Running DOOM on a Video Walkie-Talkie is Possible, Just Not Practical

People buy video walkie-talkies to facilitate short-distance communication among children and / or family members. Aaron Christophel recognized an opportunity to bring DOOM onto one of these low-cost gadgets. These devices have small color screens, built-in cameras, microphones, speakers, and rechargeable batteries. Models retail online for between fifteen and twenty euros and rely on the TXW818 system on a chip to function. This chip has processing capacity comparable to some wireless modules and supports external memory, as well as four megabytes of PSRAM.

Christophel began by closely inspecting the electronics, noting that the devices differed: some had two megabytes of external flash memory, while others had four. Stuffing chip markings and scrambling flash memory is a typical practice that manufacturers use to make it difficult to meddle with their goods. Not to mention the specific tools required to delve deep into things: a USB to UART converter, a Blue Pill board posing as a J-Link clone, and a box called a profiler that allows you to monitor power consumption as devices perform different things.

The majority of the project included reverse engineering. Christophel dumped the original firmware and examined it with Ghidra, an app that is quite useful for this type of task. The software development kit that came with the device did not include drivers for the screen or camera, so Christophel had to create his own. Things only got tougher from there, since the stock firmware disabled the debug interface immediately after booting up. Christophel needed to find a means to keep it on, so he devised a mechanism that allows him to maintain access to the debug capabilities by connecting specific capacitors and communicating with the flash chip at startup.

With that resolved, Christophel began work on a new firmware, which did a variety of cool things such as initialize the screen and make the buttons functional. He also created a function that allows the device to recognize the amount of flash memory it has on board and alter its settings accordingly. Things got a lot more complicated when he attempted to port the game DOOM over. The original game data file alone takes up approximately one and a quarter megabytes, which was a problem for devices with only two megabytes of flash. Christophel had to reduce the file size to five hundred kilobytes. When the device boots, the firmware unpacks this data into PSRAM, which is exactly what is required to run the game.

The gameplay is rather straightforward, with you using the walkie-talkie buttons to move ahead and backward, as well as to turn around. When things begin rolling, an entertaining visual effect appears in the center of the screen, which is simply the front camera feed with the player’s face staring back at themself. Given the limited hardware, the performance is adequate, and the little screen does an excellent job of displaying the action, allowing you to have some fun with the device. Christophel has made the whole source code available on GitHub, so if you’re intrigued, you can now build the binaries and try it for yourself.
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