1What to flash
2Connect and flash
Plug the board in over USB, choose what to flash, then click Connect and pick the serial port.
If the chip isn't detected, hold BOOT while connecting.
Flashed. Now give the board a job.
The same browser tab can turn this ESP32 into a Synacl gateway: Modbus and RS485 meters, GPIO, I²C and 1-Wire sensors straight to live dashboards and alerts, with over-the-air updates that keep your settings, remote configuration, and live logs from anywhere.
You just flashed the Synacl gateway firmware. It's waiting for Wi-Fi and account credentials: create a free account, open Gateways → Add gateway, skip the flash step, register the gateway and provision it over this USB cable.
- OTA updates with settings kept. One click updates a gateway over the air; Wi-Fi and MQTT stay put.
- Remote config and live logs. Change sensors, intervals and pins from your desk, watch the boot log from your phone.
- 21 protocols to dashboards. Modbus RTU/TCP, MQTT, I²C, SPI, 1-Wire, UART, GPIO, CAN, M-Bus, SNMP and more.
- Free for five devices. No card, no sales call. Rules, alerts and 60 days of history included.
How it works, and what to do when it doesn't
Everything below is the same advice we give people flashing Synacl gateways, because the mechanism is identical: a browser, a USB cable and Espressif's own flashing code.
Where do my firmware files go?
Nowhere. The page reads the .bin from your disk and writes it to the chip over Web Serial using Espressif's esptool-js, entirely inside your browser. There is no upload and no server in the loop. The only download is the optional Synacl gateway firmware, which comes from api.synacl.com.
Which boards does the ESP32 web flasher support?
The whole ESP32 family: ESP32, ESP32-S2, S3, C2, C3, C5, C6, C61, H2 and P4, plus the ESP8266. The chip is detected automatically when you connect, and its name, MAC address and flash size are shown before anything is written.
Which flash offset should I use?
- Merged / factory image (Arduino's
.merged.bin,esptool merge_binoutput):0x0. - Application only (PlatformIO
firmware.bin, Arduino.ino.bin):0x10000. This works when the board already has a bootloader and partition table, which any board that has run once does. - Full set: bootloader at
0x1000on the classic ESP32 or0x0on S2, S3, C3 and C6; partition table at0x8000;boot_app0at0xE000; the app at0x10000.
My serial port is not listed. What now?
Install the USB-UART driver for your board's bridge chip (Silicon Labs CP210x or WCH CH340 cover almost every dev board), use a data cable rather than a charge-only one, and close anything else holding the port, such as the Arduino Serial Monitor or a PlatformIO monitor. Only one program can open a serial port at a time. Boards with native USB (S2, S3, C3) show up without a driver but may need the BOOT button to enter download mode.
It says failed to connect or timed out.
Hold the BOOT button on the board while you click Connect, and release it once the chip is detected. Some boards also need a tap of EN or RST while BOOT is held. If the flash itself fails partway through, lower the baud rate to 115 200 and try again; long or thin USB cables and some CH340 clones don't cope with 921 600.
Does it work in Firefox or Safari?
No. Web Serial is only available in Chromium browsers on desktop: Chrome, Edge, Opera and similar. Mobile browsers don't expose it. There is nothing to install in those browsers; the API is simply built in.
Can I flash the Synacl gateway firmware without an account?
Yes. The board does nothing until it is provisioned with Wi-Fi and account credentials, which you do later from the app: create a free account, open Gateways → Add gateway, skip the flash step, register the gateway and provision it over the same USB cable. The gateway setup guide walks through it.
When should I tick Erase entire flash?
Only when switching from a firmware with a different partition layout, or when the settings partition is corrupted and the board boot-loops. It wipes everything on the chip, including saved Wi-Fi credentials. A normal flash writes only the regions your files cover and leaves the NVS settings partition alone, which is why re-flashing a Synacl gateway keeps its provisioning.