Joshua Vasquez writes:
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.