esp32: add API to get reset reason

components/esp32/include/esp_system.h

@@ -35,6 +35,25 @@ typedef enum {
 #define FOUR_UNIVERSAL_MAC_ADDR 4
 #define UNIVERSAL_MAC_ADDR_NUM CONFIG_NUMBER_OF_UNIVERSAL_MAC_ADDRESS
 
+
+/**
+ * @brief Reset reasons
+ */
+typedef enum {
+    ESP_RST_UNKNOWN,    //!< Reset reason can not be determined
+    ESP_RST_POWERON,    //!< Reset due to power-on event
+    ESP_RST_EXT,        //!< Reset by external pin (not applicable for ESP32)
+    ESP_RST_SW,         //!< Software reset via esp_restart
+    ESP_RST_PANIC,      //!< Software reset due to exception/panic
+    ESP_RST_INT_WDT,    //!< Reset (software or hardware) due to interrupt watchdog
+    ESP_RST_TASK_WDT,   //!< Reset due to task watchdog
+    ESP_RST_WDT,        //!< Reset due to other watchdogs
+    ESP_RST_DEEPSLEEP,  //!< Reset after exiting deep sleep mode
+    ESP_RST_BROWNOUT,   //!< Brownout reset (software or hardware)
+    ESP_RST_SDIO,       //!< Reset over SDIO
+} esp_reset_reason_t;
+
+/** @cond */
 /**
   * @attention  application don't need to call this function anymore. It do nothing and will
   *             be removed in future version.
@@ -68,17 +87,6 @@ esp_err_t esp_register_shutdown_handler(shutdown_handler_t handle);
   */
 void esp_restart(void) __attribute__ ((noreturn));
 
-/**
-  * @brief  Internal function to restart PRO and APP CPUs.
-  *
-  * @note This function should not be called from FreeRTOS applications.
-  *       Use esp_restart instead.
-  *
-  * This is an internal function called by esp_restart. It is called directly
-  * by the panic handler and brownout detector interrupt.
-  */
-void esp_restart_noos() __attribute__ ((noreturn));
-
 /**
   * @brief  Restart system.
   *
@@ -87,6 +95,12 @@ void esp_restart_noos() __attribute__ ((noreturn));
   */
 void system_restart(void) __attribute__ ((deprecated, noreturn));
 
+/**
+ * @brief  Get reason of last reset
+ * @return See description of esp_reset_reason_t for explanation of each value.
+ */
+esp_reset_reason_t esp_reset_reason(void);
+
 /**
   * @brief  Get system time, unit: microsecond.
   *

components/esp32/include/rom/rtc.h

@@ -68,6 +68,7 @@ extern "C" {
 #define RTC_XTAL_FREQ_REG       RTC_CNTL_STORE4_REG
 #define RTC_APB_FREQ_REG        RTC_CNTL_STORE5_REG
 #define RTC_ENTRY_ADDR_REG      RTC_CNTL_STORE6_REG
+#define RTC_RESET_CAUSE_REG     RTC_CNTL_STORE6_REG
 #define RTC_MEMORY_CRC_REG      RTC_CNTL_STORE7_REG