An “unbusy” USB-C Port Doubles-up For JTAG Programming

Board space is a premium on small circuit board designs, and [Alvaro] knows it. So instead of adding a separate programming port, he’s found a niche USB-C feature that lets him use the port that he’s already added both¬†for its primary application¬†and for programming the target microcontroller over JTAG. The result is that he no longer needs to worry about spending precious board space for a tiny programming port; the USB-C port timeshares for both!

In a Twitter thread (Unrolled Link), [Alvaro] walks us through his discovery and progress towards an encapsulated solution. It turns out that the USB-C spec supports a “Debug-Accessory Mode” specification, where some pins are allowed to be repurposed if pins CC1 and CC2 are pulled up to Logic-1. Under these circumstances, the pin functions are released, and a JTAG programmer can step in to borrow them. To expose the port to a programmer, [Alvaro] cooked up a small breakout board with a USB-C plug and separate microcontroller populated on it.

This board also handles a small quirk. Since [Alvaro’s] choice of programming pins aren’t reversible, the USB-C plug will only work one of the two ways it can be plugged in. To keep the user informed, this breakout board sports a red LED for incorrect orientation and a green LED for correct orientation–nifty. While this design quirk sacrifices reversibility, it preserves the USB 2.0 D+ and D- pins while also handling some edge cases with regard to the negotiating for access to the port.

Stick through [Alvaro]’s Twitter thread for progress pics and more details on his rationale behind his pin choices. Who knows? With more eyes on the USB-C feature, maybe we’ll see this sort of programming interface become the norm?

[Alvaro] is no stranger to Hackaday. In fact, take a tour back to our very first Supercon to see him chat about shooting lasers at moving targets to score points on a DEFCON challenge in the past