Added support for RTC user memory in ESP-specific APIs. (#1836)

libraries/esp8266/examples/RTCUserMemory/RTCUserMemory.ino

@@ -0,0 +1,52 @@
+// Example: Storing struct data in RTC user memory
+//
+// Struct data with the maximum size of 512 bytes can be stored in the RTC user memory using the ESP-specifc APIs.
+// The stored data can be retained between deep sleep cycles. However, the data might be lost after power cycling the ESP8266.
+//
+// Created Mar 30, 2016 by Macro Yau.
+//
+// This example code is in the public domain.
+
+typedef struct {
+  byte data[512];
+} rtcUserMemory;
+
+rtcUserMemory mem;
+
+void printMemory(bool readFromRtc) {
+  char buf[3];
+  Serial.print(readFromRtc ? "Read:  " : "Write: ");
+  for (int i = 0; i < sizeof(mem); i++) {
+    sprintf(buf, "%02X", mem.data[i]);
+    Serial.print(buf);
+    Serial.print(" ");
+  }
+  Serial.println();
+}
+
+void setup() {
+  Serial.begin(115200);
+  Serial.println();
+
+  // Read struct from RTC user memory
+  if (ESP.rtcUserMemoryRead((uint32_t*) &mem, sizeof(mem))) {
+    printMemory(true);
+  }
+
+  // Generate new data set for the struct
+  for (int i = 0; i < sizeof(mem); i++) {
+    mem.data[i] = random(0, 128);
+  }
+
+  // Write struct to RTC user memory
+  if (ESP.rtcUserMemoryWrite((uint32_t*) &mem, sizeof(mem))) {
+    printMemory(false);
+  }
+  
+  // Enter deep sleep mode for 10 seconds
+  ESP.deepSleep(10e6);
+}
+
+void loop() {
+  
+}