Sebastian "dos" Krzyszkowiak's Debubo Is a USB PD Gadget Designed to Unlock USB SBU Debug Consoles

Developer Sebastian "dos" Krzyszkowiak has designed a tool that aims to make it easier to experiment with USB Power Delivery, offering the ability to sniff the connection or act as a man-in-the-middle device — or to access a USB Sideband Use (SBU) debug console on otherwise-uncooperative gadgets: the Debubo.

"[Debubo is] a four-layer PCB with [STMicroelectronics] STM32G071CB microcontroller equipped with two USB Power Delivery interfaces and [WCH Electronics] CH342 dual UART to USB (CDC ACM class) converter," Krzyszkowiak explains. "[The] STM32 can communicate via Power Delivery protocol over both USB-C plug and 'DATA/PWR' USB-C receptacle, which are directly connected to each other to pass power and USB 2.0 data through."

At its simplest, the Debubo sits in the middle of a USB Power Delivery connection — a relatively recent addition to the USB standard, which allows devices to negotiate higher voltages and amperages than a traditional USB connection can offer — and allows the communications to be monitored. Flip modes, though, and a secret power is engaged: the ability to access devices' debug consoles.

"[The] CH342 provides access to 3.3V UART on [the] USB-C plug's SBU [Sideband Use] pins," Krzyszkowiak writes. "This allows to access debug console on devices that only expose it on SBU pins after entering an alt-mode or sending custom VDMs [Vendor-Defined Messages] over PD [Power Delivery]. Debubo can also be used as a Power Delivery sniffer/injector (as a man-in-the-middle)."

There are a few caveats to the board's design, though: Krzyszkowiak warns that it requires that both sides agree on the negotiated voltage in order to avoid damage, you can't send or pass-through Fast Role Swap signaling though it can be received, and there's no shortage protection on the CC lines — which are only 5V tolerant, and could result in damage when using higher voltages.

Krzyszkowiak has released the Debubo design, as a KiCad project and Gerbers for production, on GitLab under the reciprocal GNU General Public License 3.