brownout detector: enable by default

Because of errata related to BOD reset function, brownout is handled as follows:

- attach an ISR to brownout interrupt
- when ISR happens, print a message and do a software restart
- esp_restart_nonos enables RTC watchdog, so if restart fails,
  there will be  one more attempt to restart (using the RTC
  watchdog)

components/esp32/brownout.c

@@ -0,0 +1,61 @@
+// Copyright 2015-2017 Espressif Systems (Shanghai) PTE LTD
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdbool.h>
+#include "sdkconfig.h"
+#include "soc/soc.h"
+#include "soc/cpu.h"
+#include "soc/rtc_cntl_reg.h"
+#include "rom/ets_sys.h"
+#include "esp_system.h"
+#include "driver/rtc_cntl.h"
+#include "freertos/FreeRTOS.h"
+
+#ifdef CONFIG_BROWNOUT_DET_LVL
+#define BROWNOUT_DET_LVL CONFIG_BROWNOUT_DET_LVL
+#else
+#define BROWNOUT_DET_LVL 0
+#endif //CONFIG_BROWNOUT_DET_LVL
+
+static void rtc_brownout_isr_handler()
+{
+    /* Normally RTC ISR clears the interrupt flag after the application-supplied
+     * handler returns. Since restart is called here, the flag needs to be
+     * cleared manually.
+     */
+    REG_WRITE(RTC_CNTL_INT_CLR_REG, RTC_CNTL_BROWN_OUT_INT_CLR);
+    /* Stall the other CPU to make sure the code running there doesn't use UART
+     * at the same time as the following ets_printf.
+     */
+    esp_cpu_stall(!xPortGetCoreID());
+    ets_printf("\r\nBrownout detector was triggered\r\n\r\n");
+    esp_restart_noos();
+}
+
+void esp_brownout_init()
+{
+    REG_WRITE(RTC_CNTL_BROWN_OUT_REG,
+            RTC_CNTL_BROWN_OUT_ENA /* Enable BOD */
+            | RTC_CNTL_BROWN_OUT_PD_RF_ENA /* Automatically power down RF */
+            /* Reset timeout must be set to >1 even if BOR feature is not used */
+            | (2 << RTC_CNTL_BROWN_OUT_RST_WAIT_S)
+            | (BROWNOUT_DET_LVL << RTC_CNTL_DBROWN_OUT_THRES_S));
+
+    ESP_ERROR_CHECK( rtc_isr_register(rtc_brownout_isr_handler, NULL, RTC_CNTL_BROWN_OUT_INT_ENA_M) );
+
+    REG_SET_BIT(RTC_CNTL_INT_ENA_REG, RTC_CNTL_BROWN_OUT_INT_ENA_M);
+}