ESP8266-based web flasher for STM32 built-in ROM (System Memory) Bootloader over UART.

This library turns an ESP8266 into a standalone “browser flasher”:

• Connect a target STM32 (UART + BOOT0 + NRST)
• Open a web UI in your browser
• Upload update.bin to ESP8266 LittleFS
• Run bootloader commands (detect/connect, erase, program, jump to app)

• Browser-only workflow: upload + flash from any device on the same WiFi/LAN.
• Uses STM32 ROM bootloader USART protocol (ACK/NACK-based command frames).
• Embedded UI: firmware upload, control buttons, command log.
• mDNS access: http://<mdns-host>.local/
• LittleFS storage: stores firmware file on ESP8266 (default /update.bin).
• Simple access control: username/password + session bound to client IP (LAN use).

• Supported: ESP8266 (Arduino core)
• ESP32: not supported as-is. It can be ported, but requires a separate implementation layer (different web server, mDNS, FS, and serial handling).

• ESP8266 TX → STM32 RX (cross)
• ESP8266 RX → STM32 TX (cross)
• GND ↔ GND
• ESP8266 GPIO → STM32 BOOT0 (recommended with proper driver/level shifting)
• ESP8266 GPIO → STM32 NRST (recommended with proper driver/open-drain transistor)

• ESP8266 logic is 3.3V. Avoid direct 5V UART.
• Many STM32 boards need BOOT0 = 1 during reset to enter system bootloader.
• Many STM32 ROM bootloaders use 8E1 (8 data bits, even parity, 1 stop bit).

Put the library folder here (Windows): C:\Users\Documents\Arduino\libraries\STM32RomWebFlasher\

Inside it you should have:

STM32RomWebFlasher/
├─ library.properties
├─ examples/
└─ src/

• ZIP: Sketch → Include Library → Add .ZIP Library… (zip must contain the library root)
• Manual: copy the folder to Documents/Arduino/libraries/
• Restart Arduino IDE after installing.

Create a sketch like this:

#include <STM32RomWebFlasher.h>

static const char* WIFI_SSID = "YOUR_SSID";
static const char* WIFI_PASS = "YOUR_PASS";
static const char* MDNS_HOST = "stm32flasher";

static const char* WEB_USER = "admin";
static const char* WEB_PASS = "admin";

STM32WebFlasherConfig cfg = 
{
  WIFI_SSID,                 // WiFi SSID
  WIFI_PASS,                 // WiFi PASSWORD
  MDNS_HOST,                 // Web Accessable Address http://MDNS_HOST.local
  WEB_USER,                  // Webpage Login Username
  WEB_PASS,                  // Webpage Login Password
  80,                        // Port
  D1,                        // STM32 Target BOOT0 Pin
  D2,                        // STM32 Target Reset Pin
  115200,                    // UART Baudrate
  true,                      // UART Swap feature
  1000,                      // Sync Timeout
  "/update.bin"              // Firmware Update Path
};

STM32WebFlasherESP8266 Flasher(Serial, cfg);

void setup()
{
  if (!Flasher.begin())
  {
    while (true) delay(1000);
  }
}

void loop()
{
  Flasher.loop();
}

• Via IP: http://<esp-ip>/
• Via mDNS: http://stm32flasher.local/

Step	Description	Screenshot
1	Connection Page
2	Login
3	Main Control Panel
4	Connect / Detect Target
5	Device Information
6	Logs / Error Handling
7	Firmware Upload

1. Login
2. Connect Target
   • ESP8266 drives BOOT0/NRST and attempts ROM bootloader sync/detect
3. Upload firmware
   • Upload a .bin file; it is saved to LittleFS as /update.bin
4. Run commands
   • Erase / Program / Full Update / Jump to App / Read Info

The UI sends: GET /cmd?c=<letter>

Returns the Login page or Main UI depending on session.

Body: user=<...>&pass=<...>
JSON:

• { "ok": true }
• { "ok": false, "error": "..." }

Logs out and clears session.

Tries to enter ROM bootloader and detect target.

Exits bootloader / jumps to application.

JSON status (connected, hasFile, flashKB, devId, desc).

Multipart firmware upload → saved to LittleFS as /update.bin.

Runs command X.

Constructor:

STM32WebFlasherConfig(
  const char* wifiSsid,
  const char* wifiPass,
  const char* mdnsHost,
  const char* webUser,
  const char* webPass,
  uint16_t httpPort,
  uint8_t boot0Pin,
  uint8_t resetPin,
  uint32_t uartBaud,
  bool uartSwap,
  uint32_t syncTimeoutMs,
  const char* updatePath
);

• wifiSsid, wifiPass
  Wi-Fi network credentials.

• mdnsHost
  The .local mDNS host name (example: stm32flasher → stm32flasher.local).

• webUser, webPass
  Web UI login credentials.

• httpPort
  HTTP server port (usually 80).

• boot0Pin
  ESP GPIO pin connected to STM32 BOOT0 (controls boot mode).

• resetPin
  ESP GPIO pin connected to STM32 NRST (controls reset).

• uartBaud
  UART baud rate for STM32 ROM bootloader communication.

• uartSwap
  ESP8266 UART pin swap enable/disable.

• syncTimeoutMs
  Timeout for sync/ACK operations (milliseconds).

• updatePath
  Firmware file path inside LittleFS (example: "/update.bin").

Constructor and main methods:

STM32WebFlasherESP8266(HardwareSerial& serial, const STM32WebFlasherConfig& cfg);
bool begin();
void loop();

• begin() configures WiFi, LittleFS, mDNS, web routes, and UART.
• loop() must be called continuously.

This is a protocol-level failure: the STM32 did not ACK the last expected step.

• BOOT0 not asserted correctly during reset
• NRST timing incorrect
• Wrong UART settings (many STM32 need 8E1 parity)
• TX/RX swapped or missing common GND
• Target not actually in system bootloader mode
• Baud mismatch (try 57600 / 38400)

• BOOT0 HIGH → pulse NRST → attempt sync
• Confirm correct STM32 bootloader UART pins for your MCU
• Confirm ESP UART config matches the bootloader (including parity)
• Validate hardware with STM32CubeProgrammer or stm32flash first

• STM32 ROM bootloader commands overview (ST Community):
  https://community.st.com/t5/stm32-mcus/how-to-utilize-stm32-system-rom-bootloader-commands/ta-p/605286

• AN3155 — USART protocol used in the STM32 bootloader (ST Application Note):
  https://www.st.com/resource/en/application_note/an3155-usart-protocol-used-in-the-stm32-bootloader-stmicroelectronics.pdf