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This Arduino library eases the storage of variables in reset-safe RTC memory.
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This library eases the storage of variables in reset-safe RTC memory. Variables stored there survive all kinds of resets as long as there is no hard reset. It provides boilerplate code to securely store relevant state data in RTC memory so it may survive (unexpected) reboots or deep sleeps. Supports ESP only at this time, will change in the future.

Aim of the library is to make an integration into existing projects as easy as possible.

Help wanted

My own main target is the ESP8266 platform. I am happy owner of a brand new ESP32 so expect support for it soon. Other plattforms can be supported too, but since its basic function relies on a real time clock (RTC) with additional RAM, it would require a plattform including such a real time clock or using an external module for it. I don't have that at home right now, so it would help a lot, if you own an rtc module, if you contact me or develop the integration yourself and make a pull request.


At the moment, this library is not part of the "official" repository within the arduino gui. Therefore you need to download the library as a .zip-file and install it by hand using the arduino gui.

Quick examples

Pseudo code

The major aim of the library is seamless integration but it still requires a little change to your existing programming pattern. In pseudo code it would look like this:

// pseudo code example
global variable definition: a, b, c

setup {
  if tryToLoadFromRTC==true then
    do some work to recreate/enter saved state
    do "cold boot" work

loop {
  do loop work
  if stateRelevantChangeOf(a) then

The magic lies in the first if statement. If the loading of a valid set of values works, you will (most likely) need to add some code, so that your look will enter the correct state.

Basic example

Let's see some real action:

#include <RTCVars.h>
RTCVars state; // create the state object

int reset_counter;                      // we want to keep these values after reset
int program_step;

void setup() {
  Serial.begin(115200);                 // allow debug output
  state.registerVar( &reset_counter );  // we send a pointer to each of our variables
  state.registerVar( &program_step );

  if (state.loadFromRTC()) {            // we load the values from rtc memory back into the registered variables
    Serial.println("This is reset no. " + (String)reset_counter);
    state.saveToRTC();                  // since we changed a state relevant variable, we store the new values
  } else {
    reset_counter = 0;                  // cold boot part
    Serial.println("This seems to be a cold boot. We don't have a valid state on RTC memory");
    program_step = 0;

void loop() {
  // do your work here - try to reset your chip externally or internally but don't power off...
  Serial.println("Current state is " + (String)program_step);
  program_step = (program_step == 7) ? 1 : program_step + 1;
  Serial.println("New state is " + (String)program_step);
  state.saveToRTC();                    // there are no parameters because it only needs the vars' definiton once 

First the library is included by #include <RTCVars.h>. Then an RTCVars object named state is created via RTCVars state;. This object now is the interface to all functionality. Be sure to register all variables before reading the old state from RTC memory. The state is invalid if the registered variables differ in total size to the saved state. It seems good practice to use globally defined vars only and register all of them in the setup() function. If you really need to keep track of different, state-specific variables, look at the advanced usage.

Registering does nothing but keeping track of where the variables are to find in memory. Of course reading the state from RTC memory would require to save the values in the corresponding variables. Therefore you need to register them even before you call state.loadFromRTC(). If everything works out, the call returns true. If not there are problems with the state or it has been a cold boot. See advanced usage for further information on error handling.

Later on, everytime a change is made to some of the vars registered and have a consistent state is esablished, state.saveToRTC() is called to push these values to RTC memory.

Supported types and size

Please note, that there are two limits in this lib: memory and the number of managed variables. Due to the fact how storage of vars is organized, there is a fixed upper boundry other than memory. By default, the limit is 32 variables, the types are byte, char, int, long, float. Since these types do not exceed 8 bytes each, the RTC memory is not the limit here (512 bytes - 28 starting offset - 7 header/checksum = 477 bytes). See advanced usage for more functionality to control these settings.

Advanced Usage

This part will show some functionality seldomly used (e.g. different states) or typically integrated at a later time (e.g. error handling). See the "fullblown" example at the end.

Error handling

The same goes here...

Multiple sets of state vars

Basically it is a bad idea to use different sets of variables to store in RTC memory. Typically this is not needed but due to memory constraints sometimes there is the need for it. Problem is, to setting up a state starts by registering the vars. Restoring these only works if vars are registered with the same type in the same order. What if these are different between two states: state one needs to store 30 ints and state two 12 floats. It's needed to know before setting up vars, which state is to be restored. This is what /state id/s are for: To keep two (or more) distinct sets of vars for different states, instantiate an additional RTCVars object and register different vars. Initally all RTCVars objects have the state_id 1. This can be changed by calling the objects method setStateId(byte new_state_id). Using different ids is essential for a correct recovery after reset. The state_id of an RTCVars object is returned when calling getStateId() function. In a program's setup() method, after creating the RTCVars object, getStateIdFromRTC() function should be called. It reads the state_id of the state in RTC memory ahead of other checks or the need to register vars. Be aware though, that this call might fail (see error handling) because of wrong checksum or other reasons (e.g. cold boot). Check the returned state_id against valid states and register the vars relevant for this state. Afterwards it is needed to set the object's state_id to the one in the RTC (for safety reasons). You can then load the state from RTC via loadFromRTC().

