lse_bypass": {
+                "help": "1 to use an oscillator (not a crystal) on 32k LSE",
+                "value": "1"
             }
         },
         "macros_add": [