DIY Watchman Dongles
Background
If you are using a lighthouse based tracking system, chances are you have encountered watchman dongles, either directly (Vive/Tundra trackers) or indirectly, mounted inside of your HMD.
These watchman dongles are USB radio receivers that communicate wirelessly with Lighthouse-compatible devices using Valve’s proprietary protocol. Watchman dongles are typically based on the Nordic NRF24LU1+ chipset, which is a USB-capable 2.4 GHz transceiver with a embedded microcontroller. In setups involving trackers like Tundra or HTC Vive, each dongle manages communication with a tracker or controller.
Unfortunately, these dongles are quite hard to get ahold of, without modifying steam controller dongles, it is reasonably straight forward to create your own.
In this semi-guide (lol) i will be showing how I was able to modify and flash a off-the-shelf development kit with Valve's own firmware.
Parts
When creating your own dongles, you can either choose to get a NRF24LU1+ development board or a board that has a NRF52840, which is a more advanced and newer integrated transciever. This is the one that sits inside of the Valve index and some newer headsets. NRF52840 development kits are more widespread these days since the NRF24LU1+ platform is being phased out.
I opted to go with a Makerdiary MDK52840 USB Dongle. These can be found for anywhere between 20-30€ in the EU. Whats great with these particular dongles is that they come preloaded with a UF2 Bootloader, which makes the flashing process quite straightforward.
Modifying the firmware
I cant share a modified firmware file, as that is Valves own IP. But i can show you how you can get one and modify it yourself. This guide is based HEAVILY on the work of Ugokutennp on github but instead of creating a UF2 file, I create a DFU package instead and flash it using Adafruit's modified NRFUtil.
Start by locating your SteamVR install directory. Im on Linux, so i could find it under ~/.local/share/Steam/steamapps/common/SteamVR but YMMV.
Now go to the drivers/indexhmd/resources/firmware/radio directory, in here you will find two archives. You will want to copy gd_1558748372_dfu.zip to a working directory of choice.