data-storage-dat

• memory-dat - NVS-dat - SPIFF-dat - SD-dat - EEPROM-dat

• ESP32-SDK-dat

data storage

• NVS-dat - SPIFF-dat - SD-dat

Method	Size Limit	Persistent after reset/power off?	Use Case
Global variables (RAM)	Up to available RAM (~500KB)	❌ No	Temporary data while program is running
RTC RAM	~8 KB	✅ Across reset, ❌ across power off	Store small state info between deep sleep cycles
NVS (Non-Volatile Storage / Preferences)	Key–value pairs, ~4 KB per partition	✅ Yes	Config values, Wi-Fi credentials, small settings
SPIFFS / LittleFS	Depends on flash partition (100KB–1MB typical)	✅ Yes	Store files, logs, calibration data
SD Card	GBs	✅ Yes	Large data logging, images, audio
External EEPROM	Varies (e.g. 4KB–1MB chips)	✅ Yes	Additional storage if onboard flash is insufficient

BLE Data Update Workflow (ESP32)

• BLE-dat

1. ESP32 runs a BLE server

   • Creates a characteristic for data transfer.

2. Phone/PC app connects via BLE

   • Splits the file into small BLE packets (~20 bytes each).
     
   • Sends them to ESP32 characteristic.

3. ESP32 receives packets

   • Reassembles them into a buffer.
     
   • Writes the buffer into SPIFFS/LittleFS (or NVS for small key-values).

4. Program reloads data

   • Next time your code runs (or immediately after update),
     it reads the updated file/config from storage.

BLE server code example (Arduino):

 #include <BLEDevice.h>
 #include <BLEUtils.h>
 #include <BLEServer.h>
 #include <SPIFFS.h>

 #define SERVICE_UUID        "1234"
 #define CHARACTERISTIC_UUID "5678"

 File dataFile;
 String buffer = "";

 class MyCallbacks: public BLECharacteristicCallbacks {
 void onWrite(BLECharacteristic *pCharacteristic) {
    std::string rxValue = pCharacteristic->getValue();
    if (rxValue.length() > 0) {
       buffer += rxValue.c_str();
    }
 }
 };

 void setup() {
 Serial.begin(115200);
 SPIFFS.begin(true);

 BLEDevice::init("ESP32");
 BLEServer *pServer = BLEDevice::createServer();
 BLEService *pService = pServer->createService(SERVICE_UUID);
 BLECharacteristic *pCharacteristic = pService->createCharacteristic(
    CHARACTERISTIC_UUID,
    BLECharacteristic::PROPERTY_WRITE
 );
 pCharacteristic->setCallbacks(new MyCallbacks());
 pService->start();
 BLEAdvertising *pAdvertising = BLEDevice::getAdvertising();
 pAdvertising->addServiceUUID(SERVICE_UUID);
 pAdvertising->start();

 Serial.println("Waiting for BLE data...");
 }

 void loop() {
 // Example: once data transfer finished, write to SPIFFS
 if (buffer.endsWith("EOF")) {   // simple marker
    dataFile = SPIFFS.open("/data.txt", FILE_WRITE);
    dataFile.print(buffer);
    dataFile.close();
    buffer = "";
    Serial.println("File updated!");
 }
 }

ref

• memory-dat