What are JTAG and SWD debug interfaces?

JTAG (Joint Test Action Group) is a standard debug interface using 5 signals: TCK, TMS, TDI, TDO, and optional TRST. Supports daisy-chaining multiple devices, boundary scan testing, and flash programming. SWD (Serial Wire Debug) is an ARM-specific 2-pin alternative: SWDIO (data) and SWCLK (clock). SWD offers same debugging capabilities with fewer pins, ideal for space-constrained designs. Both support: breakpoints, single-stepping, register/memory access, and trace. SWD is preferred for ARM Cortex-M; JTAG for multi-device chains and non-ARM processors.