A convenient way to store data into Flash memory on the ATSAMD21
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readme.md

FlashStorage library for Arduino

The FlashStorage library aims to provide a convenient way to store and retrieve user's data using the non-volatile flash memory of microcontrollers.

The flash memory, due to it's properties, is generally used to store the firmware code, but it can also be used to store user data.

Supported hardware

Currently, only ATSAMD21 cpu is supported (and consequently every board based on this cpu like the Arduino Zero or Aduino MKR1000).

Limited number of writes

Flash memory has a limited amount of write cycles. Typical flash memory can perform about 10000 writes cycles to the same flash block before starting to "wear out" and begin to lose the ability to retain data.

So BEWARE: IMPROPER USE OF THIS LIBRARY CAN QUICKLY AND PERMANENTLY DESTROY THE FLASH MEMORY OF YOUR MICRO, in particular you should avoid to call the write() function too often and make sure that in the entire life of the micro the number of calls to write stay well below the above limit of 10000 (it's a good rule-of-thumb to keep that number in mind even if the manufacturer of the micro guarantees a bigger number of cycles).

The same caution must be taken if you're using the EEPROM API emulation (see below) with the EEPROM.commit() function.

Usage

First of all you must declare a global FlashStorage object for each piece of data you intend to store in the flash memory. For example if you want to store the age of a person you must declare an age_storage like this:

FlashStorage(age_storage, int);

this instruction means "create a FlashStorage to store an int variable and call it age_storage". Now you can use age_storage as a place to safely store an integer:

void readAndStoreUserAge() {
  Serial.println("Please enter your age:");
  String age = Serial.readStringUntil('\n');

  age_storage.write(age.toInt());  // <-- save the age
}

after a reset of the microcontroller you can retrieve the stored age by using:

int user_age = age_storage.read();

Using the alternative EEPROM-like API

If you include FlashAsEEPROM.h you'll get an EEPROM emulation with the internal flash memory. See EmulateEEPROM sketch for an example.

The API is very similar to the well known Arduino EEPROM.h API but with two additional functions:

  • EEPROM.isValid() returns true if data in the EEPROM is valid or, in other words, if the data has been written at least once, otherwise EEPROM data is "undefined" and the function returns false.
  • EEPROM.commit() store the EEPROM data in flash. Use this with care: Every call writes the complete EEPROM data to flash. This will reduce the remaining flash-write-cycles. Don't call this method in a loop or you will kill your flash soon.

License

This library is released under LGPL-2.1.

FAQ

Can I use a single FlashStorage object to store more stuff?

Yes, you can declare a struct with more fields and create a FlashStorage object to store the entire structure. See the StoreNameAndSurname sketch for an example on how to do it.

The content of the FlashStorage is erased each time a new sketch is uploaded?

Yes, every time you upload a new sketch, the previous content of the FlashStorage is erased.

Do you recommend to use FLASH instead of EEPROM?

No. If your micro provides an EEPROM it's almost always better to use that because it's a kind of memory designed with the specific purpose to store user data (it has a longer lifetime, number of write cycles, etc...).

In the absence of an EEPROM you can use this library to use a piece of the flash memory as an alternative to EEPROM. However, you must always keep in mind of it's limits.