EEPROM on Flash

STM32-based chips don't have nonvolatile EEPROM memory (which can be changed one byte at a time). Instead, they have Flash memory in which 32 bit words can be written one at a time, but not overwritten.

To overwrite some data, you need to erase an entire flash page, which could be as much as 128kB long!

To work around this, We've provided a 'Fake EEPROM' module called FlashEEPROM (About Modules).

This stores a 'journal' of all written data. When you write something new to it, that data goes on the end of the journal (if it's too long, everything is read into RAM, the page is erased, and only the latest values are written back).

You use it as follows:

/* No arguments mean use the default, which will work on the Original Espruino or Espruino Pico
 You can supply extra arguments to choose which flash page you'll use, and even whether you
 use external flash memory - but be careful when doing this. You can accidentally overwrite
 Espruino itself! */
var f = new (require("FlashEEPROM"))();

f.write(0, "Hello");
f.write(1, "World");
//.. you can write to any address between 0 and 255, with any data up to 65536 chars long

f.read(0)
// returns new Uint8Array([72, 101, 108, 108, 111])

E.toString(f.read(1))
// returns "World"

f.readMem(0)
// returns "Hello" - this is a special string that is accessed directly from ROM
// (and is not loaded into RAM first)

f.readAll();
// returns [
//  new Uint8Array([72, 101, 108, 108, 111]),
//  new Uint8Array([87, 111, 114, 108, 100])
// ]

f.write(1, "Espruino");
// has now overwritten 'World'

E.toString(f.read(1))
// returns "Espruino"

/* You can tidy up the journal if you want, removing any values that were
overwritten by subsequent writes. It can take around a second though */
f.cleanup();

// Or clear absolutely everything out of memory
f.erase();

Speed

This module can be quite slow as it will scan all of the flash page for data. The more times you change the data, the bigger the journal gets, and the slower both reads and writes will get.

You can explicitly call f.cleanup() which will re-write the data to just what is needed, but you should be aware that flash memory has a fixed number of write cycles (see the IC's datasheet) so you should try and do this as rarely as possible.

Another option is to limit the amount of Flash memory that can be used. This is a good idea if you're using something like the Espruino Pico that will default to using the full 128kB page!

// Use only 1024 bytes
f.endAddr = f.addr+1024;

Reference

/* Internal function - return an object containing:
    * addr: the address in memory of the key (must be between 0 and 255), or -1 if it doesn't exist 
    * end: the address of the next free entry in memory
*/
FlashEEPROM.prototype.getAddr = function (addr) { ... }

/* Read the current value of a key (key must be between 0 and 255).
 This will return a Uint8Array. It can be converted to a string
 with E.toString()
*/
FlashEEPROM.prototype.read = function (addr) { ... }

/* Read the current value of a key (key must be between 0 and 255).
 This will return a 'Memory Area' string, which directly references the
 data in flash memory rather than loading it into RAM first. This is 
 useful when you've written a lot of data and you're trying to save
 RAM.
*/
FlashEEPROM.prototype.readMem = function (addr) { ... }

// return the current values of all keys in an array
FlashEEPROM.prototype.readAll = function () { ... }

// Internal function to write a data block
FlashEEPROM.prototype._write = function (n, addr, data) { ... }

/* Write a new value.  Addr must be between 0 and 255, data 
 can be a string or uint8array but cannot be longer than 255 bytes
*/
FlashEEPROM.prototype.write = function (addr, data) { ... }

// Read all data, erase the page, and write it back (removing duplicates)
FlashEEPROM.prototype.cleanup = function () { ... }

// Erase everything
FlashEEPROM.prototype.erase = function () { ... }

/* Create a new Flash EEPROM.

 `addr` - addr to start at (if left off, this will be the address 
   after the last page of Flash. In the Original Espruino and 
   Espruino Pico, this is an flash page that isn't supposed to
   be in the chip - but which is on every one we have tested.

 `flash` - the flash object to use (it will use the internal 
   flash if this undefined). If an object implements read, write,
   erasePage and getPage then it can be used
*/
function (addr, flash) { ... }

Using

No tutorials use this yet.

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