How can I access the debug interface of the EFM32?
The Serial Wire Debug (SWD) is a two-wire protocol for accessing the ARM debug interface. Its physical layer consists of two lines, a bidirectional data line (SWDIO) and a clock driven by the host (SWCLK). Connecting to these pins allows an external device (such as a debug probe) to communicate directly with the Serial Wire Debug Port (SW-DP). The SW-DP can then access one or several Access Ports (APs) that give access to the rest of the system.
Fig: Serial Wire Debug Interface
Read the complete application note here to know more about the SWD protocol and how to lock and unlock debug access.