Memory and variable accounting

As stated in Supported types and size, there are two limits to check for. First one is the RTC memory. This is a fixed size depending on the RTC type used. On the ESP8266 e.g. it is 512 bytes with a known good offset to use the memory freely of 28 bytes. Additionally the library uses 7 bytes for management purposes. At any time available RTC memory can be checked by using the getFreeRTCMem() call of the RTCVars instance. It returns the number of free bytes as int. Second limit is the number of registered vars which is set within the lib as 32. To change that limit, a change in the RTCVars.h file is needed for now, as this limit is set by a #define statement. Use the getFreeRTCVars() function of the RTCVars instance. It returns the number of unused slots for registered vars.

Advanced example

Aim of this example is to show all features developed so far: extern "C" { #include <user_interface.h> } #include <TimeLib.h>

// By default, the maximum number of variables is 32, it can be set to a higher value if needed
// More needs more RAM, (about 6 Bytes per Variable), is a definition in lib's .h file
// number is limited by total RTC memory
// Use getFreeRTCMem() and getFreeRTCMem() to check at runtime

#include "RTCVars.h"

RTCVars state;
time_t oldTime;

// test variables
// variables for RTC memory should be global or valid at any time saveToRTC() is called 
int i = 1234;
int j = 23456;
byte k = 0;
long l = 123456789;
float m = 3.141592683;
int int_array[] = {1, 2, 0};

int autoreset = 20; // this will count down to zero and reset the esp

void setup() {
  // let's inform the user about what type of reset has happened
  rst_info *resetInfo;
  resetInfo = ESP.getResetInfoPtr();
  Serial.print("Reset Reason was: ");

  // register each variable by sending a pointer to it. valid types are: byte, char, int, long, float
  // register preferably in setup call
  // Note: 
  // Registered vars cannot be "thrown away" (e.g. unregistered).
  // If you really need a different set of vars to be saved in RTC mem, create another RTCVars object
  // and throw away the first one. These different sets have different signatures so to recover those
  // you need to identify the state set with a byte number:
  // state.setStateID(3);

  // Since recovery needs to know the kind of state in memory, request the saved ID first:
  // byte id_in_rtc = state.getStateIDFromRTC();
  // Afterwards you can register this state's variables and read all from RTC.
  // Be aware that you need to set the state ID (setStateID) to the ID in RTC memory, otherwise the
  // call to loadFromRTC() will fail. 
  byte id_in_rtc = state.getStateIDFromRTC(); // =255 if no valid signature is found
  Serial.print("The RTCVars state set in RTC memory has the id ");
  // since we change the id for demonstration purpose (!) every boot, let's accept the id in RTC memory
  if (id_in_rtc != 255) state.setStateID(id_in_rtc);

  j = 0;
  // arrays can be inserted by registering all elements in a loop
  for (int n = 0; n < 3; n++) state.registerVar(&(int_array[n]));
  // double registering is senseless (it is the same memory address!) but possible

  // debug output
  Serial.println("This is the set of vars BEFORE trying to load these from RTC"); 

  // try to load from RTC memory
  // be aware that you should keep registering the same variables in the same order between to resets
  // otherwise you'll end up with data written in wrong variables
  if (state.loadFromRTC()) {
    Serial.println(F("Data successfully read from RTC"));
  } else {
    switch (state.getReadError()) {
      Serial.print(F("Couldn't load an old state because of "));
        Serial.println(F("wrong magic bytes."));
      case RTC_ERROR_SIZE:
        Serial.println(F("a different state size than expected."));
        Serial.println(F("errors while reading RTC memory."));
        Serial.println(F("a wrong checksum."));
      case RTC_ERROR_STATE_ID:
        Serial.println(F("a different state id."));
      case RTC_ERROR_OTHER:
        Serial.println(F("unknown reasons."));

  // once again let's do some debug output to see what we loaded here
  Serial.println("And this is the set of vars AFTER trying to load these from RTC"); 

  // once per reboot action
  // some fiddling in the array
  for (int n = 0; n < 3; n++) {
    // next element of fibonacci sequence with every reboot 
    int_array[int_array[2]] = int_array[0] + int_array[1];
    int_array[2] = (int_array[2] + 1) & 0x01;

  // simulate different state ids
  // note: don't do this at home. only use different ids if you use a different setup of vars
  k = (k + 1) & 0x7f;

void loop() {
  // do some action every second
  if (now()!=oldTime) {
    oldTime = now();

    // let us change one of the saved vars
    j += 1;
    // show results
    Serial.print(F("This counter will reset soon: "));
    Serial.print(F(", this won't: "));
    // make sure to call this function after each essential change of state variables
    // if you don't the RTC memory does not reflect the latest changes

    // perform a reset after defined number of seconds
    if (autoreset--<=0) ESP.reset();
  // this one changes approx. 20 times per second
  i += 1;

Obviously the use of the state_id does not make any sense here but its basic handling is still correct.

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