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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallTo keep Arduino settings after a restart or power loss, store them in the persistent storage supported by your board: use the EEPROM API on boards that provide it, and use Preferences for Arduino-ESP32. Load saved values in setup(), use defaults if no valid configuration exists, and save only when a setting changes.
Choose storage for your specific board
Arduino-compatible boards do not all implement storage the same way. Check the documentation for your exact board and core before using an API or assuming a particular capacity.
| Platform | Storage API | How data is organized | Best fit |
|---|---|---|---|
| Arduino boards with EEPROM support | EEPROM |
Bytes at addresses; get() and put() handle typed values and structures |
Small settings stored as bytes or typed data |
| ESP32 with Arduino-ESP32 | Preferences |
Key-value pairs in nonvolatile storage (NVS) | Many small scalar values, strings, and byte arrays |
| ESP32 with larger data sets | Filesystem library such as LittleFS | Files | Larger data than is suitable for Preferences |
EEPROM on supported Arduino boards
The Arduino EEPROM guide covers byte-level read() and write(), as well as typed get() and put(). Its Uno example lists 1 kB of EEPROM; capacity varies by board. The guide states a 3.3 ms write operation and a limit of 100,000 write cycles per location in its EEPROM discussion, so avoid needless writes. update() skips writing a byte if its value is unchanged, and put() uses update semantics.
Preferences on ESP32
For new Arduino-ESP32 applications, Espressif describes Preferences as the replacement for EEPROM. The Preferences API reference says the library is unique to Arduino-ESP32 and should be considered the replacement for the Arduino EEPROM library. It stores key-value data in NVS and retains it across restarts and power loss. Namespace and key names are ASCII, case-sensitive, and limited to 15 characters each. Supported values include scalar types, strings, and byte arrays. Espressif recommends a filesystem such as LittleFS for larger data.
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Load settings and establish defaults at startup
Use setup() to open storage and choose between the saved configuration and factory defaults. A known marker key or validity indicator prevents uninitialized bytes or missing values from being treated as real settings. Espressif’s Preferences tutorial demonstrates using a predetermined key to decide whether to load defaults or last-run settings.
- Select the board-specific API. Confirm the target board and core support the EEPROM API, or use Preferences when programming ESP32 with Arduino-ESP32.
- Open the storage interface in
setup(). For Preferences, open a namespace; for EEPROM, use the implementation available on the selected board. - Check for a valid configuration. Test a marker or another validity indicator before trusting stored fields.
- Load or initialize. If the marker is valid, read the saved values using the same data types used to write them. Otherwise, apply defined defaults and save the initial configuration if appropriate.
Reading uninitialized EEPROM data can produce invalid values. The Arduino guide warns, for example, that an EEPROM string without a null terminator may print garbage and a float may display invalid output. Validate data before using it, and fall back to sensible defaults when it fails validation.
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Save settings when they change
Save after a user changes a setting, not unconditionally on every pass through loop(). Frequent writes waste EEPROM write life; the Arduino guide’s update behavior is specifically useful for avoiding writes when a byte is unchanged. Check return or status values when the selected API exposes them, so the program can detect a failed save.
For Preferences, pair each read and write with the same data type: for example, use the matching typed getter and setter for a value. Keep namespace and key names within the 15-character limit, and remember that capitalization matters.
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Persisting a structure requires a format check
EEPROM.get() and EEPROM.put() can transfer structures, but saving raw structure bytes couples stored data to the structure’s layout. Changing field order or types, or changing compiler padding, can make older saved bytes incompatible with new firmware.
As an implementation safeguard, include a format marker or version and validate loaded fields before accepting them. If the saved version is unknown or the values fail validation, initialize defaults and save the new format. Arduino’s guide demonstrates transferring structures but does not prescribe a universal migration method.
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Common mistakes to avoid
- Assuming EEPROM works identically everywhere: verify support and capacity for the exact board and core.
- Trusting storage without checking it: use a marker and validate values before applying them.
- Writing on every loop: save only after a change and use update behavior where available.
- Using mismatched Preferences types: read a value with the same type used to write it.
- Choosing Preferences for large files: use a filesystem such as LittleFS for larger ESP32 data.
- Changing a stored structure without compatibility handling: version its format and validate it when loading.
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