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Custom firmware for the Xiaomi Thermometer LYWSD03MMC and Telink Flasher via USB to Serial converter.

Hardware revision B1.4, B1.6 and B1.9 is now fully compatible

This repo is made together with this explanation video:(click on it)


Victor @pvvx did some very nice refinings of this custom firmare so i really suggest on cheking it out and even use his version as it offers many more functions including non-volatile storage and a better low power management

It is possible to update the Firmware of the Xiaomi Thermometer OTA with this WEB Tool I wrote:

The web flasher works for many devices that uses the Telink TLSR82** MCUs and it can also be used to reflash the Stock firmware back to the device.

You can support my work via PayPal: this keeps projects like this coming.

Tutorial in spanish: Thanks to Eduardo Ruiz


How to flash the custom firmware:

  • Download the ATC_Thermometer.bin file from the latest release assets and open the Web Flasher.
  • Connect to the Xiaomi thermometer; searching may take a while as it broadcasts not so often for better battery life.
  • After the connection is successful click on "Do Activation" to Authorize the Connection. While it's doing so you can already select the firmware file. Be careful to select the right one as it's not possible to check the firmware further.
  • Click on start flashing to flash the new firmware to the Thermometer.

After the flashing is done, the device should reboot. If the screen stays off, pull the battery out for a short amount of time.

To flash the stock firmware back to the Thermometer, just open the corresponding file (link in the section "Stock firmware" at the bottom of this document) for flashing.


How to flash the custom firmware or unbrick the device:

To flash a new firmware via an standard USB to UART adapter, simply connect the Thermometer as seen in the picture Mi_SWS_Connection.jpg to the USB to UART converter and run the tool with the first parameter being the name of the file you want to flash.

Example: "python3 ATC_Thermometer.bin"

If the flashing fails or no valid COM port can be found, you can edit it in the Python script. Also try to increase the ResetTime, I will try to make that nicer in the future! So far it turned out that flashing via MAC does not work correctly. I think it's because the data will not get pushed out in real time so the Emulated SWS protocol gets interrupted.

The UART flasher software uses code base from Thanks to pvvx for the awesome work on this!

Custom firmware:

  • To build the custom firmware on your own, follow this guide to get a working TC32 Compiler environment ready where you can add the Custom Mi firmware.

Original guide in Chinese:

Guide translated to English with Google Translate:

  • Try to "make" the blink example included in the SDK once to see if the compiling works as it should.
  • You can then copy the folder "ATC_Thermometer" into the example folder and go into that with the terminal.
  • Now do a "make" and it will build the custom firmware.
  • The newly created .bin file can then simply be flashed by either the Web Flasher or the USB to UART method.

Because of the OTA dual bank update method a firmware can be maximum 256kB in size.

The MCU used in the Thermometer is the TLSR8251 the datasheet can be found here:

Getting the MAC of your Thermometer:

On boot the custom firmware will show the last three bytes of the MAC Address in the humidity display part on the LCD for 2 seconds each, the first three bytes are always the same (A4:C1:38) so not shown. Also the BLE name will include the last three bytes of the MAC Address.

Settings in custom firmware:

The following settings can be sent to the RxTx Characteristics 0x1F10/0x1f1f

These settings can be stored in the device by clicking the "Save current settings in flash" button or can be reset to default by clicking the "Reset settings to default" button.

The default value is denoted by the option being written in bold

Show battery level in LCD :

Will show Battery % (when the battery symbol is displayed at the bottom of the screen) or Humidity % (when the battery symbol is NOT displayed at the bottom of the screen) alternatively every 5~6 seconds.

0xB1 = Enabled

0xB0 = Disabled

Change display to °F or °C:

0xFF = Temperature in °F

0xCC = Temperature in °C

Blinking smiley:

0xA0 = Smiley off

0xA1 = Smiley happy

0xA2 = Smiley sad

0xA3 = Comfort Indicator

0xAB = Smiley blinking

Advertising type:

0xAE = Custom

0xAF = Mi Like

Advertising interval

byte0 0xFE

byte1 0x06 - value times 10 seconds = interval 60 seconds default.

Temp and Humi offset

byte0 0xFA = Temp offset

byte0 0xFB = Humi offset

byte1 as an int8_t

so Temp = range -12,8 - + 12,8 °C offset Humi = range -50 - +50 % offset

Temp or Humi instant advertising

When the temp or Humidity changes too fast between the main loop (5 seconds interval), the Advertising will be instant for that one.

byte0 0xFC = temp_alarm_point // value divided by 10 for temp in °C

byte0 0xFD = humi_alarm_point

byte1 as int8_t

Temp alarm from 0,1°C to 25,5°C Range <- 0,5°C Default

Humi alarm from 1% to 50% Range <- 5% Default

Advertising format of the custom firmware:

The custom firmware sends every minute an update of advertising data on the UUID 0x181A with the Tempereature, Humidity and Battery data.

The format of the advertising data is as follow:

  • Byte 5-10 MAC in correct order

  • Byte 11-12 Temperature in int16

  • Byte 13 Humidity in percent

  • Byte 14 Battery in percent

  • Byte 15-16 Battery in mV uint16_t

  • Byte 17 frame packet counter

Example: 0x0e, 0x16, 0x1a, 0x18, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xaa, 0xaa, 0xbb, 0xcc, 0xdd, 0xdd, 0x00

Using custom firmware with ESPHome

The LYWSD03MMC sensors work out of the box with ESPHome. There are three ways to use them:

  • With the original firmware by using the xiaomi_lywsd03mmc sensor platform. For this, you need to obtain the bindkey.
  • With the custom firmware from this project, either by
    • using platform: atc_mithermometer in ESPHome when the firmware is configured to an advertising type of “custom” (the default) or
    • setting the firmware to “Mi Like” advertisement and using platform: xiaomi_lywsd03mmc with 32 arbitrary hex digits as the bindkey, e.g. eef418daf699a0c188f3bfd17e4565d9. (This works because the values in the broadcast are not encrypted when using this firmware, but bindkey is still a required parameter.)

platform: xiaomi_lywsd03mmc is available since ESPHome 1.15, platform: atc_mithermometer since 1.16.

Using custom firmware with OpenMQTTGateway

The LYWSD03MMC sensors work out of the box with OpenMQTTGateway. You can directly upload OMG to the ESP32 from your web browser here, choose esp32dev-ble or esp32dev-ble-cont (for continuous scanning).

  • With the original LYWSD03MMC firmware, OMG will connect to the sensor so as to retrieve the sensors values and publish to MQTT.
  • With the custom LYWSD03MMC firmware from this project, OMG will detect automatically the sensor and publish to MQTT. In both cases the sensor will be auto discovered in Home Assistant per default (no configuration needed if you are already using the MQTT integration with auto discovery). You can also follow this tutorial( for the Home Assistant integration. You can also integrate to OpenHAB or other MQTT compatible controllers

Stock firmware:

This .zip file contains the stock firmware to go back:

Building manual for docker:

Many thanks to:




Custom firmware for the Xiaomi Thermometer LYWSD03MMC and Telink Flasher via USB to Serial converter






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