Reverse-engineered Gravity 800 grill now runs open source firmware — thanks to a hidden ESP-32

A tinkerer known as PRBS23 has released open source firmware for the Gravity 800 charcoal grill after reverse-engineering its original controller. What started as a simple wire repair turned into a full-blown firmware replacement project.

Opening the grill's control unit revealed an ESP-32 microcontroller and a clearly labeled programming port. The rest of the board was equally straightforward: connectors for four thermistor temperature sensors, a lid open/close switch, a fan driver, a 16-segment LCD, a piezo buzzer, and several physical input buttons.

The thermistor and physical inputs are connected to a 16-pin chip that communicates with the main MCU over a UART link at 9,600 baud. PRBS23 could not identify an ADC chip matching these specifications, so the most likely explanation is that the chip is a cheap microcontroller programmed to act as a simple analog front end.

The well-labeled programming header made it easy to dump the original firmware using the espflash utility. Analyzing the dump revealed a peculiar ADC correction function in the original code. Its purpose remains unclear — it only adjusts readings by a maximum of two to three degrees Fahrenheit, and the necessity of such a small correction is questionable.

Most other features proved easier to reverse-engineer. The CS1621 segmented display driver was already well documented. The temperature sensor and fan driver protocols were also decoded without major difficulty. The open source firmware now provides full control over the grill's cooking functions, including temperature monitoring, fan speed regulation, and display output.

The project demonstrates how even a seemingly simple appliance can contain a surprisingly capable microcontroller like the ESP-32, which is commonly used in IoT devices. PRBS23 has published the firmware and reverse-engineering notes on GitHub, allowing other owners of the Gravity 800 to flash their own controllers and potentially customize or improve the grill's behavior. The work also highlights the growing trend of hobbyists reclaiming control over consumer electronics through open source firmware replacements.