Open Source Firmware for the MasterBuilt Gravity 800: Controller Reverse Engineering
A maker known as PRBS23 has freed the MasterBuilt Gravity 800 Grill controller from proprietary firmware. The result is an open source firmware that brings PID temperature control, over-the-air updates, and a real-time web interface. The work starts from a complete reverse engineering of the original firmware, making the device far more transparent and customizable.
The original controller was carefully opened, revealing an ESP32 and a well-labeled programming header. Thanks to this, the author dumped the original firmware using the espflash utility. Analysis of the dump then showed a non-standard ADC correction function, an important detail for temperature reading. Additionally, the driver for the CS1621 segmented display was implemented using available documentation.
The new firmware includes PID temperature control, which keeps the coals stable with precision. Over-the-air updates simplify maintenance, and the real-time web interface lets you monitor the grill from any device. It all runs on an ESP32, the same board as the original controller. For those who want to redo the work on a bench, an ESP32 development board with 32 GPIOs mounts the same microcontroller as the original controller, with all pins accessible.
Step-by-step reverse engineering
The reverse engineering process was methodical and well documented. After opening the controller, the programming header allowed the firmware to be dumped without difficulty. Analysis of the dump revealed the non-standard ADC correction, which had to be replicated in the new firmware to get accurate readings. Finally, the driver for the CS1621 segmented display was written from scratch, based on public documentation.
Communication with the display runs at 9600 baud, a detail that emerged from analysis of the original firmware. Additionally, the ADC correction involves an offset of 40 degrees Fahrenheit, which must be applied to align readings with reality. These numbers are essential for anyone wanting to replicate or extend the project. The work shows that freeing an IoT device from proprietary firmware is an excellent use of reverse engineering.
What the new firmware offers
The open source firmware does more than replicate the original functions. PID temperature control is far more precise than the original on-off control. Over-the-air updates eliminate the need to physically connect the board for every change. The real-time web interface makes grill monitoring convenient and immediate. All these features run on an ESP32, which can be programmed with standard tools.
- PID temperature control for stable coals
- Over-the-air updates for remote maintenance
- Real-time web interface for monitoring
- CS1621 driver implemented from scratch
The project is a perfect example of how reverse engineering can improve a commercial product. It also opens the door to customizations the manufacturer never planned. Temperature reading is replicated with a 10 kohm NTC thermistor, the same type of probe the grill uses in four places. The code is available and well commented, so even less experienced users can learn.
Why this project matters
Freeing an IoT device from proprietary firmware is an important act. It makes the device more transparent, more secure, and easier to maintain. It also lets users add features the manufacturer never planned. PRBS23’s work on the MasterBuilt Gravity 800 is a model to follow for other similar devices. Careful reverse engineering can turn a closed product into an open platform.
The project also shows that no special hardware is needed to get started. A standard ESP32, a segmented display, and some patience are enough to achieve remarkable results. For those who prefer a ready-made solution, a four-digit 7-segment display can replace the original without rewriting the CS1621 driver. Finally, the documentation released by the author is an excellent starting point for anyone wanting to dig deeper.
Source: https://gitlab.com/prbs23/freefall_800/
