The Serial Wire Debug (SWD) interface or Serial Wire Debug Port (SW-DP) is one of the features of the debug and trace technology Arm CoreSight™. First implementations of SWD are realized in the derivatives of the Cortex-M3 core Stellaris of Luminary Micro (now Texas Instruments) and in the derivatives of the STM32 family by STMicroelectronics. The known JTAG Debug Port (JTAG-DP) is supported furthermore. Both debug ports, the SWD and the alternative JTAG debug port can be combined to the Serial Wire JTAG Debug Port (SWJ-DP), the CoreSight standard port.
When using SWD, the TDO signal can provide trace event messages via the Serial Wire Output (SWO). This behaviour can be used by the Serial Wire Viewer (SWV) to output system events via a single pin: