JTAG and SWD (debug ports)

JTAG, standardized as IEEE 1149.1, began as a way to test the connections on assembled boards (boundary scan) and became the common interface for loading firmware and stepping through code on a running chip. Serial Wire Debug (SWD) is a two-signal alternative common on modern microcontrollers that frees up pins.

These ports matter three times over: during development (debugging), in the factory (programming and testing every board), and in the field, where an open debug port can let anyone read out or replace your firmware.

Where it bites: leaving out test pads or a debug connector on a dense board makes bring-up and production programming painful, and forgetting to lock the port in production exposes firmware and keys.