Best practice for STM32 communicating with ESP32 over I2C (No UART traces)

[Maddy_esp32]

Hi everyone,

I am working on a custom PCB designed , but I’ve run into a hardware design constraint and would appreciate some architectural advice on the best way forward.

The Setup:

Main MCU: STM32L476 (Programming via STM32CubeIDE / HAL)

Coprocessor/WiFi: ESP32-WROOM (Intended to handle AWS IoT connections)

The Constraint: The PCB designer connected the STM32 and ESP32 exclusively via an I2C bus. There are no physical UART traces connecting the two chips.

The Original Goal: The initial plan was to use the STM32 to write firmware directly to the ESP32 over UART, or to use a UART software bridge, and then use UART for passing sensor data to the ESP32 for AWS publishing.

The Dilemma: Because the ESP32 ROM bootloader requires UART0 (TX0/RX0) for flashing, I realize a software bridge from the STM32 over I2C to flash the ESP32 is physically impossible.

My Questions for the Community:

The Initial Flash: Aside from tack-soldering temporary wires to the ESP32's TX0/RX0 pins to perform the initial flash, is there any advanced or obscure method to flash an ESP32 via an STM32 strictly over an I2C bus?

The Software Architecture: Assuming I tack-solder the board to flash a custom program onto the ESP32, what is the most robust architecture for runtime communication? Should I configure the STM32 as an I2C Master and the ESP32 as an I2C Slave to pass the sensor data, or is there a better paradigm for this specific Master-Coprocessor relationship using the HAL libraries?

Any advice on handling this missing-UART scenario would be hugely appreciated. Thank you!