To be able to reliably store persistent configuration in EEPROM, we need to be able to split up the available space for plugins to use. We also want to make sure that we notice when the EEPROM contents and the firmware are out of sync. This plugin provides the tools to do that.

It does not guard against errors, it merely provides the means to discover them, and let the firmware Sketch handle the case in whatever way it finds reasonable. It’s a building block, and not much else. All Kaleidoscope plugins that need to store data in EEPROM are encouraged to make use of this library.

Using the plugin

There are a few steps one needs to take to use the plugin: we must first register it, then either let other plugins request slices of EEPROM, or do so ourselves. And finally, seal it, to signal that we are done setting up. At that point, we can verify whether the contents of the EEPROM agree with our firmware.

#include <Kaleidoscope.h>
#include <Kaleidoscope-EEPROM-Settings.h>

static uint16_t settingsBase;
static struct {
  bool someSettingFlag;
} testSettings;

KALEIDOSCOPE_INIT_PLUGINS(EEPROMSettings, /* Other plugins that use EEPROM... */);

void setup () {

  settingsBase = EEPROMSettings.requestSlice(sizeof(testSettings));


  if (!EEPROMSettings.isValid()) {
    // Handle the case where the settings are out of sync...
    // Flash LEDs, for example.

  }, testSettings);

Plugin methods

The plugin provides the EEPROMSettings object, which has the following methods:


Requests a slice of the EEPROM, and returns the starting address (or 0 on error, including when the request arrived after sealing the layout).

Should only be called before calling seal().


Sets (or returns, if called without an ID) the default layer. When the keyboard boots up, it will automatically switch to the configured layer - if any.

Setting it to 126 or anything higher disables the automatic switching.


Controls whether the hardcoded layers (in PROGMEM) are ignored or not.

When not ignored, the custom layes (in EEPROM) extend the hardcoded ones. When ignored, they replace the hardcoded set.

Returns the setting if called without arguments, changes it to the desired value if called with a boolean flag.

This setting is exposed to Focus via the keymap.onlyCustom command implemented by the [EEPROM-Keymap][] plugin.

Defaults to false.


Seal the EEPROM layout, so no new slices can be requested. The CRC checksum is considered final at this time, and the isValid(), crc(), used() and version() methods can be used from this point onwards.

If not called explicitly, the layout will be sealed automatically after setup() in the sketch finished.


Updates the EEPROM header with the current status quo, including the version and the CRC checksum.

This should be called when upgrading from one version to another, or when fixing up an out-of-sync case.


Returns whether the EEPROM header is valid, that is, if it has the expected CRC checksum.

Should only be called after calling seal().


Invalidates the EEPROM header. Use when the version does not match what the firmware would expect. This signals to other plugins that the contents of EEPROM should not be trusted.


Returns the current version of the EEPROM settings. It’s the version of the settings only, not that of the whole layout - the CRC covers that.

This is for internal use only, end-users should not need to care about it.


Returns the CRC checksum of the layout. Should only be used after calling seal().


Returns the amount of space requested so far.

Should only be used after calling seal().

Focus commands

The plugin provides two - optional - Focus command plugins: FocusSettingsCommand and FocusEEPROMCommand. These must be explicitly added to KALEIDOSCOPE_INIT_PLUGINS if one wishes to use them. They provide the following commands:


Sets or returns (if called without arguments) the ID of the default layer. If set, the keyboard will automatically switch to the given layer when connected. Setting it to 126 or anything higher disables the automatic switching.

This is the Focus counterpart of the default_layer() method documented above.


Returns the actual, and the expected checksum of the settings.


Returns either true or false, depending on whether the sealed settings are to be considered valid or not.


Returns the version of the settings.


Without argument, displays the full contents of the EEPROM, including the settings header.

With arguments, the command updates as much of the EEPROM as arguments are provided. It will discard any unnecessary arguments.

Returns the amount of free bytes in EEPROM.


  • (Kaleidoscope-FocusSerial)[]

Further reading

Starting from the example is the recommended way of getting started with the plugin.