Serial Wire Debug (SWD) is a 2-pin electrical alternative JTAG interface that uses the same JTAG protocol on top. SWD uses an ARM CPU standard bi-directional wire protocol, defined in the ARM Debug interface V5, enabling the debugger to become another AMBA bus master with access to system memory and peripheral or debug registers. SWD uses fewer pins than JTAG, but has a higher transfer rate and is more stable. It is a useful interface for ARM devices or IP core testing and is gaining acceptance as the preferred interface for firmware burning. This presentation will introduce the SWD interface protocol, describe how to implement it by pattern and offer tips for online debugging.